memory mapping would be a mite silly, wouldn't it, given that the HERC uses
64K of refresh RAM. I'd say the way to do it is to use a strategy put forth
by the guys who designed the 6545, the '65xx' version of the 6845. It had
an update register which essentially allowed you to write an address to the
chip and then send a stream of characters, kind of like the cursor
addressing on a terminal.
Some scheme like that would work even in the limited I/O space of a Z-80.
Another option might be to bank the BIOS such that when it's writing to or
reading from the video page, it has a space equal to the disk buffers, which
works as a window in the display RAM. The refresh of the display would keep
the entire video memory refreshed, while the banking scheme could minimize
the memory consumption of the video device.
The PC certainly has made for cheaper serial/parallel interfaces than we
ever saw for the S-100. It's too bad there were never any truly general
purpose parallel ports use commonly enough on the PC to make them cheap.
It's only since the adoption of the 1284 standard that bidirectional I/O via
the printer port has been practical. It's too bad there aren't
i8255-equivalents which can drive something. The problem with my IMSAI
PIO-6 is that it won't even drive an LED directly.
Dick
-----Original Message-----
From: Allison J Parent <allisonp_at_world.std.com>
To: Discussion re-collecting of classic computers
<classiccmp_at_u.washington.edu>
Date: Saturday, April 03, 1999 8:54 PM
Subject: Re: homemade computer for fun and experience...
><What might be fun would be an S-100 card to serve as an interface to a
><Monochrome/Hercules equivalent card and an IBM-style keyboard, since these
>
>They do exist! Hercules made cards for the s100 machines that ran DOS!
>
><Fire). One of those little switch boxes would serve just fine. The 8.0
MH
><Z80 wouldn't be sufficient to drive a VGA, so no need for anything fancy.
>
>Why not? I'm currently planning to use ISA16 and herc video for a z280
>system. Z280 has a 16 bit bus mode. The only thing needed to support
>herc using a z80 is a MMU to allow mapping the display out of the address
>space so that you have room for CP/M. I've done some testing to verify
>this will work.
>
><It could even support two short ISA cards.
>
>More if you buffer the bus!
>
>Allison
>
Received on Sat Apr 03 1999 - 23:45:17 BST