Marko Mńkelń (Marko.Makela@FTP.FUNET.FI)
Pekka Pessi (Pekka.Pessi@HUT.FI)
April 17th, 1994 (*)
[Last modified December 22nd, 1999]
* This document is partially based on Pekka Pessi's two articles describing an 256ákB internal memory expansion for the Commodore 64. The articles were originally published in the largest Nordic and Finnish home computer users' magazine, MikroBITTI, in its first two issues in the year 1987. Six years later, they were translated to English and edited by Marko Mńkelń, with help from Pekka Pessi. The translation should be available via anonymous FTP from FTP.FUNET.FI.
August 1996: Thanks to Wolfgang Scherr from Austria, who noticed my mistake in the schematic diagram. The inputs of the 74LS153 chip were mixed, which caused the address block decoding to fail.
By now, I know of two Commodore 64 (mine and Pekka Pessi's) where this expansion has been built, and of four Commodore 128s (of which two expanded by me, and the other two by Wolfgang Scherr and Matthias Kirchner from Germany). The expansion never became a success, although the banked concept is technically better than e.g. the Commodore REU. Meanwhile, there is one more program that supports the expansion: my transfer system prlink.
As the Commodore 128 was first introduced, 128 kilobytes feeled like an unbelievably big amount of memory. Nowadays even plain terminals and game consoles have more, and you can expand even a Commodore 64 to 256 kilobytes, twice as much as its big brother has by default.
There are several commercial memory expansions for the Commodores 128 and 64, but they are rather expensive, and most if not all of them are not being manufactured any more.
This article introduces three different memory expansions for the Commodore 128 and 128D computers. With these instructions, you can expand your computer to 256, 512 or 1024 kilobytes of internal memory. The 1024 kilobyte expansion is actually a combination of the two former ones, and it is fully compatible with both of them. When built by oneself, the 1024ákB expansion can remain cheaper than 200 Finnish marks.
One Americal dollar (USD) is equivalent to five or six Finnish marks (FIM). My expansion costed about 240 FIM, but I bought some parts in vain, and could have bought the memory chips somewhat cheaper. This time they costed 117 FIM.
I set three goals to the expansions. The bigger expansions should be fully compatible with the internal 256ákB expansion for the Commodore 64 when the computer is in C64 mode, and the 256ákB and 1024ákB expansions should be downwards compatible with a commercial internal 256ákB expansion for the Commodore 128. Finally, an expanded computer should be fully downwards compatible with an unexpanded one.
The design aims to a hardware that supports programming. The 256ákB and 1 MB expansions enhance the capabilities of the MMU in a way that the engineers at Commodore must have planned, and the other memory management logic is even easier to program.
You can retrieve this document in PostScript or LaTeX format via HTTP or anonymous FTP. The files are at FTP.FUNET.FI in the directory /pub/cbm/documents/projects/memory/c128/1028.
This article describes two memory expansions: an expansion that adds two new memory banks to the Commodore 128, doubling its memory space, and another expansion which expands each bank to 256 kilobytes, quadrupling the memory space. The former is the 256ákB expansion, from here on the MMU expansion, and the latter is the 512ákB expansion, or the PIA expansion. Combining these two expansions gives you four banks of 256 kilobytes each, that is 1024 kilobytes.
When I made the 256ákB expansion to my Commodore 64, I renamed the computer to 2564 --- 256ákB C64. The first three digits specify the amount of memory, whereas the last two ones tell the machine type number. A logical choice for the name of a 512ákB C128 is 5128 --- the first three digits tell the amount of memory in kilobytes, and the last three expose the original machine type. Unfortunately the 256ákB and 1024ákB expansions for the C128 cannot be named so nicely. I have baptized my C128D to C1028D, though.
The subsections 1.1 through 1.4 of this section apply for the PIA expansion. You can skip them, if you want to save some trouble and money and are going to expand your machine only to 256 kilobytes. Similarly, the section 1.5 can be skipped if you aim only to the 512 kilobyte expansion.
The processor of Commodore 128, MOS 8502, has an 8-bit data bus, and its address bus is 16 bits wide. Like other 8-bit processors, it can address only 64ákB of memory at a time. In most 8-bit computers, the memory is limited to these 64 kilobytes. How could one add memory above this limit?
The solution is simple: the memory is divided into banks of no more than 64ákB, which are switched on and off. Some processors have been added a special circuit for this purpose, in which case the executing program can be in its own 64ákB bank and the processed data in another bank. For example, MOS 6509, a fellow processor of MOS 8502, works in this way, enabling access to one megabyte. The Commodore 128 uses a sophisticated chip, MOS 8722 MMU (Memory Management Unit), which lets you to activate one 64ákB memory bank of a total of two memory banks at a time.
The PIA expansion expands each C128 memory bank to 256 kilobytes. The extended memory is divided to sixteen blocks of sixteen kilobytes each. The processor can address up to four of them at a time. Every four 16ákB segment of the address space can be mapped to any 16ákB block. Figure 1 shows the mapping right after startup.
However, the video chip VIC-IIe --- MOS 8566 (8564 for NTSC) --- retrieves its data from the memory outside the normal bus. The internal address registers of VIC-IIe are 14 bits wide, so it can address only 16ákB without external logic. The required two extra bits for accessing the whole 64ákB video bank are provided from the second CIA chip, and the video bank is selected by the MMU. Our extra logic provides additional two address bits for accessing the whole 256ákB of the selected video memory bank.
MPU's RAM RAM pool (bank 0) VIC-IIe's RAM +--$10000 --+ +-- $40000 --+ +--$10000 --+ | | /--->| Segment F |<---\ | | | Block 3 |-/ +-- $3C000 --+ \-| Block 0 | | | /--->| Segment E |<---\ | | +-- $C000 --+ | +-- $38000 --+ | +-- $C000 --+ | | |/-->| Segment D |<--\| | | | Block 2 |-/ | +-- $34000 --+ | \-| Block 1 | | | |/->| Segment C |<-\| | | +-- $8000 --+ || +-- $30000 --+ || +-- $8000 --+ | | || | Segment B | || | | | Block 1 |--/ | +-- $2C000 --+ | \--| Block 2 | | | | | Segment A | | | | +-- $4000 --+ | +-- $28000 --+ | +-- $4000 --+ | | | | Segment 9 | | | | | Block 0 |---/ +-- $24000 --+ \---| Block 3 | | | | Segment 8 | | | +-- $0000 --+ +-- $20000 --+ +-- $0000 --+ | Segment 7 | +-- $1C000 --+ | Segment 6 | +-- $18000 --+ | Segment 5 | +-- $14000 --+ | Segment 4 | +-- $10000 --+ | Segment 3 | +-- $0C000 --+ | Segment 2 | +-- $08000 --+ | Segment 1 | +-- $04000 --+ | Segment 0 | +-- $00000 --+
Figure 1. Memory mapping right after power-up
4164 41256 +--------+ +--------+ NC | 1 \/ 16| VSS MA8 | 1 \/ 16| VSS D | 2 15| -CAS D | 2 15| -CAS -W | 3 14| Q -W | 3 14| Q -RAS | 4 13| MA6 -RAS | 4 13| MA6 MA0 | 5 12| MA3 MA0 | 5 12| MA3 MA2 | 6 11| MA4 MA2 | 6 11| MA4 MA1 | 7 10| MA5 MA1 | 7 10| MA5 VDD | 8 9| MA7 VDD | 8 9| MA7 +--------+ +--------+
Figure 2. The Dynamic Random Access Memory Chips 4164 and 41256
Commodore 128 uses 64ákB dynamic RAM chips of JEDEC standard. In 1982, when the Commodore 64 was introduced, they were most modern technology, they needed only one operating voltage supply instead of traditional three.
The semiconductor memories have developed fast, however, and now a chip in a DIP of equal size can hold 256 kilobits. The pinout of these 256ákB chips differs minimally from the 64ákB ones. The smaller 64ákB chips, at least the ones used in C64 and C128, have one unused contact. The address line to handle three times bigger memory is tied to this pin. In the DRAMs the address lines are multiplexed: two address bits use the same pin successively.
In the MikroBITTI article Pekka wrote that 256ákB chips are rather cheap, and the price would lower as the production rate increases. Nowadays the production must have almost stopped. When Pekka bought his chips between March and April of 1986, they costed about 50 FIM each. When the original article was published, they costed less than 20 FIM. After that the prices rose due to a memory shortage. But nowadays the chips don't cost practically anything, if you're lucky. Many users of IBM PC compatibles want to upgrade their system memory with 1 Mb chips or alike and would like to get rid of their old 256ákB chips. I bought eight second-hand chips with total 35 FIM, and later 36 chips with 117 FIM, including shipping. The lowest price of unused chip I encountered was 13 FIM a piece and the highest was 30 FIM, almost 10 times the price I paid!
The 256ákB chips don't consume significantly more power, so there is no need for a bigger power supply. However, devices that take their power directly from the computer can cause problems. You can find this out by experimenting.
The speed of the chips doesn't prevent the replacement either. According to their schematics diagrams, Commodore 64 and 128 can use chips with access time of 200 nanoseconds. (Besides, the oldest Commodore 64 I have uses 300áns chips housed in ceramic packages.) Even the slowest 256ákB dynamic RAMs are not that slow.
It might be wise to replace the bypass capacitors near the memory chips with bigger ones, at least if you are going to make the MMU expansion (see Section 1.5). On the other hand, my machine works well with the default 220ánF capacitors. If your computer starts to work unreliably, too small bypass capacitors could be the culprit.
The dynamic RAM chips are organized in rows and columns. In 64ákB chips, a row is 256 bits wide, and in 256ákB ones it is 512 bits wide. Also the memory address is divided into row and column addresses. When a bit is being accessed in the dynamic RAM, the row address is asserted before the column address.
First the interfacing circuitry puts the row address on the Multiplexed Address bus, while the video chip asserts the -RAS (Row Address Select) signal for a short period. After that, the video chip pulls the MUX line low, and the interfacing circuitry places the column address on the bus while pulling low the -CAS (Column Address Select) signal of the selected RAM bank. After all this, the bits in that position can be read or written.
