Re: Rational behind removing register forms of inc, dec?


Rod Pemberton wrote:

"Charles Crayne"
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?

They haven't been removed at all, they always existed in two forms:

40..4f are used as REX in 64 bit mode [still INC/DEC in 32 bit mode].
FF C0..CF work identical in both 32 and 64 modes anyway and are part of
the FE/FF 00/08 INC/DECb,w,q r/m group.

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?

If I look at the 64's instruction layout, it seems the developers were
forced (by merchant's due) to keep existing things as they are and so they
just added and merged a set of 16 registers into octal addressed gates.
I think they duplicated the octal gate and had to add the REX-bits
somewhere.

__
wolfgang



.

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