21 Mar
2012
21 Mar
'12
3:57 p.m.
No, the A20 "issue" was due to a compatibility issue with some software
(can't remember at this point) where the SW was relying on the fact that addresses
would wrap around the 1M point on the 8086/8.
They were doing something akin to:
mov ds, 0ffffh
mov si, 501h
mov ax, 0(si)
That would cause ax to be loaded from 500h. Don't know why they were doing it but it
was some important SW and it wouldn't work on the 286 without the "hack"
since the 286 had 24 bit addressing.
Compatibility?the bane of us all.
TTFN - Guy
On Mar 19, 2012, at 4:18 PM, David Riley wrote:
On Mar 18, 2012, at 6:02 PM, Fred Cisin wrote:
On Sun, 18 Mar 2012, Chuck Guzis wrote:
I suspect the 286's A20 "oopsie" was due to someone writing
"0-20"
in a highlevel spec somewhere instead of "0-19". Caused no end of
grief, and IBM's workaround was a pretty gross hack at best.
- Dave
The problem with the 765 core and 128 byte MFM is
that the whole
sector doesn't get transferred--only the first 80 (decimal) bytes.
I can't help speculating whether that could be something as ridiculously
improbably simple as an engineer using too high-level a tool, and leaving
off the 'h' in "80h"