7 Jun
2002
7 Jun
'02
6:21 a.m.
--- Ben Franchuk <bfranchuk(a)jetnet.ab.ca> wrote:
8 bit cpu's... most undefined instructions
are mostly harmess.
There is one undefined instruction on the 1802 - $68 IIRC. It's in
the group of I/O instructions, but there are only 7 defined ports (with
the N-lines indicating 1-7 at the appropriate cycle). The later,
extended versions of the 1802 used $68 as an opcode extender.
The reason for bringing it up is that I remember playing on the Elf
20 years ago, and noticing that if I accidentally executed a $68, the
CPU would go berserk and whomp on random patches of memory. If it
happened, I was guaranteed to have to retoggle the program in from
scratch.
-ethan
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