23 Apr
2007
23 Apr
'07
12:11 a.m.
On Apr 22, 2007, at 11:52 AM, dwight elvey wrote:
Most any machine can be setup to shadow the ROM. On a different
machine,
I wrote a few lines of code that would simply read and then write
ROM data. I set it up so that writes would go to RAM that on boot were
shadowed behind the ROM.
After the ROM had been copied, the code was still running from ROM.
I had a 7474 that would switch the enables on the ROM and RAM
when it saw the first I/O operation.
In your case, you'll either need to write separate I/O code for CP/M
or rige some way such that on each I/O action of the CP/M, you
switched the ROM back into memory. This should work so long as you
don't
expect to buffer the I/O data where to ROM was. Both are relatively
simple to do.
Uhhh...but what if the address lines aren't handled such that the
lower address space isn't remappable? It's my impression that this
sort of scheme isn't really all that common. I mean, sure it's easy
to do (I did it on one of my SBCs) but I don't think many
commercially-produced machines were done that way.
-Dave
--
Dave McGuire
Port Charlotte, FL
BTW Can TRS-80 Model 1's or Memotech's run
CP/M (I had an "SDX"
disc pack for my Memotech at one time, but that didn't run CP/M;
rather it had a basic "disc filing system" in ROM)?
I'm pretty sure the TRS-80 Model I is incapable of running
unmodified CP/M because it has ROM in low memory that cannot be
remapped...is this correct?