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Undocumented x86 opcodes can accept operands without having defined meanings

devblogs.microsoft.com by Raymond Chen 3 min read Editor score 7/10

The piece distinguishes opcodes with no assigned meaning from architecturally undefined instructions, using gaps in the x86 opcode table to show how bytes can reach the decoder without a defined operation. The distinction helps explain the behavior of low-level code and instruction decoding.

What

The x86 opcodes ud0 and ud1 have no assigned instruction meaning, though their encodings can include operands.

How

The decoder can parse operands before raising an invalid-opcode exception; when 0F FF became PSUBQ, PADDQ had to move to 0F D4.

Why it matters

This illustrates how reusing an opcode can break programs that relied on its prior invalid-instruction behavior.

Read on devblogs.microsoft.com →

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