Rational behind removing register forms of inc, dec?

"Charles Crayne" <ccrayne@xxxxxxxxxx> wrote in message
news:20071023133450.2ae6de9c@xxxxxxxxxxxxxxxxxx

Long mode is 64-bit mode, but, fortunately, the r/m inc/dec instructions
have not gone away -- just the register only forms. Unless you code at
the machine code level, you won't even notice the difference.


Anyone know what the rational behind removing the register forms of inc, dec
is?

I'd think that if one was forced to remove instruction modes from
instructions, one would start with the memory modes since a mov or lea could
be used to replicate the removed mode. I'd also think that since hand coded
x86 code can be heavily register based, sometimes avoiding memory forms of
instructions completely (using mov or lea exclusively), that one would
choose to preserve register based instructions. The two opcode frequency
documents I have indicate that inc (but not dec) is also a high usage
instruction comparable to push. It's listed in the top five used
instructions. Was it really that hard to find a few bytes when they
elimated most of the obsolete instructions from the single byte instruction
table?


Rod Pemberton

.

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