6 May
2008
6 May
'08
6 a.m.
Ethan asks:
From some recent reading of various materials I've
been finding
as I do my research, it does seem that a lot of post-S-100 systems
had 56K or 62K of RAM and 2K or 4K of ROM, still. Since I don't
know specifics, let me ask a general question in the hopes that
someone can figure out what machines and answer accordingly... for
those 2K/4K ROM systems, did any portion of those ROMs get accessed
once CP/M was up, or were they vestigal at that point? I can imagine
some sort of monitor program or such being mapped in for when the
OS got lost in the weeds, but what was that ROM space still good for?
You mention boot ROM's, but there's more uses that
chew up "useful" address space too:
A certain fraction of CP/M machines had memory-mapped
video and this stopped them (or only through mapping trickery
they got there) from having all 64K of RAM available to the user.
For example, 24x80 of character-cell video will take up 2kbytes - add
some video attribute bits (flash, blink, underline, bold for common
video chips), and then the software (which has to be in either RAM
or ROM, doesn't matter which because it will eat up some memory
space in either event) to drive the memory-mapped graphics and
you can have 4K or more taken up by the overhead of memory-mapped
video.
That 2K or 4K bytes doesn't have to actually eat up user RAM if
it is accessed only via I/O space mapping.
Some systems made good use of shadow-mapped ROM that
was mapped in to do the actual BIOS (disk and terminal I/O) work
and then mapped itself back out to give the user more RAM.
This only affects a fraction of the "middle-aged" CP/M systems because
the oldest assume a real video or hardcopy terminal at an I/O port,
and the newest assume that a PC-clone is doing all that work for them,
again at an I/O port :-).
Tim.