together a breadboard of TTL (mostly latches/counters and
muxes) which, with suitable software support on the host, lets me
read ROMs via a parallel port. [...]
That would be of interest to me.
Okay, I'll see if I can't put together a schematic.
Also of interest would be something similar that would
to a RAM chip that would appear as read-only to an 8-bit target
system. I have *lots* of TTL on hand here... :-)
I did that, once. I have a TVI955 (what would now be called a dumb
terminal, but in its time was called a smart terminal). It includes
three ROMs. One is the terminal firmware, one is a character
generator, and one is a user-supplied firmware (there's an escape
sequence in the standard firmware that jumps to the user-supplied
firmeware). I once put together a simple game that ran entirely in the
terminal, in the user-supplied-firmware ROM....
I pulled the character-generator ROM and replaced it with flying wires
to a breadboard on which I had a similar amount of RAM, plus logic to
allow a host to write to it via a serial line. This gave me
downloadable characters. It wasn't a very good setup; something about
it (the flying wires, at a guess) produced lots of read errors,
appearing as snow on the screen. And the way the host wrote to it was
cycle-stealing; while the host was writing, the read circuits got
garbage - 0xff, or the data being written, probably - which produced
more artifacts on the screen. Definitely not suitable for firmware,
for which read errors would be fatal, but tolerable for hacking around
with character bitmaps.
Unfortunately that breadboard circuit has been torn down long since.
But I still have the ROM-reader circuit; I'll put together a schematic
and parts list for that and drop a note here when it's done.
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