CODEWITHSUNDEEP
64-bitmachine-code
Ryan Brown
Ryan Brown

Reputation: 1095

x64 opcodes and scaled byte index

I think I'm getting the Mod R/M byte down but I'm still confused by the effective memory address/scaled indexing byte. I'm looking at these sites: http://www.sandpile.org/x86/opc_rm.htm, http://wiki.osdev.org/X86-64_Instruction_Encoding. Can someone encode an example with the destination address being in a register where the SIB is used? Say for example adding an 8-bit register to an address in a 8-bit register with SIB used?

Also when I use the ModR/M byte of 0x05 is that (*) relative to the current instruction pointer? Is it 32 or 64 bits when in 64 bit mode?'

Is the SIB always used with a source or destination address?

Upvotes: 0

Views: 545

Answers (1)

user555045
user555045

Reputation: 64913

A memory address is never in an 8-bit register, but here's an example of using SIB:

add byte [rax + rdx], 1

This is an instance of add rm8, imm8, 80 /0 ib. /0 indicates that the r field in the ModR/M byte is zero. We must use a SIB here but don't need an immediate offset, so we can use 00b for the mod and 100b for the rm, to form 04h for the ModR/M byte (44h and 84h also work, but wastes space encoding a zero-offset). Looking in the SIB table now, there are two registers both with "scale 1", so the base and index are mostly interchangeable (rsp can not be an index, but we're not using it here). So the SIB byte can be 10h or 02h.

Just putting the bytes in a row now:

80 04 10 01
; or
80 04 02 01

Also when I use the ModR/M byte of 0x05 is that (*) relative to the current instruction pointer? Is it 32 or 64 bits when in 64 bit mode?

Yes. You saw the note, I'm sure. So it can be either, depending on whether you used an address size override or not. In every reasonable case, it will be rip + sdword. Using the other form gives you a truncated result, I can't immediately imagine any circumstances under which that makes sense to do (for general lea math sure, but not for pointers). Probably (this is speculation though) that possibility only exists to make the address size override work reasonably uniformly.

Is the SIB always used with a source or destination address?

Depends on what you mean. Certainly, if you have a SIB, it will encode a source or destination (because what else is there?) (you might argue that the SIB that can appear in nop rm encodes nothing because nop has neither sources nor destinations). If you mean "which one does it encode", it can be either one. Looking over all instructions, it can most often appear in a source operand. But obviously there are many cases where it can encode the destination - example: see above. If you mean "is it always used", well no, see that table that you were looking at.

Upvotes: 0

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