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Index > Main > mov rax in fasm 1.71.16

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ToTamir



Joined: 11 Jan 2014
Posts: 1
ToTamir
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?
Post 11 Jan 2014, 09:49
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tthsqe



Joined: 20 May 2009
Posts: 729
tthsqe
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.
Post 11 Jan 2014, 10:06
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