The computer has two circuitries that take care of this multiplexing. The multiplexers U14 and U15 form the address when either one of the C128's two processors has the bus, whereas the video chip produces the row and column addresses itself when it needs its screen data.
You could access a set of nearby locations faster, if you specified the row address only once, and then produced only the column addresses for each location. This technique is supported in the Acorn Archimedes computer on the processor level, and some other computers utilize it with external circuitry as memory interleaving. The Commodore, however, does not have to hassle with this, as its system clock rate is so slow. As a matter of fact, it actually uses the least significant processor address bits (A0--A7) as a row address and the most significant bits (A8--A15) as the column address.
A dynamic memory chip stores the data bits as charged tiny capacitors, which discharge among the time. The data must be refreshed periodically, every 2--4 milliseconds, by recharging the capacitors.
If the whole contents of the memory was refreshed simultaneously, the power peak would cause enormous problems. Only a block of one or two rows can be refreshed at a time. The 64ákB chips have 128 blocks to be refreshed, which implies a 7-bit refresh counter (2^7 equals 128).
In order to avoid disturbance, 256ákB chips must have more blocks. Thus they require a longer refresh counter (8 bits). As the amount of refresh cycles has increased, the capacitors' ability of keeping charge has been improved. The 64 kilobit DRAMs required 128 refresh cycles every 2 milliseconds, now the 256ákB chips need 256 cycles but every 4 ms.
Whenever you select a row address (see Section 1.3), the block to which the row address belongs gets refreshed. As the 64 kilobit chips have a 7-bit refresh counter, the lowest seven row address bits specify the row address, and the highest bit is ignored. The 256 kilobit memory chips have an 8-bit counter, so they ignore the 9th row address bit and determine the block to be refreshed by the eight lowest bits.
The VIC-IIe chip refreshes the memory systematically, 5 rows in the end of each screen scan line. It does this by selecting a row address determined by its internal counter, and then increases this counter by one. The video chip could have only 7-bit refresh counter, and it would still operate with 64ákB chips, but fortunately it has an 8-bit counter, so all of the 256ákB chips get refreshed.
Newer memory chips can be refreshed using a CAS-before-RAS technique. In this technique, you pull first the -CAS signal low, and then the -RAS signal. The memory chips recognize this as a memory refresh condition, and they refresh a block and increase their internal refresh counter. However, this technique was not available when the Commodore 64 and its video chip were designed.
The Commodore 128 has two memory banks, numbered 0 and 1. The banks are switched in and out by a custom chip called MOS 8722 MMU (Memory Management Unit). The chip has the registers for handling four memory banks, but there are only two hardware lines for bank selection, named -CAS0 and -CAS1. They are connected to the -CAS signal (see Section 1.3) of the memory chips in banks 0 and 1, respectively.
The MMU expansion adds two new memory banks to the computer. It adds another 8722 MMU chip to the system, routing some signals so that the chip considers bank 2 as bank 1. The -CAS0 and -CAS1 outputs of the two MMU chips will be combined to form the -CAS signals for all four memory banks. The logic glue involved is very simple, and designing it was quite straight-forward.
Although this procedure worked perfectly for me, I cannot guarantee that anyone else can perform this upgrade without damaging their computer. I therefore disclaim any responsibilty for any damage that may occur as a result of attempting this upgrade. It will also void any warranty on your computer.
On a more positive note, there is no reason why someone who is experienced in wielding a soldering iron, and has done some electronic construction or troubleshooting, should not be able to perform this upgrade successfully.
A termostate soldering iron, desoldering pump or other desoldering tool, a screwdriver, a spoon and a continuity tester are the only tools needed. The spoon is for removing the chips. A bottle top remover is not suitable for that.
A tiny screwdriver is equally good. Just insert the screwdriver tip under one end of the chip and wound it a bit in upward angle so that the chip moves slightly. Then insert it to the other end of the chip and try to lift it a bit. You may have to repeat this procedure. Be careful not to wound the pins too much.
The continuity tester is vital for checking suspicious connections. If your tester does not automatically select proper measuring range, use the coarsest (mega-ohm) range, as it uses smallest current, which shouldn't damage any chips on the board.
The installation begins of course by opening the machine and removing the keyboard and LED cables (and internal drive and power supply in the C128D). It is useful to memorize, photograph or draw how the parts were initially connected.
After removing the cables, open the screws that hold the metal RF shield and the motherboard with the case, and remove the shield and the board.
If you are going to expand your machine only to 256 kilobytes, skip the following subsection. If you aim to a whole megabyte, expand your computer first to 512 kilobytes, and then make the MMU expansion.
The PIA expansion consists of one daughter board, which contains most of the new logic, one piggy-backed chip, and a spaghetti of wires.
In Figure 3, there is a schematics diagram of the daughter board for the PIA expansion. If you cannot read it, try to get the LaTeX or PostScript version of this document.
Figure 3. The schematics diagram of the PIA expansion. See text.
There are some signals that you must wire to the mother board. You can take the -I/O2 and A7 signals from the cartridge port, or from some through-put location near the daughter board. The -I/O2 signal should be on the pin 7 of the chip U3 (74LS138). The A7 can be also taken from the MMU's (U7, MOS 8722) pin 23, in which case the address range of the PIA will be limited to $DFC0--$DFFF instead of $DF80--$DFFF, or from the 8502's pin 14. It is also on the multiplexor U14 (74LS257A), in pin 3.
The MA8 signal is the new Multiplexed Address line for the memory chips and should be soldered to the pin 1 of each chip. All the remaining five signals on the right edge of the diagram interface to the multiplexor chip U14. The MUX signal goes to pin 1. To interface the address lines A14, A15, B14 and B15, you have to desolder two pins of the multiplexor, 2 and 5. The signal A15 should then be wired to the mother board contact under the multiplexor pin 2, or to the 8502's pin 23, and the signal B15, the relocated address line should be soldered to the multiplexor's pin 2. Similarly, the contacts A14 and B14 should be connected to the system bus line A14 and the U14's pin 5, respectively. Figure 4 shows the pinout of the multiplexer chip U14.
U14 74LS257A +--\__/--+ MUX -|1 16|- Vdd TA15 -|2 15|- -AEC A7 -|3 14|- TA12 VMA7 -|4 13|- A4 TA14 -|5 12|- VMA4 A6 -|6 11|- TA13 VMA6 -|7 10|- A5 Vss -|8 9|- VMA5 +--------+
Figure 4: Pin-out for the multiplexer chip U14
+-----\___/-----+ +-----\___/-----+ Vss -|1 40|- CNT Vss -|1 40|- CA1 PA0 -|2 39|- SP PA0 -|2 39|- CA2 PA1 -|3 38|- RS0 PA1 -|3 38|- -IRQA PA2 -|4 MOS 37|- RS1 PA2 -|4 MC 37|- -IRQB PA3 -|5 36|- RS2 PA3 -|5 36|- RS0 PA4 -|6 6526 35|- RS3 PA4 -|6 6821 35|- RS1 PA5 -|7 34|- -RESET PA5 -|7 34|- -RESET PA6 -|8 CIA 33|- D0 PA6 -|8 PIA 33|- D0 PA7 -|9 32|- D1 PA7 -|9 32|- D1 PB0 -|10 31|- D2 PB0 -|10 31|- D2 PB1 -|11 30|- D3 PB1 -|11 30|- D3 PB2 -|12 29|- D4 PB2 -|12 29|- D4 PB3 -|13 28|- D5 PB3 -|13 28|- D5 PB4 -|14 27|- D6 PB4 -|14 27|- D6 PB5 -|15 26|- D7 PB5 -|15 26|- D7 PB6 -|16 25|- Phi2 PB6 -|16 25|- E PB7 -|17 24|- -FLAG PB7 -|17 24|- CS1 PC -|18 23|- -CS CB1 -|18 23|- -CS2 TOD -|19 22|- R/-W CB2 -|19 22|- CS0 Vdd -|20 21|- -IRQ Vdd -|20 21|- R/-W +---------------+ +---------------+
Figure 5: The CIA and the PIA
+----------------------------------------------+ | Electronic Components | +----------+-----------------------------------+ | Symbol | Description | +----------+-----------------------------------+ | IC1 | MC6821 | | IC2 | 74LS153 (or 74LS253) | | IC3 | 74LS151 (or 74LS251) | | IC4 | 74LS05 | | U9--U12, | 80256 or | | U21--U24 | compatible | | C1 | 100ánF polyester capacitor | | R1 | 33 ohm resistor | | R2 | 4.7 kilo-ohm resistor | +----------+-----------------------------------+ +----------------------------------------------+ | Other Parts | +----------+-----------------------------------+ | Quantity | Quality | +----------+-----------------------------------+ | 2 pcs | 10-pin flat cable connector pair | | ca. 15 cm| 10-wire flat cable | | 1 pc | 16-pin piggyback socket | | 2 pcs | 16-pin WW-socket | | or | one 16-pin piggyback socket and | | | two normal 16-pin sockets | | 1 pc | 40-pin WW-socket | | or | one 40-pin piggyback socket and | | | a normal 40-pin socket | | 10 pcs | 16-pin socket | | 1 pc | 14-pin socket | | plenty of| connection wire | +----------+-----------------------------------+
Table 1. Parts list for the PIA expansion
First you have to remove the memory chips U38 through U53. If you look at the mother board from the front of the computer as if you were typing, the chips are on the front left in two rows of eight chips. They are of type 4164 (or 3164 or 6665 or 6664 or 8064 or...). You could install the new chips into sockets, but I thought that it is a waste of money.
If these memory chips are already on sockets, the most of the work is done for you. It helps a lot, if you remove the bypass capacitors before removing the chips. Removing the components is easiest with a desoldering pump. It becomes even easier, when you first solder the pins with fresh solder, so that the hartz from it makes the removal of old solder easier.
Using much power is questionable, as the copper folio comes off the board in a surprisingly easy way. As usual, I used a screwdriver like a crowbar, and the through-coppering got lost from several places. This was not crucial, as those pins were connected only to the down side of the board. However, three or four routes broke on the top side also. This made it far more difficult (and slower) to solder the new chips in, but I succeeded on the first try.
After you have removed the 4164s, you can solder the 16-pin sockets (or the 41256 memory chips) into their places. You can solder the capacitors back as well, if you removed them.
