🚀 go-pugleaf

storm@cs.mcgill.ca (Marc WANDSCHNEIDER) writes:

>I've noticed that most of the x86 chips I have programmed have a Parity
>bit.  I was just wondering if they actually compute this every time an
>operation is done, or if they just get this from memory (ie, is the parity
>bit valid after EVERY operation, or just loads from memory...?)

The parity flag indicates the parity of the low 8 bits of the result
of the last arithmetic or logical operation - it has nothing to do with
memory parity and since move instructions do not affect the contents of the
flags register a load from or a store to memory will not change the flag.
However most arithmetic and logical instruction on the X86 will cause it
to be updated - the only exceptions seem to be AAA, AAS, MUL, IMUL and the
bit scan and bit test and set/clear/complement instructions after any of
which the parity flag is undefined.