MITS 8800B CPU Board

[Randy McLaughlin]

From: "Tom Jennings" <tomj at wps.com>
Sent: Tuesday, June 14, 2005 1:51 PM


> On Mon, 13 Jun 2005, Allison wrote:
>
>> Yes, you will have to have rom at 0000h though and Mwrite
>> was derived and delivered from the front pannel if memory
>> serves.  So that means ram will have to be set up to not
>> require Mwrite.  Either that or you will have to through
>> something small together to create Mwrite.
>>
>> As to mapping rom out once operational, thats a later problem.
>> It's fairly easy to do.
>
> My S100 system has a homemade EPROM card that powers up into the
> address space, on top of RAM, where the Z80 jumps to it and
> executes from it.
>
> The first thing the EPROM code does is copy itself, in place. This
> reads from EPROM, writes to RAM (well, to EPROM too, but the write
> line is ignored by the EPROM board).
>
> There is a flip-flop set by the RESET line that enables the EPROM
> board.  Any I/O instruction (eg. address not decoded) resets the
> flip-flop, disabling the board.
>
> Therefore, right after the EPROM monitor copies itself to RAM, it
> jumps to the serial port init code, and any old IN or OUT
> instruction disables the EPROM card, leaving the full 64K address
> space available.
>
> My EPROM monitor occupies the top 2K? of the first 32K of address
> space. It fits in the lower 32K static memory card. Hitting reset
> does not disturb CP/M (up at ef00h), and leaves "most" TPA
> programs intact.
>
> The board was designed/made by Tom Campbell, who I worked for way
> back when. It's elegantly simple.
>
>
>
>  di ; everyone shut up, please
>  lxi h, epromstartaddress ; EPROM address
>  lxi d, epromstartaddress ; likewise, I'm sure,
>  lxi b, 2048 ; BC=EPROM size
>  ldir ; whirl!
>  in 0 ; disables EPROM
>  ...

Please note that by using current RAM chips that are >= 64K address decoding 
is extremely simplified.  To carve out space from ROM not only butchers the 
address map but adds complexity.

As I already said either an I/O instruction or an address line can 
accomplish the same task.


Randy
www.s100-manuals.com