[x64] Question about sign-extension

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yoshimitsu

Joined: 07 Jul 2011
Posts: 96

yoshimitsu 07 Jan 2012, 10:17

Hello,
I don't quite get x64 operation encodings and how x86 and x64 operands interact.

Quote:

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

for example, how do I achieve such an instruction?
"mov rax,12345678h" gives "mov rax,0000000012345678h" and something like "mov rax,dword 12345678h" is invalid.
"mov qword [var],12345678h" writes a zero-extended value into var, as expected.
But "mov qword [var],87654321h" fails to assemble because of the value being out of range, although I was expecting sign-extension.
Am I misunderstanding this at all?
And which instructions do actually sign-extend?

Additionally, I've got another question in reference to LEA:
Is LEA encoded RIP-relative only if the ModR/M byte is 05, means just a disp32-value was provided?
And if e.g. a register is added, then the disp32-value is interpreted as an absolute value?
Thanks

07 Jan 2012, 10:17

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8349
Location: Kraków, Poland

Tomasz Grysztar 07 Jan 2012, 12:08

yoshimitsu wrote:

"mov rax,12345678h" gives "mov rax,0000000012345678h" and something like "mov rax,dword 12345678h" is invalid.

"mov rax,12345678h" assembles for me to form with sign-extended 32-bit immediate (because it's shorter). What version of fasm have you used?

yoshimitsu wrote:

But "mov qword [var],87654321h" fails to assemble because of the value being out of range, although I was expecting sign-extension.
Am I misunderstanding this at all?

When you write assembly language instruction, it focuses on what the instruction has to do, not how is it going to be encoded. So if you write what value you want to put into qword variable, you should really specify the value that you want to put there and not the value that you expect to get in the encoding. So the correct form would be:

Code:

mov qword [var],0FFFFFFFF87654321h

Code:

mov qword [var],-789ABCDFh

yoshimitsu wrote:

Additionally, I've got another question in reference to LEA:
Is LEA encoded RIP-relative only if the ModR/M byte is 05, means just a disp32-value was provided?
And if e.g. a register is added, then the disp32-value is interpreted as an absolute value?

Yes.

07 Jan 2012, 12:08

yoshimitsu

Joined: 07 Jul 2011
Posts: 96

yoshimitsu 07 Jan 2012, 18:44

Sorry, I did not use "mov rax,12345678h", but "mov rax,87654321h" in which case you can't take the sign-extended 32-bit immediate, because it'd have a false result which wouldn't match the actual instruction.

Tomasz Grysztar wrote:

[...] you should really specify the value that you want to put there and not the value that you expect to get in the encoding [...]

That's a point, I was wondering, though, because the intel docs is talking about a 32-bit immediate and 0FFFFFFFF87654321h wouldn't fit this.
So FASM throws "value out of range" because "87654321h" isn't explicitly negative?

07 Jan 2012, 18:44

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