IBM PC hacking

Chris M chrism3667 at yahoo.com
Sat Oct 1 12:22:08 CDT 2005


--- 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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