You must connect the pin 1 of each memory chip (or socket). It is the extra address line (MA8) to the switcher. The best way is to solder a Wire-Wrap wire to each contact under the mother board, but any thin and pliable uni-strand wire should do. The wire does not affect in any way the computer's operation with 64ákB chips.
After the pins have been connected together, they must be temporarily connected to +5áV, which is in the pin 8 of the memory chips. Comparing to TTL chips, the operating voltages are `reversed' in dynamic memories.
Now the new 256ákB memory chips can be installed to the sockets (preferably right-side forward), and you can try switching the power on. You do not have to connect anything except the power cable and the cable to the TV set or monitor. Ensure that the ``40/80 display'' key is up, and that the monitor is set to display the 40 column screen, too. It is a good idea to turn on the monitor first and let it warm up, so that it will show the picture from the very beginning.
If the screen shows up normally, you may not (yet) have made any mistakes. If it does not show up at all, you have to find possible cut-outs and shorts. Multi-colored `@'s show up usually because of too small bypass capacitors. Another cause is that the pin 1 is not connected to +5áV. In this case the screen may come up normally, but a little disturbance in the operating voltage locks the computer up. Now the computer should operate exactly like an unexpanded C128, so any previously working program should work with it.
Next you remove U14 (74LS257, to the right of the memory chips) and U1 (MOS 6526, near the keyboard connector). Either or both of these chips may already be on sockets, and you must remove the rest. Reinsert the chips and check if the machine boots up.
If the computer does not work on first try, remember to disconnect any cables from it before trying to fix the problem. The soldering iron may occasionally give little electric pulses to the computer, and this might burn some expensive chips, especially if the computer is hooked to a wall outlet or a television set.
When you have completed the preparations, you can start building the control logic. You could build the whole expansion by piggy-backing chips, that is, by soldering new chips on the top of old ones, bending some feet to the side, and connecting messy wires all over your computer. However, the best way is to put most of the chips on a daughter board. I used only a small daughter board, and piggy-backed five or six chips, but you can be wiser and put all new chips on the daughter board.
My daughter board interfaces the heart of the expansion, MC 6821 PIA, to the bus of the computer through the pins of U1, the MOS 6526 CIA near the keyboard connector. The CIA is raised on the board, and its pins are lenghtened with two through-put socket halves, so that they can reach the socket on the mother board. I built the daughter board on an uncoppered prototype board, a plastic board with holes punched in it at a 1/10 inch grid.
The room reserved for the mother board in the C128 and C128D is very shallow, about one third of the height in the C64. In addition to that, the front edge of the mother board must be even shallower, as the metal shield has an angle in it. Due to this, you cannot use any sockets in the CIA daughter board, and you have to choose the chip layout very carefully. My daughter board has the PIA chip on the left side of the CIA. To leave room for the MMU expansion, I could put only two chips (IC2 and IC3) horizontally next to the notched end of the PIA (IC1) and CIA (U1). I placed the inverter (IC4) with the 100ánF bypass capacitor near the other end of the PIA chip.
A far better way is to interface the daughter board to the socket of U7 (MOS 8722 MMU). There are not so terrible space limitations, the RF shield is higher near the rear edge of the machine than on the front edge. In addition to that, the keyboard cable of the flat C128 is not so likely to damage that daughter board than the CIA board, which would be next to the keyboard connector. The MMU daughter board would allow you to make an easily removable expansion, as no chips would be piggy-backed. You could even make an option for installing a second MOS 6581 (8580 for the 9ávolt version) SID (Sound Interface Device) on the board to get stereo sound. However, this board should be etched, as the PIA and MMU pin layouts differ very much from each other. See Figure 6 for pinouts for the MMU. Pinouts for the PIA and CIA are presented in Figure 5.
Building the CIA daughter board was a real pain. I had to solder the CIA directly to the piggy-back socket pins, and I even bent the CIA pin ends aside, so that I could make it about 1 mm shallower. I put the CIA pins and the through-put socket halves to the same holes and started soldering. To keep the socket halves parallel, I put one half against the outside of the CIA pins, and the other half against the inside. It was very easy to solder the half whose contacts were outside the pins, but the other half was a real pain. It could be done by heating a CIA pin, inserting some solder from the side, and hoping that it connects the piggy-back socket pin. I had to solder those pins four or five times.
After raising the CIA on the daughter board, it is a very good idea to insert the board to the socket and check if the machine boots up. If some of the right side pins (21--40) are loose, the machine can jump to ML monitor due to an unexpected interrupt, or it can misread the keyboard. In the C64 mode, it will probably jam.
The next step is to add the PIA on the board. The contacts from the CIA except the operating voltages may be difficult to route. I solved the problem by putting the wires through the very small holes that were left between the biggy-back socket halves and the down surface of the daughter board. It was very painful, but the design is very compact. After soldering all CIA contacts to the PIA, I wired the inverter and the rest of the chips. To increase reliability, I used thin multi-strand wire, as uni-strand wire gets easily loose when you push it.
Since I had finished the daughter board, I bent up the pins 2 and 5 of the U14 multiplexer chip, and connected its pins 1--3 and 5--6 to the daughter board with wires. First I inserted the wires for A14 and A15 directly to the chip socket, but as it turned out to be unreliable, I located a through-put place for each line, and soldered the wires there instead.
When you have wired the multiplexer U14, remove the jumper wire between MA8 and +5áV and connect that address line to the daughter board. Then connect the PIA's -CS line to -I/O2, which is in U3's pin 7 (or one of the through-put places along the trace's path to the cartridge port), and insert the daughter board to the socket. Switch the power on and pray that your dear computer works.
If you get only crap consisting of @'s or some randomly changing graphics on the 40 column screen, check that all CIA pins have a good contact to the piggy-back socket, and that the wires from U14 and its socket are firmly connected. If it doesn't help, you have to check all daughter board connections with the continuity tester. Don't panic, you can ensure that the computer works by connecting the MA8 line back to +5áV, by bending the U14 lines back down, and by inserting a spare CIA chip to the CIA socket.
After you have installed the boards to your machine, it is time to test the connections. You can connect LED, keyboard and probably disk drive in addition to the power cable and the TV cable, but do not fasten the mounting screws yet. If the 40 column screen shows up and if the machine seems to operate, input the following test program:
10 PB=57282 20 POKE PB,255:POKE PB+1,4:POKE PB,255 30 PRINT"PRESS A KEY AFTER THIS HAS DISAPPEARED": FOR I=0 TO 3000:NEXT 40 POKE PB,14:WAIT 198,15:GET A$:POKE PB,255
On the line 10 a variable PB is set up. It is the address of the peripheral and data direction registers for the 6821 port B, and the block selection register of the segments 2 and 3 and the VIC-IIe.
The line 20 contains initialization of PIA: the lines PB0--PB7 are set outputs, the data direction register is switched to data register with `POKE PB+1,4', and the PB lines are set high.
On the line 40 VIC-IIe is given block 0 ($00000--$0FFFF) of the default bank (0), and then the program waits for a keypress and restores the block F ($30000--$3FFFF).
If this test program works as expected, the screen will be filled with `@'s and other random characters.
At this point, you may want to switch to the C64 mode and to run the TEST program, which is among the distribution files. (See Section 5.1.) Also, you can try the PIAGLOBE.128 program to test almost all of the 512ákB or 1024ákB memory.
The PIAGLOBE.128 program is based on Georg Schwarz's globe spinner GLOBE.64 that uses two graphics screens. He has made a slightly faster version for the C128, utilizing the 2 MHz mode in the screen border. But compared to it, PIAGLOBE.128 is from other planet. Depending on the amount of memory available, it calculates 112 or 56 pictures of the globe and then uses them in a continuous animation. One revolution will last approximately 2.23 seconds on PAL systems and 1.87 seconds on NTSC. As the calculation phase lasts more than a minute, the program changes the screen color between each picture.
On NTSC systems, the edge of the globe might not display correctly. The edge is rounded with 24 sprites, which are moved around by a raster interrupt routine, starting from $4801. I did not bother to think about the timings, since I had enough troubles with relocating the program and the tables, and in trying to get all that graphics data to fit in the memory. In the distant future I might make a better looking version of the globe spinner, who knows. There is over 80 kilobytes of unused memory when running the program on a C1028.
This MMU expansion is far easier to understand than the PIA expansion, and maybe faster to build, too. You have to solder the new MMU and a couple of logic chips on top of some chips laying on the mother board, or to install them on a daughter board. I chose the piggy-backing method.
The biggest problem with this expansion is that the MOS 8722 MMU is a custom chip from Commodore, and it is only used in the Commodore 128, which has not been manufactured for ages. If you do not happen to have a wreck C128 lying around, you can try ordering the chip from Jameco Electronics. The chip shouldn't cost more than 10 USD. You can reach them at:
Orders (phone): 1-800-831-4242 Orders (fax): 1-800-237-6948 Fax (overseas): +1-415-592-2503 Mail: Jameco Electronics 1355 Shoreway Road Belmont, CA 94002 U.S.A.
In Figure 6, there is a wiring diagram for this expansion. It is a bit tight, and needs some clarification. The U9 74F32 is a quad OR chip (each C = A OR B) that takes the -CAS0 and -CAS1 outputs from the original MMU and lets them through to the original memory chips as -RAMCAS0 and -RAMCAS1 if and only if both the -CAS output from the video chip and the -CASENB output from the PLA are active. IC9, the 74F32 next to U9, ``hooks'' the -RAMCAS outputs and lets them through when the -CAS0 output of the new MMU is active, that is, when the banks 0 or 1 are being accessed. The IC9 also generates the -CAS signals for the two new memory banks, -RAMCAS2 and -RAMCAS3. The 68 ohm resistors protect the IC9, as the inputs of the DRAM chips are not fully TTL compatible.
The logic glue at the lower edge of the picture take care of feeding correct values to the new MMU's data leads D6 and D0. See Section 3.1.3 for a complete description. The IC8 4066 takes the data lines D0, D1, D6 and D7 from the system bus and feeds two of them to the new MMU's data lines D0 and D6 at a time. In my implementation the D0 and D1 lines come from the U20 4066 and the remaining two data leads from the original MMU. The U20 4066 is not really needed at all, as the same lines are on the original MMU, but I chose it as it was easy to piggy-back.
