Why is the x86 undefined instruction called ud2? Why 2?

Automated excerpt

The ud2 instruction is an architecturally undefined instruction, guaranteed to raise an “invalid opcode” exception. So people who wanted to force an invalid opcode exception went looking for some byte sequence that reliably raised the invalid opcode exception when executed. Somebody found that the 0F FF sequence led to an invalid opcode exception. Maybe they tried introducing a new instruction that uses 0F FF. Or maybe it was still undefined but just performed some random operation instead of raising the invalid opcode instruction.

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