9 Apr
2010
9 Apr
'10
12:29 p.m.
There is in
most cases (assuming thr Apple ][ referecne manual is
correct), but it's not as obvious as you might think. It's not always the
same bit that changes.
That a different bit changes for the different forms of inc/dec instructions
is irrelevent to the conjecture. The primary candidates see only a 1-bit change. The conjecture doesn't strictly hinge on the 1-bit
aspect, regardless, just
makes it more likely.
Sure. A single-bit failure is more likely than a 2-bit failure, etc. I
wonder if one direction of failure is more likely than another (in the
case of most EPROMs, as they contaiin FFs when erased, bit rot gerneally
causes 0's to become 1's, for example)
If so, it
would be sensible to compare them with
ROM dumps fro ma working machine.
Glad you see it my way.
Someday I might get back to working on it. Getting ROM dumps from a working
machine shouldn't be difficult even for the soldered-in ROMs (a little PEEKing
program or such), they may be out on the web already. Unfortunately, in the broken machine the socketed ROM - if it is something
optional - is not likely
to contain the pertinent code.
Ithink we can disocunt the possibllity of a bad socket too ;-)
It might be easiest to desolder the 6502 processor and find some way to
drive the address connections on its PCB position and monitor the data
lines. Compters with what I grew up calling 'user ports' are not common
these days [1] althoguh it shouldn't be hard to make somthing (I hate to
say this, but I would guess a microcontroller with a USB slave interface
could do this very easily on a modern PC.
[1] I personalluy like the HP98622 card in an HP9000/200 machine.
-tony