On the lower right corner is U3, a 74LS138 which takes care of producing the -CS (Chip Select) signals for the address ranges $D400--$DBFF and $DC00--$DFFF. The IC6 does not actually need any signals from it, not even the -IOCS that is on the contact -G2, but it was easier to piggy-back so. If you build a daughter board for this expansion too, you can wire the IC6's G1 to +5áV and the -G2 and -G3 to ground. The IC6 together with the IC7, a quad Negative-AND chip, inputs the four lowmost address bus bits and produces the Enable signals for the IC8 4066.
Figure 6: The schematics diagram for the MMU expansion logic.
MOS 8722 MMU +--------------+ Vdd | 1 \__/ 48 | 40/-80 -RESET | 2 47 | MS3 TA15 | 3 46 | -EXROM TA14 | 4 45 | -GAME TA13 | 5 44 | -FSDIR TA12 | 6 43 | -Z80EN TA11 | 7 42 | D7 TA10 | 8 41 | D6 TA9 | 9 40 | D5 TA8 | 10 39 | D4 -CAS1 | 11 38 | D3 -CAS0 | 12 37 | D2 MS2 | 13 36 | D1 MS1 | 14 35 | D0 MS0 | 15 34 | Vss AEC | 16 33 | 0in MUX | 17 32 | R/-W A0 | 18 31 | A15 A1 | 19 30 | A14 A2 | 20 29 | A13 A3 | 21 28 | A12 A5/A4 | 22 27 | A11 A7/A6 | 23 26 | A10 A8 | 24 25 | A9 +--------------+
Figure 7: The MOS 8722 Memory Management Unit
+----------------------------------------------+ | Electronic Components | +------------+---------------------------------+ | Symbol | Description | +------------+---------------------------------+ | IC5 | MOS 8722 | | IC6 | 74LS138 | | IC7 | 74F00 | | IC8 | 4066 | | IC9 | 74F32 | | IC10--IC25 | 80256 or compatible | | R3, R4 | 68 ohm resistor | +------------+---------------------------------+ +----------------------------------------------+ | Other Parts | +-----------+----------------------------------+ | Quantity | Quality | +-----------+----------------------------------+ | 1 pc | 14-pin socket | | plenty of | connection wire | +----------------------------------------------+
Table 2. Parts list for the MMU expansion
74LS138 74F00 4066 +--------+ +--------+ +--------+ A | 1 \/ 16| Vdd I1 | 1 \/ 14| Vdd A1 | 1 \/ 14| Vdd B | 2 15| -Y0 J1 | 2 13| I4 B1 | 2 13| E1 C | 3 14| -Y1 K1 | 3 12| J4 B2 | 3 12| E4 -G3 | 4 13| -Y2 I2 | 4 11| K4 A2 | 4 11| A4 -G2 | 5 12| -Y3 J2 | 5 10| I3 E2 | 5 10| B4 G1 | 6 11| -Y4 K2 | 6 9| J3 E3 | 6 9| B3 -Y7 | 7 10| -Y5 Vss | 7 8| K3 Vss | 7 8| A3 Vss | 8 9| -Y6 +--------+ +--------+ +--------+ Kn = NOT(In AND Jn)
Figure 8. The logic glue chips 74LS138, 74F00 and 4066
First bend up the pins 3--15, 35 and 41 of the new 8722 MMU chip (IC5) aside. Bend the rest of the pins slightly to the opposite direction so that they are perpendicular to the surface of the chip. If the old MMU is in socket, pop it off. Push the new MMU on the back of the old one. Solder the pins together, but be careful not to heat the chips too much. Now you can insert the MMU pair to the socket.
After wiring those pins, you must build the logic for interfacing the system data bus to the new MMU. Locate U3, 74LS138, it is to the left of the processor (U6, 8502). Take another 74LS138 (IC6) and bend its pins 1--3, 7 and 9--15 aside. Solder the rest of the pins to corresponding U3 pins. Connect the pins 1--3 of the new chip to the processor's pins 7--9, respectively.
Before connecting the IC6's required outputs, you have to solder IC7 and IC8 on the board. First locate U20, the 4066 to the right of the RF shield covering the video circuitry. Get a new 4066 (IC8) and bend its pins 1, 4--6 and 8--13 to the side and solder the four downwards pointing pins to the U20. Wire the pins 1 and 4 together and connect them to the new MMU's pin 35, the data lead D0. Similarly, connect the pins 9 and 10 together, and solder them to the topmost MMU's pin 41, D6. Solder the IC8's pins 8 and 11 to the system data bus lines D6 and D7, respectively. They are available in the original MMU's pins 41 and 42. It is a good idea to locate through-put places for these signals and solder the wires there.
The next chip to be mounted is IC7, the 74F00. A good place for it is the 14-pin chip next to the IC6 and the processor. Leave only the pins 7 and 14 down and solder them to the chip on the motherboard. Connect the pins 13 and 12 from the 74F00 to the IC6's pins 15 and 13, and connect the 74F00's pins 11 and 10 together. Then solder the pin 9 to the 8502's pin 10, A3. Solder the IC7's pin 8 to its own pins 4 and 5, and connect it also to the IC8 4066's pin 13. Connect the IC7's pin 6 to the IC8's pin 5. Lead the 74LS138's pin 10 to the 4066's pin 6 and to the 74F00's pins 1 and 2. Lead the 74F00's pin 3 to the 4066's pin 12.
After all these piggy-backings, it is wise to check if the computer powers up any more. If not, check the solderings. When I built this second version of the MMU expansion, I swapped the 4066 pins 9 and 10 by mistake, which resulted in a miserably black screen each time I desperately tried to power the computer up.
If the first stage succeeded, you can build the new logic for deriving the -CAS signals for the two new RAM banks. Locate U9 (74F32) and lift it on a socket, if it was directly soldered to the system board. If the computer works after this operation, you can continue with the piggy- backing. Take a new 74F32 (IC9) and bend its all pins except 7, 9, 12 and 14 to the side. Bend the pins 8 and 11 of the old 74F32 up, so that they can be connected to the IC9's pins 9 and 12, respectively. Connect the pins 7 and 14, too. Solder a short length of stiff uni-strand wire to the new 74F32's pins 8 and 11, in order that these pins reach the socket on the motherboard. Connect the pin 12 of the new MMU to the IC9's pins 13 and 10.
Now the computer should work just as earlier, except that when you try to access the banks 2 or 3, the processor will read randomly changing garbage. You can verify this by moving the cursor to the top left-hand corner of the screen and typing ``M 20400'' or ``M 30400'' in the machine language monitor a couple of times.
To generate the signals -RAMCAS2 and -RAMCAS3, connect the topmost MMU's pin 11 to the IC9's pins 1 and 4, and solder the new 74F00's pins 2 and 12 as well as the pins 5 and 9 together. Then mount R3 and R4, the 68 ohm resistors to the new 74F00's pins 6 and 3. On their free ends will be the -RAMCAS2 and -RAMCAS3 signals, respectively.
After ensuring that the computer works, you can prepare for the final step. Take the sixteen memory chips (4164's or similar if you are aiming to the 256ákB memory expansion; 41256's or similar for the 1024ákB expansion) and bend their pins 15 (-CAS) up. Solder the remaining pins on top of the sixteen memory chips on the motherboard. Then combine the -CAS signals of the eight new memory chips at the front edge of the system board and connect them to R3 or R4. Similarly, connect the pins 15 of the remaining memory chips and wire them to the remaining 68 ohm resistor.
Power the computer up again and pray until it works. If you can access the banks 2 and 3 as expected, congratulations! When re-assembling the chassis, be very careful with the RF shield. Especially measure that the -CAS signals for the new memory banks have no contact with the shield.
There are four new micro chips in the PIA expansion. The most important of them is the PIA chip MC 6821, which holds the values of the block selections. The PIA has two 8-bit ports set up in the addresses 57280 and 57282. The upper and lower four bits (nybbles) of each port determine which 16ákB block is mapped to each 16ákB segment of the processor's address space. IC2 and IC3 participate in forming the memory block control signals.
There is a chip equivalent to the PIA even in Commodore's own 6500 series, but it is not suitable for this connection, as it is not TTL compatible. The 6821 from Motorola 6800 series, which contains also processors reminding those in the CSG (Commodore Semiconductor Group; former Mostek or MOS Technologies) 6500 and 8500 series, is bus compatible and suitable for this purpose.
Commodore 128 asserts the 16 bit addresses to the original 64ákB chips in two parts. First it asserts the lower eight bits, then the higher eight. The 256ákB chips require two additional address bits, so the chips are given nine bits at a time. Due to this address multiplexing, the block selection bits cannot be directly input to the memory chips, but they must be lead through the multiplexer circuitry of IC2, IC3 and U14.
IC4 contributes to the operation during power-up. It ensures that the C128 gets reasonable memory blocks to its different segments. In the beginning the segments are filled with four upmost memory blocks.
The PIA's data bus and E, -RESET and R/-W signals have been connected directly to the 6526 chip. Similarly are the RS0 and the RS1, which select a PIA register, connected to A0 and A1.
The I/O block decoder (U3) tells us when the second I/O block is selected. This block resides in the area $DF00--$DFFF. The signal -I/O2 is connected to the PIA's chip selection pin -CS, and it forms most of its addressing. The address line A7 limit PIA's area in I/O2 to $DF80--$DFFF, because it will be tied to the CS pin.
As the address space has been divided to four segments of 16ákB, the A14 and A15 cannot be lead directly to the memory chips, but they participate in the block selection. These two address bits determine which of the four blocks is in use. For each segment, the PIA ports tell which memory block to map. Original A14 and A15 are connected to IC2 and IC3, which select the right output lines of PIA. For each 16ákB segment there are 4 output lines which form the block address for the segment.
IC2 selects two lowest bits of the block address and feeds them to the address multiplexer chip U14 as B14 and B15. They are practically equivalent to the A14 and A15 signals. After the address bits A0--A7 have been asserted during the first addressing cycle, IC2 asserts B14 and B15 during the second (CAS) cycle.
