28 Jun
2007
28 Jun
'07
9:37 p.m.
On 28 Jun 2007 at 20:59, Chris M wrote:
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.)
I've done a bit of that myself--and it's extremely difficult to get
such emulation perfect. There's always some idiot programmer who
used a feature or mode that you never thought *anyone* would use.
F'rinstance--how about port 61H with the "refresh" bit toggling every
50 or so microseconds? It's pretty difficult to emulate that
accurately with a software trap scheme.
Cheers,
Chuck