1 Oct
2005
1 Oct
'05
10:22 a.m.
--- Jim Leonard <trixter at oldskool.org> wrote:
What was special about it wasn't necessarily what
it
was, but what it was
trying to be. Very interesting design choices and
direction for IBM.
--
Jim Leonard (trixter at oldskool.org)
Didn't the Peanut employ some sort of video gate
array that made the video memory, actually existing
low in ram, appear to be @ b800:0000? I read something
to that effect in "the IBM Personal Computer from the
Inside Out".
A gentleman I'm in contact with attempted to develop
a mod for the Tandy 2000 that enabled PC compatiblity.
I can understand some degree of what's being said
(though not necessarily how to put it all together),
but maybe my placing it here will provoke discussion.
Sounds kind of sort of like what was going on with the
Jr.
And actually, I thought a memory parity error always
generated a NMI, or at least most PC's were designed
to.
I was working on a mod, never completed, for the T2K
that would make it
after the O/S was loaded, 100% PC-compatible.
Required 1 hardware chip,
and customized software. Whereupon any DOS
application would run on the T2K,
even stuff doing serial-port manipulation, direct
video-memory writes, DMA,
etc.
The one-chip mod was a PAL that monitored the address
lines from the
CPU,
when It saw an I/O instruction in 'low'
address->space, it generated an
NMI (not used at all by the T2K), then the software
service routine
for NMI unwound the stack to find the offending I/O
instruction, and
re-mapped the "PC" functionality to the T2K
hardware. Coupled with a
timer-tick 'refresh' routine that copied data >from
"PC video
memory" to
the T2K video memory (remapping attributes, etc. as
required.)
The chip actually had 2 modes of operation -- NMI
active, as described
above, and 'NMI inactive', where it pretty much did
nothing -- except
listen for the 'magic words' that made it go active,
that is. :) This
enable/disable mode switching was necessary, to
allow "T2K DOS"
internals
(and/or the 'sofware service routine) to access the
T2K hardware
directly.
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