30 Sep
2001
30 Sep
'01
8:54 a.m.
Maybe part of the confusion arises because many
processors (including the
8080) complement the carry flag at the end of a subtraction, so that it can
be used directly as a "borrow" flag in multibyte subtractions. Others
(like the 6502) don't do that.
I'm not sure what you mean by this, but on my C128,
1300 ad 01 04 lda $0401
1303 38 sec
1304 e9 08 sbc #$08
1306 8d 01 04 sta $0401
1309 ad 00 04 lda $0400
130c e9 00 sbc #$00
130e 8d 00 04 sta $0400
1311 60 rts
does what you would expect if, say, $0401 had a seven in it: it decrements
$0400 as well, and does a proper borrow.
--
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Cameron Kaiser, Point Loma Nazarene University * ckaiser(a)stockholm.ptloma.edu
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