29 May
2010
29 May
'10
1:42 p.m.
Jim Brain wrote:
The UI- command does not exist in the 1540 ROM. Commodore did not
think that far ahead.
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.
Eric