Minimal CP-M SBC design - test-drb@ccmp.vtda.org

5 May 2008

Hi Andrew,

Just sitting here wondering why you're not using one of the enhanced-
functioning Z80 chips.  Even going with the 64180 or Z180 would give 
you 2 UARTS and an MMU, in addtion to 2 DMA channels and a timer.  
Later version of this product line, will of course, give you more 
instructions and functionality.

But the MMU can make the whole process of bootup very easy.  Simply 
use the MMU to map the ROM out after the system's been started 
(you've got a 1MB physical address space).  Duck soup.

But if you want to stick with the "real" Z80, I've seen two methods 
of getting around the reset to 0000.

The first is to simply force three bytes onto the system bus to 
perform a jump to whatever address you desire after a reset.  You'll 
only do this once per reset, so the circuitry's pretty simple.

Another way to is to start out with an EPROM mapped in and then 
disable it using an I/O instruction.  You can leave RAM mapped in for 
write cycles, so that only reads will come from the ROM and writes 
will go directly to RAM.  That way, you can set up locations starting 
at 0000 from a ROM.

In any case, the ROM needn't be very big.  I think Don Tarbell used a 
little bipolar 82S123 PROM.  Gives you 32 bytes to do what you need, 
which, in Don's case was enough to get the first sector of an 8" 
floppy read.

CP/M BIOSes for 2.2 and below are easy--they're poll-mode with 
clearly described inputs and outputs.  About the only thing you may 
find confusing is the IOBYTE convention, but that's optional and 
fairly well documented.

I've written a CP/M BIOS without resorting to assembly, doing the 
whole thing in machine code.  It's not a big thing and you can start 
with the basic set of disk and console I/O routines.  There are two 
boot entry points in the BIOS jump vector--the "cold start" entered 
by a jump to 0000 that (re)loads the entire CP/M BDOS and CCP, and 
the "warm start" that simply reloads the CCP.

Disk I/O is done in 128 byte "sectors", so if your physical sectors 
are longer than that, you'll need to set up blocking and deblocking 
routines.  

All of this is covered in the CP/M System Alteration guide in pretty 
fair detail, along with a couple of samples.

CP/M 3.0 or MP/M is more involved, taking advantage of bankswitching. 
 Interrupt-driven I/O is required for MP/M--and the I/O system is 
more elaborate.

Cheers,
Chuck