The 256ákB memory chips still need two extra address bits. The expansion must multiplex them with IC3, which is a `one-of-eight' multiplexer. Its eight inputs are tied to the two upmost bits of the four block addresses. A14 and A15 are connected to the IC3, but it needs yet another control signal to handle all eight input bits. This signal is MUX, which controls multiplexing other address bits (MA0--MA7) as well.
While the MUX signal is low and the memory chips are fed the lowest bits (A0--A7) of the address, the IC3 selects the third bit of the block address determined by A14 and A15. This bit is called address bit A16, and it is asserted to the `extra' address line MA8 simultaneously with the lowmost bits. When MUX is high, the upper address bits are fed, and IC3 selects the fourth bit of the block address determined by A14 and A15. It corresponds to the address bit A17 and is fed through the same MA8 with all the other upmost bits.
When the video chip accesses the bus, the address and data lines from the processors are in high-impedance state, driven to logical `1' level with very weak current, so that the video chip can change their state easily. As the address lines A14 and A15 (or actually TA14 and TA15 in the C128) are not connected to the VIC-IIe, they remain as logical `1' whenever the video chip has the bus. Thus, the switcher logic `thinks' that the address range $C000--$FFFF is being addressed, and it selects the block for that segment also for the VIC-IIe.
The resistor on the MA8 line protects the IC3, because the inputs of dynamic memories are not fully TTL compatible.
The new MMU must be fooled so that it mistakes the memory bank 2 for bank 1. This can be done by connecting its data bit D6 to the system data line D7 whenever the RAM Configuration Register (location $D506, see Table 10) or any of the Configuration Registers ($D500--$D504 and $FF00--$FF04, see Table 8) are being accessed. In addition to this, the page relocation registers must be taken into consideration, or you could relocate the processor pages 0 and 1 only to banks 0 or 3. For this reason, the MMU data line D1 must be connected to system data line D0 when the locations $D508 or $D50A are being accessed. There is a commercial 256ákB or 512ákB expansion for the C128 that does not take care of this.
Actually the logic rules can be relaxed, and the MMU pin D6 can be connected to system D7 for most time. The only register that needs the D6 line to be connected to system D6 is the Mode Configuration Register ($D505, see Table 9).
Connecting the D1 and D6 pins is performed by three chips: 74LS138, 74F00 and 4066. Consult Figure 8 for their pinouts. The 74LS138 inputs a 3-digit binary number through its A, B and C pins and converts it to an octal digit by activating one of its eight output lines -Yn, provided that the chip has been selected with the G inputs. The 74F00 is a quad Negative-AND chip, and the 4066 is a quad analog switch. It connects its A and B contacts together whenever the respective E pin is in the logical `1' state. The IC6 74LS138 and the IC7 74F00 generate the E signals for the IC8 4066 from the system address lines A0--A3.
In order to enable the operation of the machine, each of the four segments must be mapped to a unique memory block. The Commodore 64 Kernal tests the lowmost continuous area of writeable memory and would hang up, if the same block was mapped to both $0000 and to $4000, for instance. Modifying the startup routines would cure this problem, but in that case the Kernal ROM chip should be changed. Also the C128 Kernal would need some patching, if you don't want to run any initializer program every time you start the computer up.
The bootup state can be achieved otherwise. The -RESET signal sets all the PIA port lines to inputs. As input a line has an impedance of several megaohms. A TTL chip reads such a signal as a logical `1'. IC4 can force four block selection pins (PA0, PA1, PA5 and PB0) low, so that the memory segments of C64 point to the four upmost memory blocks in ascending order. The port A contains the bits 1101 1100 and the port B 1111 1110. As the IC4 has open collector outputs, it doesn't disturb the port's operation when outputting high state. That is why the initialization routines (see Section 3.4) write the value 52 to the address 57281, which forces the IC4 outputs high by lowering the CA2 line.
The address space of Commodore 128 consists of four 16ákB segments which are at the address ranges $0000--$3FFF, $4000--$7FFF, $8000--$BFFF and $C000--$FFFF. A PIA expanded C128 uses the topmost four 16ákB blocks of each bank after startup. It considers them as its whole world and does not know anything of the other memory blocks. Figure 1 describes the situation. A total of twelve memory blocks of each memory bank are left out of the C128's world.
With expanded memory we can cheat the C128 by writing a suitable number to a known address, in order to make it consider the lowmost block as the second segment, for example. Then all operation that the C128 does at the second segment's area alter in fact the lowmost block, although the computer has no idea of it. This is the idea behind the whole PIA expansion circuit.
What is the benefit of it? The second segment (segment 1) is actually a good example of the function, because it resides in the middle of the RAM reserved for C64 BASIC programs. If we make a little C64 BASIC program that holds an array exactly in the third segment, we can switch another memory block to that area while the program is running, and we have another 16 kilobytes to expand our table. In this manner all unused memory blocks (12 * 16 kilobytes) of the memory bank selected for the C64 mode (0 by default) can be taken in use, and the memory is able to hold enormous arrays, which can be accessed simply by switching the memory block. See Example 4.1 for an example of this technique.
Another and more useful way to exploit the extra blocks is to use them as a RAM disk. A RAM disk means that you can copy even a whole disk to these blocks and consider it as a new disk drive, from which you can load program and data at a very fast speed. For a RAM disk you need a smart program that redirects disk commands and executes them on the expanded memory. (See Section 5.4.)
The critical addresses of the device are 57216--57343 ($DF80--$DFFF). There is the PIA chip to which you POKE the values to switch memory blocks. The PIA does not have 128 registers, as one might think. There are sixteen copies of its 4 addresses in that memory area. For instance, the addresses 57216, 57284, 57288 and 57340 are equivalent to each other.
57280 is a memory place whose lowmost four bits (bits 0--3, low nybble) determine, which of the sixteen memory blocks is accessed through the lowmost segment (segment 0) of Commodore 128. The upper four bits (bits 4--7, high nybble) specify, which of the blocks show up at the second segment (segment 1). In a similar manner the low nybble of the address 57282 determines which block resides at segment 2, and the high nybble tells the block addressed via the upmost segment.
These addresses have even another function. They can act as data direction registers as well, i.e. tell if the port lines are inputs or outputs. However, this application uses only some of the PIA's characteristics. For normal operation, all the port lines should be set to outputs. The function of these addresses depend on the bit 2 of the next address. For instance, the function of 57280 is defined with the address 57281. If you POKE there a value with its third bit set, the values written to 57280 will go to the data direction register. Inputs have the corresponding data direction register bits reset, and outputs have them set. See Tables 3--6 for a complete description of PIA registers.
The second MMU, if any, does not need any initialization. In contrary to that, the PIA does. Before using that expansion memory, you have to first initialize the PIA. Every time when a -RESET is issued, the PIA registers change to the default state. (See Section 3.1.4.) In the beginning of your program you will initialize the PIA registers so that the default block division of memory remains:
pia .equ $DFC0 LDA pia+1 ; Select Peripheral Registers ORA #4 STA pia+1 TAX LDA pia+3 ORA #4 STA pia+3 TAY LDA #$FE ; Set the default memory bank data STA pia LDA #$DC STA pia+2 TXA ; Select Data Direction Registers AND #$FD STA pia+1 TYA AND #$FD STA pia+3 LDA #$FF ; Set the ports to output STA pia STA pia+2 TXA AND #$C7 ORA #$30 ; Set CA1 and STA pia+1 ; select Peripheral Registers STY pia+3
You may want to use an array instead. That will save both space and processing time but lose generality. Someone may have CB1 or CB2 in use (see Section 6), and changing all the command register bits causes side effects on these pins. Anyway, here is a BASIC example of using an initialization table:
10 PIA=57280 20 FOR I=11 to 1 STEP -1:READ A:POKE PIA+I,A:NEXT 30 DATA 4,254,4,220,0,255,0,255,4,254,52
Many Commodore 64 games do not like any extra hardware in the area $DE00--$DFFF, as it is used by many ``freezer'' cartridges and alike. If you need to use such software with the memory expansion, you can completely disable the PIA from the address space until a system -RESET occurs. To do this, change the last DATA value on the line 30 from 52 to 53. By the way, the PIA expansion can be easily enhanced to be the most powerful freezer cartridge. See Section 6.1 if you are interested in this.
Think it in hexadecimal numbers. There are sixteen memory blocks, numbered from 0 to F. The address $DFC0 holds two hexadecimal digits. The less significant digit, the one at right, selects the memory block for the segment 0 ($0000--$3FFF), whereas the other digit is for segment 1. The other important PIA address, $DFC2, selects the blocks for segments 2 and 3 with is low and high nybble, respectively.
For instance, if you want to switch block E ($38000--$3BFFF) to segment 1, initialize the PIA and execute the following. Note that your program must run outside segment 1 ($4000--$7FFF), or otherwise the next instruction will be fetched from the new block, thus probably crashing the processor.
pia .equ $DFC0 LDA pia ; Segments 0 and 1 AND #$0F ; Preserve segment 0 ORA #$E0 ; Select bank E for segment 1 STA pia
If you used our initialization routine before this, the memory areas $4000--$7FFF and $8000--$BFFF should now mirror each other. This is an easy way to peek under ROMs and I/O with a simple C64 mode machine language monitor that does not play with the 8502's I/O registers to switch ROMs and I/O temporarily out. Also, this technique can be used in the C128 machine language monitor to access the lowest 4ákB of other memory banks than 0.
The built-in monitor for the C128 mode ignores the bank address when reading from the addresses $0000--$03FF, and reads the data always from bank 0. You can work this around by mapping some other address range, like $C000--$FFFF, to the same block that the segment 0 uses. Then you can read from, say, $0000 in bank 2, by issuing the command M 2C000.
As the video chip's address bus is only fourteen bits wide, it can access only sixteen kilobytes directly. The two additional lines needed to address 64ákB are provided by the second CIA. Its lines PA1 and PA0 are the inverse of the VIC-IIe's address lines VA15 and VA14. The video RAM bank is selected with the two upmost bits of the MMU register $D506 (see Table 10), and it does not necessarily need to be equal with the processor bank.
