How to tell the length of an x86 instruction?
I was looking at the different instructions in assembly and I am confused on how the lengths of different operands and opcodes are decided upon.
Is it something you ought to know from experience, or is there a way to find out which operand/operator combination takes up how many bytes?
For eg:
push %ebp ; takes up one byte
mov %esp, %ebp ; takes up two bytes
So the question is:
Upon seeing a given instruction, how can I deduce how many bytes its opcode will require?
Solution
There's no hard and fast rule for x86 without a database as the instruction encoding is pretty complex (and the opcode itself can vary from 1 to 3 bytes). You can consult the Intel® 64 and IA-32 Architectures Software Developer’s Manual 2A document (Chapter 2: Instruction Format) to see how instructions and their operands are encoded:
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