Ruud's Commodore Site: C/PM cartridge for the C64 Home Email

C/PM cartridge for the C64

Some backgrounds

CP/M is an operating system developed by Digital Research and was meant for systems using Intel's 8080 processor. Some employees of Intel grounded a firm of their own and called it Zilog. One of their first products was a processor: the Z80. This processor wasn't only downwards compatible with the 8080 but had a lot of extra instructions and registers. The Z80 also had some technical advantages above the 8080 and soon computer manufactures discarded the 8080 and turned towards the Z80. Due to this fact CP/M programmers started to use the extra features of the Z80 as well.
There was a lot of software available for CP/M and Commodore hoped to benefit from this. Unfortunately the C64 had been equipped with the 6510, a processor that is 100% incompatible with the Z80. So they developed the CP/M cartridge that contained that needed Z80.
This document will describe the technical ins and outs of the CP/M cartridge. Because there are enough data books about the Z80 available, I won't describe the Z80 in detail but only those things needed to describe the cartridge.

Another processor, how is that possible?

The 6510 can be shut down completely by activating its RDY and AEC input. RDY stands for 'ReaDY' and is meant to signal the processor that it has to halt its actions for the moment. The AEC input forces the 6510 to disconnect itself from the bus.
To connect another processor to the bus, we only have to activate these inputs at the right moment and we are ready to fire up our own processor. This activation can be done by negating the DMA input at the expansion port.


Available here.

The Z80

The Z80 has 16 address lines, just like the 6510. And just like 6510, you can also tristate the Z80. This means that you can connect the Z80 straight to the C64 bus. Unfortunately there is one nasty problem. After a RESET the Z80 expects to find a program at address $0000 and as we all know, this is the I/O area of the 6510.
So the developers decided to shift the address bus for the Z80 with $1000 = 4096 bytes. This is done with the help of U10, a 74LS283. So address $0000 for the Z80 is $1000 for the 6510. Unfortunately the 74LS283 cannot be tristated so an extra 74LS245, U14, does this job. An extra 74LS245, U11, does also tristate the address lines A0..A7. Dis/enabling the address bus will be discussed later.
Personal comment: IMHO there is no reason to buffer/tristate the address lines A0..A11.

As you can see, U7, a 74LS245 and U4, a 74LS373, are used to buffer/store the data bus. Why two ICs?. As you probably know, CLK2 is used to tell all ICs inside the C64 when the data on the bus should be read. The actual reading is done at the negative flank of CLK2. However in case of the Z80, the actual reading is done at the positive edge of the RD (= ReaD) output and this edge appears about 85 ns. after CLK (at 4 MHz.). This means that the data has to be stored in the mean time and that is were the 74LS373 comes in place. This automatically explains the function of U7 because we also need a buffer to write data towards the C64. Dis/enabling these two ICs will be discussed later.

Only one signal of the control bus is used: R/W. This signal is derived from WR through U5A, a 74LS74 (D-flipflop). The moment WR becomes active (L), R/W becomes (L) as well. R/W becomes (H) at the positive edge of CLK when WR is (H) again, or when the Z80 has to WAIT (= Z80 equivalent of RDY).
Personally I don't understand the involvement of this flipflop at all. WAIT is only activated when the z80 is told to return control to the 6510 again and at that moment the complete R/W signal will be disabled so I question this function. The flipflop only delays the (L) level of R/W about 250 ns after the negative edge of CLK. So R/W remains (L) for these 250 ns during the first part of the negative halve of CLK2. But it has no function there at all, so why???

What about the other signals of the control bus? With the Z80 a designer has the possibility to separate memory and I/O by means of two lines: MREQ and IORQ (memory / I/O request). As you can see, both have not been used. This means that memory and I/O operations can have affect on the same area within the 64K-range of the C64.
The Z80 does have the equivalents of the 6510 IRQ and NMI inputs as well: INT and NMI. But both are NOT used! I don't have any idea why this, IMHO enormous, limitation has been put upon the cartridge. I was told that this cartridge was a flop. As I never have worked with CP/M on a C64 (my interest in this cartridge is purely technical at the moment) I cannot give you any hard facts why. But as programmer I think the cause of this flop is this not connecting NMI and INT with the C64; it puts a lot of stress on the programmers handling things like keyboard, RS232 and Video. As far as I know the idea was to let only the 6510 deal with the interrupts but then I still wonder how the communication between the Z80 and 6510 has been set up.

