Quiz Entry - updated: 2026.07.14
What is the rep prefix and what does rep ret mean?
rep makes a string instruction repeat %ecx times (memcpy-style); rep ret is a quirky AMD branch-prediction workaround that behaves as a plain ret.
rep with string instructions:
| Instruction | Action |
|---|---|
rep movsb |
Copy %ecx bytes from (%rsi) to (%rdi) (like memcpy) |
rep stosb |
Fill %ecx bytes at (%rdi) with %al (like memset) |
repne scasb |
Scan for %al in (%rdi), up to %ecx bytes (like strchr) |
rep ret: AMD CPUs had a branch prediction issue when ret immediately follows a conditional jump. rep ret is a workaround — the rep prefix is ignored on ret but prevents the CPU bug. Treat it as plain ret.
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