Write Mini Disassembler: Decode Byte Stream Back to Mnemonics

Decode Pipeline

A disassembler reverses assembly: bytes in, mnemonics out. A minimal x86 decoder reads one opcode byte, optionally consumes ModR/M and immediates, then prints the instruction.

cCopy
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Pattern Matching Opcodes

Start with a table for common one-byte opcodes:

• 0x90: NOP
• 0xC3: RET
• 0x01 /r: ADD r/m, r

For example, the byte stream 90 90 C3 disassembles to two NOPs followed by RET.

Extend ModR/M parsing to distinguish ADD EAX, EBX from ADD [RBX], EAX.

For this exercise, you will write a loop that advances ip correctly for each instruction length. This task asks you to never lose sync with the byte stream, because one wrong instruction boundary corrupts every following decode in a real debugger.

Working Through the Exercise

Keep the relevant datasheet, ISA manual, or architecture textbook chapter open while you implement. When your output disagrees with the reference trace on the same program, the bug is usually a mis-decoded opcode, a stale register read, or a flag bit left unchanged after arithmetic.

• Trace first: walk one failing input by hand before changing random lines
• Read the manual: skim the official doc for the mechanism named in this task
• Compare baselines: save measurements from the prior step so speedups are honest

For this exercise, you will use those habits while implementing the requirement in the starter code. Microarchitectural product names change across CPU generations, but the control ideas (fetch, bypass, cache lines, vector lanes) stay stable enough to debug from first principles.