Minimal CP-M SBC design

[Allison]

>
>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:
>> 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.

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.

 
>> 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.

Go with the 32k parts!  Hint to save wiring time stack the one atop the other
and keep CS/ seperate.

Also if you have a 32K eprom you can bank switch that in (do it as low ram/low rom)
and you can put CPm and utility stuff in there.

>> 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.

It would be but I know of no on line copy.  If you cant cind the on line 
copies of the bios from teh book I think I have them and can send to pvt email.

>> 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.

Either will work likely less hardware with the 16550 as it has BRG.


>> 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.
>

The gal can make life much easier.  The larger ramm is nice for MP/M or 
implementing a soft ramdisk.


>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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>Ethan.Dicks at usap.gov            http://penguincentral.com/penguincentral.html