问题
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://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!?
回答1:
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
来源：https://stackoverflow.com/questions/3476541/x86-opcode-encoding-sib-byte