25 Jun
2010
25 Jun
'10
9:31 a.m.
On 25/06/2010 15:12, dwight elvey wrote:
I've been thinking for some time about something
simple
to help fix old machines.
What I've been thinking of is a simpler idea that
requires
less knowledge about the machine. One just needs a
little knowledge about the processor. The idea is to make
a simple socket header to go between the processor and
the rest of the machine. All it would have is some simple
circuit to determine read or write from the processor.
It would have data buffers such that it would write
through from the processor to the machine. On reads,
it would intecept the data and replace it with an instructiion
provided by some dip switches.
I'm not sure which single instruction would be
best but
for a 8080, a push or pop are quite useful. The entire
circuit would just be a little glue logic and a few bus
buffers.
That's what we used to do for 6502s and Z80s. For a 6502, a NOP is most
useful ($EA) but a BRK is handy too and conveniently is $00. For a Z80
or 8080 etc, 0x00 is a NOP and 0xFF is RST 7, which is also useful. NOP
is handy because you can see the address bus just counting up in binary.
We didn't even bother with buffers for a Z80. Just leave the data bus
pins open :-)
For specific machines, I have a little 28-pin carrier with an EPROM
socket and a BCD rotary switch to set the base address. Replace the OS
or BIOS ROM with simple test routines -- for example, a basic one for a
BBC Micro (an Apple or PET would be very similar and even simpler) has a
short loop that does a read of each I/O device in turn.
--
Pete Peter Turnbull
Network Manager
University of York