The VIC-IIe needs another two address lines to see full 256ákB of the selected RAM bank. The PIA lines PB7 and PB6 (the uppest two bits of $DFC2) serve as VA17 and VA16. So, the VIC-IIe memory does not necessarily have to be accessible to 8502, but there is a restriction: As the block selector for the upmost segment uses the same two lines, both the VIC block and the block for segment 3 cannot be chosen freely.
For instance, if you want the VIC to `see' its RAM at $04000, the lines VA17--VA14 must be `0001'. You can select only blocks 0--3 for segment 3 to fulfill this condition. Let's assume that you want block 2 to be mapped there:
cia2 .equ $DD00 pia .equ $DFC0 LDA cia2+1 ; First set the CIA2 lines ORA #$03 ; PA0 and PA1 to output. STA cia2+1 LDA cia2 ; Then set PA1 and reset PA0. AND #$FC ; Remember, the lines VA15 and VA14 ORA #$02 ; are the inverse of them. LDA pia+2 ; Segments 2 and 3 AND #$0F ; Preserve segment 2 ORA #$20 ; Select bank 2 for segment 3 STA pia+2
If you want the video bank selection to work exactly like in a stock computer, you have four alternative memory block configurations. The addresses $DFC2 and $DFC0 must contain one of the following bytes: $FE and $DC, $BA and $98, $76 and $54, or $32 and $10. You can use the expansion to debug or examine programs that occupy full 64 or 128 kilobytes of normal memory. When you issue a -RESET, the program's memory will remain totally unaltered, if it is outside the topmost four blocks. There is no need for a `freezer' cartridge (see Section 6.1), and I'm pretty sure that such cartridges do not even exist for the C128 mode.
+------------------------------------------------------+ | Peripheral Register A | | (Peripheral Lines PA7--PA0) | +------------------------------------------------------+ | Bits Description | | 7--4 Block Selection, Segment 1 | | 3--0 Block Selection, Segment 0 | +------------------------------------------------------+ +------------------------------------------------------+ | Data Direction Register A | +------------------------------------------------------+ | Bits Description | | 7--0 Data Direction of Peripheral Lines PA7--PA0 | | When a bit is set, its corresponding | | Port A line is an output. Otherwise | | it is an input. | +------------------------------------------------------+
Table 3. The PIA address $DFC0 (57280)
+-------------------------------------------------------------------+ | Control Register A | +-------------------------------------------------------------------+ |Bit(s) Description | | 7 IRQA1 Interrupt Flag | | Goes high on active transition of CA1; Automatically | | cleared by MPU read of Peripheral Register A. May also | | be cleared by hardware -RESET. | | | | 6 IRQA2 Interrupt Flag | | When CA2 is an input, IRQA2 goes high on active | | transition of CA2; Automatically cleared by MPU read of | | Peripheral Register A. May also be cleared by hardware | | -RESET. | | | | 5--3 CA2 Control | | 00x Input, triggered on falling edge | | 01x Input, triggered on rising edge | | When x is 1, -IRQA Interrupts by CA2 active transition | | are enabled. | | 10x Output, Read Strobe for Peripheral Register A | | CA2 goes low on first high-to-low E transition following | | a read of Peripheral Register A. When x is 0, it remains | | low until next active CA1 transition. When x is 1, CA2 | | remains low for one E cycle. | | 110 Reset CA2 | | 111 Set CA2 | | | | 2 Register in address $DFC0 | | 0 Data Direction Register | | 1 Peripheral Register | | | | 1 Determine Active CA1 Transition | | 0 IRQA1 set by high-to-low transition on CA1 | | 1 IRQA1 set by low-to-high transition on CA1 | | | | 0 CA1 Interrupt Request Enable/Disable | | 0 Disable -IRQA Interrupt by CA1 active transition. | | 1 Enable -IRQA Interrupt by CA1 active transition. | +-------------------------------------------------------------------+
Table 4. The PIA address $DFC1 (57281)
+------------------------------------------------------+ | Peripheral Register B | | (Peripheral Lines PB7--PB0) | +------------------------------------------------------+ | Bits Description | | 7--4 Block Selection, Segment 3 | | 3--0 Block Selection, Segment 2 | +------------------------------------------------------+ +------------------------------------------------------+ | Data Direction Register B | +------------------------------------------------------+ | Bits Description | | 7--0 Data Direction of Peripheral Lines PB7--PB0 | | When a bit is set, its corresponding | | Port B line is an output. Otherwise | | it is an input. | +------------------------------------------------------+
Table 5. The PIA address $DFC2 (57282)
+-------------------------------------------------------------------+ | Control Register B | +-------------------------------------------------------------------+ |Bit(s) Description | | 7 IRQB1 Interrupt Flag | | Goes high on active transition of CB1; Automatically | | cleared by MPU read of Peripheral Register B. May also | | be cleared by hardware -RESET. | | | | 6 IRQB2 Interrupt Flag | | When CB2 is an input, IRQB2 goes high on active | | transition of CB2; Automatically cleared by MPU read of | | Peripheral Register B. May also be cleared by hardware | | -RESET. | | | | 5--3 CB2 Control | | 00x Input, triggered on falling edge | | 01x Input, triggered on rising edge | | When x is 1, -IRQB Interrupts by CB2 active transition | | are enabled. | | 10x Output, Read Strobe for Peripheral Register B | | CB2 goes low on first high-to-low E transition following | | a read of Peripheral Register B. When x is 0, it remains | | low until next active CB1 transition. When x is 1, CB2 | | remains low for one E cycle. | | 110 Reset CB2 | | 111 Set CB2 | | | | 2 Register in address $DFC2 | | 0 Data Direction Register | | 1 Peripheral Register | | | | 1 Determine Active CB1 Transition | | 0 IRQB1 set by high-to-low transition on CB1 | | 1 IRQB1 set by low-to-high transition on CB1 | | | | 0 CB1 Interrupt Request Enable/Disable | | 0 Disable -IRQB Interrupt by CB1 active transition. | | 1 Enable -IRQB Interrupt by CB1 active transition. | +-------------------------------------------------------------------+
Table 6. The PIA address $DFC3 (57283)
In the Commodore 128 Programmer's Reference Guide, starting at page 530, there is a list of the MMU registers. Apparently, the engineers at Commodore planned to produce successors for the MOS 8722 MMU that is used in the Commodore 128. There could have been at least two different chips: one that could access 256 kilobytes of memory (just like my MMU expansion), and another one for accessing a whole megabyte. The one megabyte memory space could have been implemented with 256 kilobit chips, whose MA8 line could have been produced in the MMU. Alternatively, they could have used the smaller 4164 memory chips. 128 memory chips would have taken quite a lot of printed circuit board space.
Commodore's plans for the one megabyte expansion were not so bright. Each bank would have been expanded to 256 kilobytes, just like in the PIA expansion, but you would have had to switch all 64 kilobytes on at once. Well, maybe they were going to use the common memory scheme in that expansion. Anyway, Commodore canned the plans for the new MMU's in a very early phase and started to produce external DMA-based RAM Expansion Units, REU's, which are supported also in the C128's operating system.
The Tables 7 to 11 describe the MMU registers. Please ignore the `RAM block' setting of the RAM Configuration Register. The System Version Register (Table 11) should always return $20 upon reading, so you have to use other techniques if you want to determine the real amount of memory banks on the system. Actually, the only difference between MMU expanded systems and unexpanded ones is that the banks 2 and 3 map to banks 0 and 1 on stock C128's.
The Configuration Register (Table 8) reflects the memory configuration state upon reading, and whenever you write to it, the new memory configuration will be selected immediately. If you have selected the I/O block at $D000, you can access the CR at $D500, but the address $FF00 is available in all memory configurations. Generally, the processor addresses $FF00--$FF04 are always mapped to the MMU.
There are also Preconfiguration registers, which let you to change configurations easier. If you write anything to a Load Configuration Register, the value in the corresponding Preconfiguration Register will be loaded to the Configuration Register, thus selecting that memory configuration. Upon reading, the Load Configuration Registers reflect the state of the corresponding Preconfiguration Register. These registers are normally used by the BASIC interpreter, so if you want your routines to coexist with the BASIC, it is probably best to leave those registers alone.
The Mode Configuration Register (Table 9) lets you to change processors and operating modes, among others. All bits in this register act as outputs, but they can be used as inputs, too. If you wrote a `1' to any bit, external hardware could pull the line down, and the bit would output `0'. For instance, you can make the operating system to think that the 40/80 key is depressed by clearing the high bit of this register. Also, if you make your own routine to select the C64 mode, you will able to assert the -EXROM or -GAME, if you want to play with different C64 mode memory configurations without having to hook anything to the computer. Also, note that you can use other banks than the default 0 in the C64 mode, if you disable common memory and remember to select the video bank.
The 64ákB video bank can be selected with the RAM Configuration Register (Table 10). Its other purpose is selecting the size and location of the Common RAM block, a memory area that defaults to bank 0 in spite of the processor RAM bank settings in the Configuration Registers.
Finally, the page relocation registers let you to relocate the processor pages zero and one (addresses $00--$FF and $100--$1FF) anywhere in the memory. Well, actually the 8502's addresses 0 and 1 always map to its built-in I/O register. In addition to that, the MMU defaults the page relocation bank to 0 if you are using common memory at bottom of memory. To relocate the zero page, first write the memory bank (0--3) to $D508, and then write the memory page number to ($D507). The stack page (page 1) uses the addresses $D509 and $D50A.
