Jim Brain wrote:
>
> The UI- command does not exist in the 1540 ROM. Commodore did not
> think that far ahead.
I wrote:
Hmmm... You must be right. Looking at a disassembly
of the 1540 ROM,
I can't even figure out how UI+ would have worked properly on it
either. Unlike the 1541 ROM, the UI vector never checks for a + or -,
doesn't set the value of zero-page location $23 (which is never used),
and jumps into the reset code. Must be something strange and very
non-obvious going on in there.
The more I look at it, the more convinced I am that
the sheet of paper I
got with my C64 and 1540 back in 1982, explaining how to use the 1540
with the C64, must have suggested something other than the UI command.
Maybe it said to turn off the C64 display when reading from the disk? I
can't remember now. Or maybe my 1540 already contained a 1541 ROM, and
the paper was explaining how to use the UI- command to use it with a
VIC-20. That's starting to seem like the most likely explanation. Darn
this unreliable memory!
Eric