15 Jun
2005
15 Jun
'05
12:51 p.m.
On Tue, 14 Jun 2005, Randy McLaughlin wrote:
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.
I think you missed the point, plus I left out one detail:
* It specifically doens't require carving the address space up AT
ALL. The machine simply has 64K contiguous RAM, from 0000h to
ffffh.
* The EPROM board used PHANTOM to disable RAM when addressed -- I
left that little detail out!
* The "top 2K of the first 32K" description was distracting in
this example, sorry. For completeness: That address was chosen
for this system because it used two 32K RAM cards; you could run
the EPROM monitor with only th elower one; plus in the S100 days a
common "convention" was to put floppy controller or whatever
address space "up high", and 7e00h avoided colliding; pressing
RESET smashed memory at that location, and not in any CP/M system
space (eg. 0 - 80h or 48K+). It obviously requires a CPU card
that does reset-vectoring, ala Cromemco ZPU.
A vanilla 8080 (eg. MITS) of course resets to 0000h; this trick is
even cleaner for that, since the "copy EPROM code to RAM" could do
so without bothering the RAM area overlaid by the EPROM during the
copy process.