+---------+--------------------------------------+ | Address | Description | +---------+------+-------------------------------+ | $D500 | CR | Configuration Register | | $D501 | PCRA | Preconfiguration Register A | | $D502 | PCRB | Preconfiguration Register B | | $D503 | PCRC | Preconfiguration Register C | | $D504 | PCRD | Preconfiguration Register D | | $D505 | MCR | Mode Configuration Register | | $D506 | RCR | RAM Configuration Register | | $D507 | P0L | Page 0 pointer low | | $D508 | P0H | Page 0 pointer high | | $D509 | P1L | Page 1 pointer low | | $D50A | P1H | Page 1 pointer high | | $D50B | SVR | System Version Register | +---------+------+-------------------------------+ | $FF00 | CR | Configuration Register | | $FF01 | LCRA | Load Configuration Register A | | $FF02 | LCRB | Load Configuration Register B | | $FF03 | LCRC | Load Configuration Register C | | $FF04 | LCRD | Load Configuration Register D | +---------+------+-------------------------------+
Table 7. The MOS 8722 MMU registers
+----------------------------------------------+ | Configuration Register format | +--------+-------------------------------------+ | Bit(s) | Description | +--------+-------------------------------------+ | 7--6 | Processor RAM bank (0--3) | | 5--4 | Contents of the area $C000--$FFFF | | | 00 Kernal ROM | | | 01 Internal Function ROM | | | 11 RAM | | 3--2 | Contents of the area $8000--$BFFF | | | 00 BASIC ROM high | | | 01 Internal Function ROM | | | 10 External Function ROM | | | 11 RAM | | 1 | Contents of the area $4000--$7FFF | | | 0 BASIC ROM low | | | 1 RAM | | 0 | Contents of the area $D000--$DFFF | | | 0 I/O registers | | | 1 RAM or character generator ROM | +--------+-------------------------------------+
Table 8. The MOS 8722 MMU Configuration Registers ($D500--$D504 and $FF00--$FF04)
+----------------------------------------------+ | Mode Configuration Register | +--------+-------------------------------------+ | Bit(s) | Description | +--------+-------------------------------------+ | 7 | 40/80 key sense | | 6 | Operation mode selection | | | 0 C128 | | | 1 C64 | | 5 | -EXROM line control | | 4 | -GAME line control | | 3 | Fast Serial bus direction | | 2--1 | unused | | 0 | Processor selection | | | 0 Z80 | | | 1 8502 | +--------+-------------------------------------+
Table 9. The MOS 8722 MMU Mode Configuration Register ($D505)
+-----------------------------------------------------+ | RAM Configuration Register | +------+----------------------------------------------+ | Bits | Function | +------+----------------------------------------------+ | 7--6 | Video RAM bank (0--3) | | 5--4 | RAM block (0--3) | | 3--2 | Common RAM selection | | | 00 Common RAM block disabled | | | 01 Common RAM at bottom of memory | | | 10 Common RAM at top of memory | | | 11 Common RAM block at both top and bottom | | 1--0 | Size of Common RAM block | | | 00 1ákB | | | 01 4ákB | | | 10 8ákB | | | 11 16ákB | +------+----------------------------------------------+
Table 10. The MOS 8722 MMU RAM Configuration Register ($D506)
+---------------------------------------------+ | System Version Register | +------+--------------------------------------+ | Bits | Description | +------+--------------------------------------+ | 7--4 | Bank version (amount of 64ákB banks) | | 3--0 | MMU chip version | +------+--------------------------------------+
Table 11. The MOS 8722 MMU System Version Register ($D50B)
It is best to use sixteen or thirty-two kilobytes of common memory on the MMU when using the PIA expansion. This way you can hold your program in bank 0 in the lowmost or highmost sixteen kilobytes, or both, and you can easily access all of the memory through two 16-kilobyte windows at $4000 and $8000. However, keep in mind that the page relocation cannot be used outside bank 0 when the low common memory block has been selected.
If you are working with graphics, switch the character generator ROM off, and you have one restriction less. The three lowmost bits of the processor's built-in I/O register (at addresses 0 and 1) have totally different function in the C128 mode. The lowmost bit, LORAM, selects one of the two color memory banks for the processor. The second bit from right, -HIRAM, selects the color memory bank for the video chip. The CHAREN bit, bit 2, dictates whether the character generator ROM is mapped to the video chip addresses $1000--$1FFF or not.
Due to the complexity of the C128's operating system, it is very difficult, if not impossible, to utilize the PIA expansion memory in BASIC 7.0. You have to leave the segments 0 and 3 alone, as they contain system variables, routines to access different RAM banks, and interrupt vectors. The other segments are not safe either, as the BASIC interpreter uses all of the RAM bank 1 for BASIC variables. If you switch other RAM blocks to a segment, you will have to ensure that no BASIC program data or variables occupy any of the address range used by that segment.
For this reason, I did not even try to find out how you could utilize the PIA expansion in the BASIC 7.0. But you can use the MMU expansion RAM very easily with the built-in BANK command. The new RAM banks have the numbers 2 and 3.
If you really want to utilize the PIA expanded memory in BASIC 7.0 programs, it is possible by using machine language subroutines to access the extra memory. Remember to disable the interrupts in your subroutine, as the BASIC interpreter uses raster interrupts.
With BASIC 2.0 the use of the extra memory is a bit limited. In the upmost segment (segment 3) there is operating system ROM, under which you can place different memory blocks, but reading them with BASIC is naturally impossible. However, in some cases writing data to this segment partially under Kernal ROM and I/O area may be a working solution. The lowmost kilobytes are free RAM, and it can be utilized by switching memory blocks. But the benefit of the extra memory decreases, as you can use only the lowmost four kilobytes of each block.
You can read and write the area $C000--$DFFF of this segment using BASIC. Accessing the area $D000--$DFFF without machine language is tricky but possible.
The highest segment but one, segment 2, is halfly under BASIC ROM, and only its lower half can be freely used.
You can always write under ROM (except in the UltiMax game cartridge configuration).
When utilizing it, you have to take in consideration that those 8 kilobytes can be under a ROM module, if one is connected, or they could hold some of the variables and tables that are stored in the top of the BASIC memory. You have to construct your programs so that they do not collide with the segment's area.
The lowmost segment, segment 0, contains Kernal's and BASIC interpreter's system variables. Normally you cannot change its contents, since the operating system would not find its status information. This can be worked around by copying those vital bytes to the new memory block and switching the block with a machine language routine.
The only segment that can be wholly used with plain BASIC is the segment 1, the second one from the bottom ($4000--$7FFF). It resides in the middle of the space reserved for BASIC programs. If you construct your BASIC programs wisely, that is short enough, and ensure that the information used by BASIC are located exactly on this area, you can switch the blocks in this segment freely and exploit all the twelwe extra blocks as a huge data storage.
Example 4.1 shows how you can create a table on this area and hold its data simultaneously in all the extra memory blocks.
10 I=0:J=0:K=0:A=0 20 DD=56576:PIA=57280 30 FOR I=11 TO 0 STEP -1:READ A:POKE PIA,A:NEXT 31 DATA 4,254,4,220,0,255,0,255,4,254,52,220 40 K=16384-7:POKE 47,K AND 255:POKE 48,K/256: POKE 49,K AND 255:POKE 50,K/256 50 DIM A%(8191) 60 FOR I=0 TO 15:POKE PIA,I*16+12 70 PRINT I":";:FOR J=0 TO 9:PRINT A%(J),:NEXT:PRINT:NEXT 80 POKE PIA,220:END
The program displays ten first integers of each memory block. The table it reserves fills the whole segment 1, because each integer (notice the % sign) takes two bytes and 8192 of them are reserved. The contents of the table A% can be changed to another memory block by simply POKEing PIA's corresponding register. On the line 40 the table is ensured to start at $4000 by changing the start and end addresses of tables in the addresses 47--50. Saving the name, size and dimensions of the table takes the seven bytes, which are subtracted from the start address.
Reserving the table to an arbitrary address has its drawbacks. The size of the program and its variables may not exceed 14 kilobytes so that they could fit to the memory before the beginning of the table. All variables must definitely be declared before allocating the table. If you do not declare them by giving them a value, the interpreter finds really exotic values for them or transfers the table off its position.
10 I=0:J=0:A=0:A$="" 20 DD=56576:PIA=57280:V=53248:COLOUR=50176 30 FOR I=11 TO 0 STEP -1:READ A:POKE PIA,A:NEXT 31 DATA 4,254,4,220,0,255,0,255,4,254,52,220 40 POKE V+24,16+8:POKE V+17,59 50 FOR I=0 TO 11:POKE PIA+2,I*16+14: POKE DD,PEEK(DD) AND 252 OR (NOT I AND 3) 60 FOR J=0 TO 999:POKE J+COLOUR,3:NEXT 70 GET A$:IF A$="" THEN 70 80 NEXT I 90 POKE PIA+2,254:POKE DD,PEEK(DD) OR 3: POKE V+24,23:POKE V+17,27
The extra memory can be used as a store of high resolution pictures as well. This program shows all twelve memory areas that could contain reasonable pictures. The pictures can be created with a BASIC extension that resides in RAM and saves its graphics under Kernal ROM. In those memory blocks that contain no pictures, you see random memory contents.
High resolution graphics is enabled on the line 40. The beginning of the line 50 switches block I to the segment 3 and switches the VIC chip to the same memory area. The second POKE statement selects the block for the video chip. `(NOT I AND 3)' filters the extra bits off and inverts the essential ones so that they can be stored to the lowmost two bits of $DD00. The line 60 sets the picture's colour to black-cyan. The next line waits for a keystroke before showing another picture. After all blocks have been shown, the original state of the I/O chips will be restored on the line 90.
As memory expansions are expensive, programs making use of extra memory are rare. Besides, different expansions are not compatible with each other. However, it does not mean that you could not fully utilize the expansion. The most obvious utilization method is a RAM disk. When using VC-1541, it is not only luxury but almost vital condition.
Pekka Pessi has made a couple of C64 programs that utilize the PIA expansion. The software is distributed in two self-extracting archive files (SFXes).
If you haven't got the files with this document, you can retrieve them via anonymous FTP from the server FTP.FUNET.FI. The directory /pub/cbm/documents/projects/memory/c128/1028 contains files related to this expansion.
The file ROS-V1.SFX (for RAM Operating System) holds the source code of the RAM disk program, and some miscellanous files. To load the archive file, use the command ``LOAD"ROS-V1.SFX",device''. Then change a blank disk to the drive before RUNning. The second archive file, UTIL256.SFX, contains the following software:
The program TEST tests the block selection and the whole memory. If it jams before reporting ``Test passed'', something has gone wrong. Its source code is in the file TEST.A, which requires a library STRING.A. I translated the executable to English by patching the binary file.
With the MULTI51200 program you can run four different programs. The program does no multitasking, it only holds four environments in the memory, each in its own memory block. MULTI.A is the source code. After loading it with ``LOAD"MULTI51200",device,1'' and initializing the BASIC pointers with ``NEW'', you can switch the environments with ``SYS51200,f,b''. The parameter f is a flag determining if the current block should be copied to the destination block (0) or not (1). The parameter b selects the destination block (0--3). The initial block is 3.
