Hand-rolling a CP/M machine

[Allison]

>
>Subject: Re: Hand-rolling a CP/M machine
>   From: "Chuck Guzis" <cclist at sydex.com>
>   Date: Wed, 25 Apr 2007 11:49:49 -0700
>     To: "General Discussion: On-Topic and Off-Topic Posts" <cctalk at classiccmp.org>
>
>On 25 Apr 2007 at 13:24, woodelf wrote:
>
>> Well if anybody is doing VDM-1's the big problem is that you can't find
>> the Character ROM any more. I like the idea of TTL display like VDM-1
>> but since this is not the late 70's a 80x24 screen is reasonable
>> and the data out is a simple VGA format, since the monitors are easy to
>> come by.
>
>At the expense of getting booed off the topic, if you need simple 
>80x24 and are using something like an 8085, the 8275/8276 CRT 
>controller with an 8257 DMA controller (in auto-initialize mode)  
>doesn't take a lot of extra glue.  The gotcha is that "special 
>effects" like bolding and underlining take up space on the display 
>(Doesn't the Wyse 50 do that?).  There is a "transparent" mode, but 
>management of the screen buffer becomes a real headache.  I've never 
>tried to interface an 8257 to a Z80, so I don't know if the timing 
>works there.

Perfectly reasonable, though at first it was a simple system.

The Z80 and 8257 (or 8237) coexist fairly well.  There are some signal 
differences for the glue TTL but it works. Back in the day it was also 
popular to do that becase 8257 was cheaper and faster then Z80 DMA and
didn't have to be clocked synchronous like the Z80 DMA.

Allison