mov rax in fasm 1.71.16

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ToTamir

Joined: 11 Jan 2014
Posts: 1

ToTamir 11 Jan 2014, 09:49

Why

Code:

mov rax,16777216

is 7 bytes ( 48 C7 C0 00 00 00 01 )

and

Code:

mov rax,4294967295

is 10 bytes ( 48 B8 FF FF FF FF 00 00 00 00 )?

I don't understand it. The same 64 bit register but different opcodes?

11 Jan 2014, 09:49

tthsqe

Joined: 20 May 2009
Posts: 739

tthsqe 11 Jan 2014, 10:06

here are the relevant entries in the intel docs:

Code:

REX.W + B8 + rd io
 MOV r64, imm64
  Move imm64 to r64

REX.W + C7 /0 io
 MOV r/m64, imm32
  Move imm32 sign extended to 64-bits to r/m64

Intel tries to compress the op-codes whenever possible, which is why they are so much work to decode.

11 Jan 2014, 10:06

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