If you don't like to switch the memory blocks manually in your favourite machine language monitor, the MON256 utility is for you. The memory is again divided to four 64ákB blocks, numbered from 0 to 3. The commands are as follows:
a nnnn cmd or Assembles instruction cmd to . nnnn cmd memory address nnnn. b bb ff Selects a block. The bb holds the block number, and ff is a flag. If it is 1, it directs all memory accessing to RAM. If it is 0, you can access the ROMs and I/O. c hhhh iiii jjjj Compares the memory area hhhh--iiii with the area beginning from jjjj. d [hhhh [iiii]] Disassembles memory. f hhhh iiii nn Fills the memory between hhhh and iiii with the byte pattern nn. g [hhhh] Executes program until a BRK is encountered. h hhhh iiii nn mm... or Hunts the memory area hhhh--iiii h hhhh iiii 'text for the byte sequence nn mm... or for `text'. j [hhhh] Calls a subroutine. l "filename"[,n] Loads a program. Default device number is 8. m [hhhh [iiii]] Hexadecimal dump of memory. > hhhh nn mm... Stores bytes in memory. r Dumps the registers (for `g' and `j'). ; Modifies the register values. s "filename",n,hhhh,jjjj Saves the memory area hhhh--jjjj to device n. t hhhh iiii jjjj Copies the memory area hhhh--iiii to jjjj. v "filename"[,n] Verifies a program. Default device number is 8. x Exits the monitor. @ [command] Sends `command' to device 8. If it begins with $, the disk directory will be read. If no command is given, the disk drive's status will be displayed.
The source code for the monitor is split in the files MON.A, CONSOLE.A, COM.A, ROUTINES.A and TABELS.A.
The most important utility is a RAM disk program, which occupies about 9 kilobytes of memory. It transfers Kernal and BASIC interpreter to RAM and patches the serial bus routines. The actual program is in the area $00800--$03FFF.
It emulates all VC-1541 functions except relative files. For example, the commands U1, U2, B-A etc. work. UI+ and UI- make no difference. The program also detects some fast loaders and works with them installed.
The loader is called RAM DISC, and the patched Kernal is in RAM.K. You need only patches to the low-level serial bus routines, so you may want to restore the original colors and keyboard definitions. To minimize incompatibility problems, you should replace the ROM chip that holds the BASIC and Kernal ROMs with an EPROM containing the patched Kernal. The RAM disk routines are in RAM.C.
The program RAM DISC COPY copies a regular 1541 disk to RAM. It utilizes the slow U1 command, and it is included as an example only. The source code DUP.A exposes the RAM disk's storage format.
A faster and more useful tool is FDUPLICATE. Using it, you can copy a regular disk to the RAM disk or vice versa. You can make multiple copies of a disk easily. This program's fast transfer routines are designed for PAL systems, and the utility cannot be used in NTSC machines without little modification. Its source code is in the two files S/SUCK and S/DSUCK.
There are a couple of unused contacts in the PIA. In addition to that, two 7405 ports are not connected. The extra PIA lines include an input, CB1, an input/output line CB2, and the Interrupt Request line -IRQB.
If you connect the -IRQB line to the -IRQ or -NMI input of your system, you can have one or two new interrupt sources, useful for interfacing your custom hardware. And if you are running out of User Port pins, the lines CB1 and CB2 can save you from designing an I/O cartridge. Besides, you can use the -IRQB as an output if you wire the CB1 line to something that you can control with software, CA2 for instance. Just remember to add a pull-up resistor to the -IRQB line if necessary.
For the Commodore 64, there are several `freezer' cartridges that let the user to halt theoretically any program (game) to alter it (make the player immortal), or more often merely to make a `back-up copy' of it. Alas, anything cannot be frozen with these cartridges. If the programmer of the game is clever enough to inhibit IRQ and NMI interrupts in his program, and if the code runs at the area $0000--$0FFF, no external cartridge will be able to halt it without asserting the -RESET signal, which would lose most status information of the computer.
You can use the PIA expansion to freeze programs, in many cases also in the C128 mode. In the C64 mode, an internal freezer can stop anything except a totally jammed program. In the C128 mode, it can stop software running on the 8502 without using any ROM at the area $C000--$FFFF.
This freezer expansion will let you to replace the program's memory with previously initialized RAM by pressing the Restore key. If the NMI interrupts are disabled, freezing will be done with the BRK instruction. As the circuit forces the -HIRAM line to logical zero, the C64 mode interrupt vectors will always be fetched from RAM. All of the freezer software can be stored to RAM, so it is easy to change, and no EPROM programming devices are needed. Another advantage is that the freezer software may freely use 128 kilobytes of memory, and there are 64 kilobytes of working storage, way more than in the best freezer cartridges. Besides, those amounts will be at least doubled or quadrupled in the C128 mode freezer, depending on whether you have built the MMU expansion.
For this expansion, you need a double ON--OFF or ON--ON switch, four 1N4148 diodes and two 10 kilo-ohm resistors. First do some preparations. Break the connection between the PIA's -RESET input (pin 34) and the system -RESET, and replace it with a diode with the marked end towards the system bus, so that the pass direction is from the PIA to the system -RESET. Then you have to solder a pull-up resistor between the PIA's -RESET line and the +5áV power outlet. Locate the U29 chip (7406) on the motherboard and desolder its pin 4. Solder a diode between the motherboard connection and the pin with the mark pointing to the chip. Finally, solder the other pull-up resistor between the U29's pin 4 and +5áV. After these modifications, the PIA should continue to reset normally, and the Restore key should remain functional.
Now you can mount the switch to the system. If you have an ON--ON switch, hold it in your hand in such an angle that you see two rows of three pins. Solder two diodes from the right-hand contacts of the switch to the U29's pin 4, with the mark pointing to the U29. Solder the PIA's -RESET line (pin 34) to either middle contact of the switch, or to either free contact if you have an ON--OFF switch. Finally, mount the -HIRAM line, which is on the 8502's pin 29, to the remaining middle contact.
The switch affects in the operation of the Restore key. When it is open, the Restore key operates normally. When the switch is closed, the Restore key also resets the PIA, switching the default memory blocks in, and forces the -HIRAM line low, so that the interrupt vectors will always be fetched from RAM. The idea of the expansion is that the program runs in some other memory blocks, say 3--0, with the PIA totally disabled from the address space. The default memory blocks ($F--$C) will be initialized mostly with null bytes, which is the opcode of the BRK instruction. You also need the freezer interrupt vectors ($FFFA--$FFFF) and a small interrupt handler that stores the processor registers and switches the main freezer program into the address space.
If you want to freeze hanged programs, too, you can do it without losing the state of the I/O chips. By using a custom Kernal ROM, you could even store all 8502 registers except the Program Counter. You just have to be able to reset the 8502 without resetting the rest of the chips. To do this, desolder the -RESET signal, pin 40 on the 8502. Connect a 1N4148 diode between the system -RESET signal and the 8502, with the mark pointing away from the processor. Add a pull-up resistor to the signal, and a switch between the logical ground and the signal.
Now, whenever you push the switch, only the 8502 will be reset. If the computer is in the C64 mode at this time, it will remain in that mode, since the MMU does not get reset. You should then use the Autostart code ($C3 $C2 $CD $38 $30 at $8004--$8008), and the Kernal will jump to the vector ($8000). Unfortunately it will destroy all registers except the Y register when searching for that code. More fortunately, if the computer is in the C128 mode, and if the Kernal ROM is not selected in the memory configuration, the processor will fetch the -RESET vector from the active RAM bank. In that case you can store all processor registers except the program counter.
Alas, I just have invented a software hack that cannot be frozen with this circuit. If a program runs on the I/O area ($DE00--$DFFF except the range where the PIA is mapped to while the I/O is switched on) and has disabled the NMI interrupts, the program will continue running even if you have activated the freezer and hit the Restore key. Should you ever encounter this type of a software hack, you can use a triple switch and add the LORAM signal (8502's pin 30) to one extra contact. Connect the remaining contact to the U29's pin 4 through a diode with the mark pointing to the U29. This will disable the I/O area for the time the Restore signal is active, so the processor can fetch a BRK from the memory underneath, and will always freeze correctly.
I haven't completed the freezer expansion yet, as the daughter-board in my faithful 2564 (C64 with the 256ákB expansion) stopped working when I opened the cover to start the surgery operation. Much time has passed, and I have lost interest in this rather time-consuming project. The freezer expansion wouldn't have much advantage over freezer cartridges. Also, many Commodore 64 emulators have some freezer features.
Naturally you also need a program to utilize the freezer circuitry. You don't need to code so much, only the routine to jump back to the freezed program and the routine that saves all registers upon freezing need to be written from scratch. You can patch existing machine language monitors, sprite editors and utilities like that, only the data fetch and data store subroutines must be rewritten. Developing the freezer software is very simple, since you can access the frozen program through a 16ákB window, and you can use the Kernal routines all the time.
In my C64, I have replaced the 2364 Kernal ROM with a 32-kilobyte EPROM. The two extra address lines are controlled by the PIA's pins CA2 and CB2. As the C128 uses a 23128 ROM for both C64 BASIC ROM and Kernal ROM, this type of expansion cannot be used. Besides, you can access only 256 kilobytes of the memory in the C64 mode. Why would you want to improve the C64 mode if you can make the C128 mode far better?
The C128 has a socket for a Function ROM for your operating system extensions. It can hold a 27256 EPROM, 32 kilobytes. If that does not satisfy your needs, you could use a 27512 EPROM instead. In this case you have to bend the A15 line up. This line could be controlled by the PIA's CB2 line. To ensure that the logic works also when the CB2 line is input, add a 4.7 kilo-ohm pull-up resistor between CB2 and +5áV.
If you have anything to ask or comment, feel free to contact me. If you complete either or both expansions, I would like to receive an E-mail message or a postcard from you, so that I know I didn't write this document in vain. Finally, if you make any software for the expansion or patch existing programs to make use of the extra memory, I would love to put the software available on the FTP site FTP.FUNET.FI.
My addresses are: