How to load programs at 1.5Mbaud
(or the Gemini EPROM board and paging)
Some time ago I took the route to disks, and after a while I had CP/M 2.2 up and
running on two 8″ drives. With the reducing costs of dynamic RAM my system memory had
grown to 64K – spread over two RAM-B boards. (The main processor board is a Nascom 2
by the way). My disk software resided from F000-F7FF with the screen at F800-FBFF. I
did not want to commit the area F000-F7FF to EPROM as it sometimes was required for
other software that I was developing. The net result was that I had a variety of tapes
that I used to load, depending on what I wanted to do at the time. The procedure for
starting CP/M on first switching on was:–
Ensure Power-on-jump switches set for 0.
Ensure Nas-Sys switched in and screen at 800.
Load Cassette tape @2400 baud to F000-F7FF (Disk & I/O routines).
Enter short machine code program ( 00 76 ).
Throw switch which dropped Nas-Sys out and moved the screen from
800-BFF to F800-FBFF. (RAM appeared at 0 as it was no longer
overlaid, and the screen now overlaid RAM at the other end of memory).
Alter the power-on-jump to F000.
Bingo CP/M going!
Once in CP/M life wasn’t so difficult. New versions of the Disk & I/O software could
be loaded over the existing ones and tried immediately, and only in catastrophic cases
would the whole start up procedure have to be repeated. Having the routines sitting
there in RAM meant that they could be patched easily, or totally changed, and I had no
worries about slow EPROMs and whether I should have the Wait state generator switched
on. However it was too much of a rigmarole and there must be a better way! Putting
everything in EPROM in the sockets on the Nascom 2 would not help as EPROM gobbles up
areas of the memory map and you have programs sitting there that you don’t want to
use. (If you’re running Basic you have no interest in Zeap or Nas-Dis have you?). I
also have the odd CP/M program that will not run in systems under 56K} Also
occasionally there are various system programs that have conflicting execution
addresses. This would all have lead to a horrific switching arrangement if I had
attempted to use the sockets on the N2.
I could see only one reasonable way out of the problem, and so gave in to the
high-pressure salesmanship that I had recently been subjected to from Interface over
the Gemini EPROM board. (Mind you I think they’d given me up by then as the first
indications I gave that I was about to conceed weren’t followed up!).
The reason for choosing the Gemini board is because it supports the page-mode scheme.
This effectively allows you to switch the board on or off by setting or resetting
certain bits of port FF. When the board is switched off it vanishes from the memory
map, leaving the full 64K of RAM free for the wanted system. (Not strictly true as the
screen is still cluttering things up at F800-FC00). RAM-A owners can carry on reading
as for the application I had in mind the RAM was specifically required NOT to respond
to the page-mode switching, as it was necessary that the RAM be present along with the
EPROM board. The two can coexist, as, when the EPROM board appears, it asserts its
“RAM DISABLE” line and so overlays any RAM that might lie in its address space.
[Note that any RAM on the Nascom 2 cannot be overlayed. The RAMDIS signal only emerges
from the N2, and is not connected back in].