Increment and Decrement Instructions in 64-Bit Mode
The INC and DEC instructions are supported in 64-bit mode. However, some forms of
INC and DEC (the register operand being encoded using register extension field in the MOD R/M byte) are not encodable in 64-bit mode because the opcodes are treated as REX prefixes.

Where is the register extension field of ModR/M byte?
Posted on 2010-08-17 05:48:58 by logicman112
In 32bit mode primary inc and dec  when addressing registers were one byte opcodes 40-4F there was a secondary encoding with 0xff(group) for addressing memory (and it could be encoded to registers too!).

The 40-4F were only defined for registers - no modrm. (by heart) it looked like this 0100 X RRR , X=0 defines inc, X=1 defines dec, rrr register

In 64bit mode they have taken this 1-byte opcode to be a prefix - REX.

You should really be reading the "instruction set reference" for the details. (by heart) 0100 remains and XRRR are the flags.
Posted on 2010-08-17 07:49:31 by drizz
You are completely right, drizz. Thank you.

But my surprise was the manual. It is saying: register operand is being encoded by a filed named, register extension field in the MOD R/M byte!! While those registers are encoded by opcode!! not MOD R/M byte!
Posted on 2010-08-18 01:38:38 by logicman112