The clock

A Z80 normally runs at a higher frequency then the 65xx family but on the other hand it uses 3 or 4 cycles for every cycle the 65xx uses. CLK is derived from the DOT clock (= ~8 MHz.). The DOT clock is divided by 2 (U1B, 74LS74) so CLK is about 4 MHz. At the moment CLK2 becomes (H), CLK is frozen. The total effect is that the Z80 runs at about 3 MHz. As you can see the output of the flipflop drives U2D, a 74LS04, and a PNP transistor. Personally I have no idea why this has been done. IMHO connecting the CLK input of the Z80 directly to the Q output should work as well.

Dis/enabling the card

Enabling the card is done through a flipflop by writing a byte to an address in the range $DE00/$DEFF with bit0 is 0. Writing with bit0 = 1 disables the card again. Disabling the card is done by activating the BUSRQ input (=BUS ReQuest) of the Z80. The moment the request is validated, the Z80 activates (H) the BUSAK (=BUS AcKnowledge). This signal is used to disable the buffers of the address and data bus as well. Enabling the card automatically disables the 6510 by activating the DMA input of the expansion connector.
Whenever the card has been enabled, the VIC must be able to disable it. So the signal BA is involved in the disabling process as well. In this case BA activates the BUSRQ input (U2C, U9B).

Testing the card

The following program is an edited version of the one in the Reference Guide, page 370. (Actually I copied it from FUNET, saved me some typing :-) )
10 rem this program is to be used with the z80 card
20 rem it first stores z80 data at $1000 (Z80=$0000)
30 rem then it turns off the 6510 irqs and enables
40 rem the z80 card. The z80 displays a message and
45 rem then the z80 card must be turned off
50 rem to reenable the 6510 system.
100 rem store z80 data
110 read b: rem get size of z80 code to be moved
120 for i=4096 to 4096+b-1:rem move code
130 read a:poke i,a
140 next i
200 rem run z80 code
205 print"[CLR]" 
206 print"6510 in command" : print
210 poke 56333,127: rem turn of 6510 irq's
220 poke 56832,00 : rem turn on z80 card
230 poke 56333,129: rem turn on 6510 irq's when z80 done 
235 print"6510 again in command" : print
240 end
1000 rem z80 machine language code data section
1010 data 102 : rem size of data to be passed
1100 rem z80 turn on code
1110 data 00,00,00 : rem our z80 card requires turn on time at $0000
1200 rem z80 task data here
1210 data 62,18,50,40,244,62,21,50,41,244,62,21,50,42,244
1211 data 62, 4,50,43,244,62,32,50,44,244,62, 9,50,45,244
1212 data 62,19,50,46,244,62,32,50,47,244,62,20,50,48,244
1213 data 62, 8,50,49,244,62, 5,50,50,244,62,32,50,51,244
1214 data 62, 2,50,52,244,62, 5,50,53,244,62,19,50,54,244
1215 data 62,20,50,55,244,62,32,50,57,244
1300 rem z80 self-turn off data here
1310 data 62,01 : rem ld a,n
1320 data 50,00,206 : rem ld (nn),a :i/o location
1330 data 00,00,00  : rem nop, nop, nop
1340 data 195,00,00 : rem jmp $0000
1350 data 00,00,00  : rem nop, nop, nop
1360 data 00,00,00  : rem nop, nop, nop
I changed the program from line 1210 on because of the fact I didn't notice any change in the behavior of the computer with the original program.

Differences with the Z80 in the C128

I had a look at the SCH of the C128 and I immediately saw two important differences: INT and IORQ were used. So the Z80 of the C128 can at least recognize interrupts. The MMU translates the use of the IORQ line to an address in the I/O area of the C128 ($D000-$DFFF). But I have no idea at this moment what the conversion algorithm is.


I put some requests on VICUG for some CP/M software for the C64 but until I haven't had any response :-( Theoretically you can use all CP/M software but I have my doubts. Bo Zimmer can provide you with a boot floppy.
Remark: the floppy doesn't boot correctly the second time after resetting the C64 using a reset button. You MUST turn your computer off and on. Don't ask me why.

Having questions or comment? You want more information?
You can email me here.