CODEWITHSUNDEEP
assemblyx86disassemblymachine-codeaddressing-mode
Zotta
Zotta

Reputation: 2603

x86 opcode encoding: sib byte

I'm currently trying to write a disassembler. I found the following list of opcodes and their meanings, so i decided to parse it at runtime: http://web.archive.org/web/20150810224114/http://mprolab.teipir.gr/vivlio80X86/pentium.txt

But i am stuck at the opcode 0x00: It is followed by a reg/modbyte. Parsing it was not much of a problem for me.

But I'm having trouble with the Scale-Index-Base byte:
If you actually specify esp as index register, it actually means that there is no index register.

The same applies for the base register with ebp. But I've tried it with C++ inline assembler: It is possible to compile: add [ebp*2+ebp],cl

So how can ebp be used as base register when using ebp as base register actually means using no base register at all!?

Upvotes: 5

Views: 3665

Answers (1)

MazeGen
MazeGen

Reputation: 106

The "missing EBP" case apply only in case ModR/M.Mod field has value 00 binary. If you need EBP as a base, the assembler changes the Mod to 01 binary and adds 8-bit displacement with value of zero:

004C6D00 add [ebp+ebp*2], cl

Upvotes: 9

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