Hand-rolling a CP/M machine

[Allison]

>
>Subject: Re: Hand-rolling a CP/M machine
>   From: Warren Wolfe <wizard at voyager.net>
>   Date: Tue, 24 Apr 2007 15:36:14 -0400
>     To: "General Discussion: On-Topic and Off-Topic Posts" <cctalk at classiccmp.org>
>
>On Tue, 2007-04-24 at 12:01 -0400, Ethan Dicks wrote:
>
>
>> Right.  That much I get... so once CP/M is running, it's ordinary not
>> to refer to the boot ROMs?  There's typically not a requirement to
>> keep some low-level BIOSy stuff in ROM?
>
>
>    No, there is no requirement to keep ANY ROM available after boot.
>It is preferable, in most cases, to have the entire address space made
>up of RAM once the system is actually running, as opposed to booting.
>Some cases involve copying the ROM contents to RAM during boot, and then
>disabling ROM.  Again, as has been implied, EVERYBODY chose their own
>method -- some were better than others, naturally enough.  As a matter
>of fact, for those systems which implement ROM at 0000 for booting, the
>ROM *MUST* be disabled for any even half-way normal CP/M system to run.
>(See the previous discussions on CP/M for Radio Shack Models)

Basically I like to call it this way.

CP/M doesn't care how it gets there, only that it does.

>> Right.  I know that there are *many* CP/M hardware configurations; I
>> am trying to get down the nub of as minimal a hardware design as
>> possible.
>
>
>    CPU, 64k RAM, disk(ette) controller, ROM able to be disabled (could
>be on controller), serial port.  Anything else is gravy.
>

I'd simplify diskette controller to mass storage (any form).

>> Sure.  For the minimal system I have in mind, I'm planning on a VT100
>> or some modern machine running a terminal emulator (Kermit, et al.) to
>> handle screen formatting.
>
>
>    Perfect.

Yep. Works well enough.

>> > A video card will chew up valuable RAM, and many of them are only 16x64,
>> > but it does let you do real-time screen updates, games etc.
>> 
>> Ah... now we are onto something - games... are there many games for
>> CP/M that require a video card, or were most happy with whatever sort
>> of TTY-type device (ANSI codes or not) was out there?
>
>    Video card manufacturers often produced games.  Probably the most
>common target video card was the VDM-1 card, as in the Sol-20 by
>Processor Technology.  I have one of those cards in my IMSAI.  16x64,
>and takes up 1 K of memory.  I've seen boots that involve a VDM-1, and
>one 2708 (1K EPROM) that leaves 62K for RAM, and uses the VDM-1 memory
>for stack during boot, IIRC.  Ugly, but it works.  VDM-1 cards require
>S-100 bus, or massive hacking.  That makes them, probably, outside the
>parameters of the "quickest and cheapest" setup you've set.

the logic for rolling your own VDM1 equivilent is 1K of ram in the 
address space and a screen refresh (H and V timing, line counters)
are about 10 or so TTL peices.  It's actiully simple if you do not 
have to build a S100 bus interface.  You can copy the screen logic 
for TRS80 as that is a basic 64x16 as well.

Real time updates are possible as the refresh/scan logic keep the video 
going and the CPU does the memory updates for games.


Allison