23 Apr
2007
23 Apr
'07
11:11 p.m.
On Apr 23, 2007, at 11:15 AM, dwight elvey wrote:
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.
If you mean that the machine wasn't designed that way and it can't
be done without modifications, then you may be right. If you are
willing to make
a few cuts and jumpers, then most any machine that I've ever seen
can be
reconfigured.
The modifiaction that I described required a few cuts, jumper and
a daughter board.
Even without this, most CPU's are socketed. It is not that hard to
trap
out specific addresses and change what is sent to the bus. This can be
doen with no modifications to the system. You seem to be lacking
imagination
on what can and can't be done.
I think you need to give the problem a little more thought.
Nono, I agree with you, it's quite simple...I know how it's done;
I've designed four Z80 systems, three with remappable lower address
spaces and one without. I'm talking about running CP/M on an un-
hacked machine...as in, walking up to a bog-standard TRS-80 Model I
and booting CP/M.
-Dave
--
Dave McGuire
Port Charlotte, FL
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?
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.