5 May
2008
5 May
'08
4:37 a.m.
Subject: Re: Minimal CP-M SBC design
From: Ethan Dicks <ethan.dicks at usap.gov>
Date: Mon, 05 May 2008 01:25:47 +0000
To: "General Discussion: On-Topic and Off-Topic Posts" <cctalk at
classiccmp.org>
On Sun, May 04, 2008 at 09:03:03PM -0400, Andrew Lynch wrote:
CP/M doesnt "require" 64K as it's entirely possible to boot and get a prompt
with less than 16k. If you wish to run any application with a hint of
usefulness you need at least 48k and 56K would be fine so ram to 0f000h serves.
I presume the F000h block is rom/Eprom which can be very useful.
Hi Ethan, I reread your email and thought I'd
try to answer some of your
questions regarding a simple do it yourself CP/M computer.
Thanks.
My first piece of advice is to ask Allison ;-)
since she has done this
dozens of times and can boot CP/M blind folded on a spark gap radio.
Sure. I was hoping she would chime in. My experience over the years
with CP/M has been somewhat tangental (I've worked on Kapros at work
back in 1980s, I own a couple of Kaypros still, and I've done a little
bit with 22disk and such under MS-DOS), but everytime Allison posts
on CP/M, I learn something.
Your basic hardware certainly sounds CP/M
capable. I assume that it can
swap RAM in to the lower pages though, right? CP/M requires RAM at $0000
through some address (depends).
Right. As it stands, the hardware probable cannot do that, but the ace
up my sleeve is that there is a 16V8 GAL that is already wired to important
bus signals so that it can act as the chip select circuit. I was
planning on using one of the output flops as the bootstrap-ROM-enable.
I've done similar sorts of things with MC68000 designs in the past.
It likes RAM all the way to $FFFF but can
live with ROM in the $F000 range. Less RAM than 48K makes things difficult
though, IMO.
That's what I was unsure about - but RAM to $EFFF and ROM from $F000 on
up is easy to implement. How I
implemented my machine was to use a memory configuration latch
(74LS273)... The schematics are all on my N8VEM page.
I'll check those out when the sats rise here.
A pair of 62256's would work but I prefer a
solution using a 512Kx8 SRAM.
That lets you use the 64K for RAM and the rest for a RAM drive. Whatever
does it for you though.
I happen to have some 62256s with me, not any 512Kx8 SRAMs, and the next
plane isn't for almost 6 months. Writing the
CBIOS is actually not that hard. I wrote one more or less based
on the one in the Andy Laird's CP/M programmers guide book. It was
recommended by Allison and is *the* reference book AFAIK. CP/M is a great
OS and is rather portable considering everything it does.
Hmm... is there a soft copy of that book anywhere? It sounds like the
perfect reading companion. I use 16550
UARTs but the CBIOS abstracts all those details away. I think
CP/M could care less what sort of serial port you use, even if you use one
at all. Just implement the CBIOS IO routines and it'll work. Same thing
for drives; you can use floppy drives, memory, IDE, hard disks, whatever
from CP/M's perspective they are all block devices.
Right... but what I _have_ is a choice between a 16550 and a 6402. One
advantage of the 6402 is that its options are hardware selected, so the
bootstrap code doesn't have to do much to be able to squirt out a message
that it's alive. It's probably impractical to put a video circuit on this
design, so I'm going with a serial console in the CBIOS I/O routines
and shifting the burden of display and keyboard input to a dedicated
device. Best of luck
with your project. Let me know if there is anything I can do
to help!
Thanks. I still have lots of reading to do, as well as a bit of work to
fiddle up some GAL equations to implement the memory map. It's going to
be somewhat trivial to roll out a 32K RAM/8K ROM design, since there's
already a pair of 28-pin sockets wired up for SRAM and EPROM. The first
big trick will be mounting a second SRAM chip. I do wish I had a 512Kx8
SRAM with me, but alas, no.
I think I have all the parts needed for a ROM emulator,
but if not, I
do have a battery-backed 8K SRAM (48Z08?) that I can program in a device
programmer and treat as a ROM for firmware development. At home, I have
a Grammar Engine PromICE, but I didn't happen to haul that along.
The ram desive will ge you there. back in the late 70s and early 80s
I did it with less (2716 and a home made programmer).
Thanks for the pointers. I'll check out your project when the 'net
comes up for us.
if I can be any help let me know.
Allison
-ethan
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