instruction-encoding

How to tell the length of an x86 instruction?

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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 … Read more

How to determine if ModR/M is needed through Opcodes?

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Intel’s vol.2 manual has details on the encoding of operands for each form of each instruction. E.g. taking just the 8-bit operand size versions of the well-known add instruction, which has 2 reg,rm forms ; a rm,immediate form ; and a no-ModRM 2-byte short form for add al, imm8 Opcode Instruction | Op/En | 64-bit … Read more