Why is the x86 undefined instruction called ud2? Why 2?
The x86 undefined instruction ud2 is called such because it originated from the 0F FF sequence, which was found to reliably raise an invalid opcode exception when executed. Initially, there was no architecturally undefined instruction on x86, so those who wanted to force this exception looked for a byte sequence that would trigger it.
The 0F FF sequence was discovered to work, but it internally decoded as if it had two parameters, although these were not actually used since the exception was raised before any usage. Meanwhile, another sequence, 0F B9, also had the same properties. This led to two groups using separate sequences, 0F FF and 0F B9, for the same purpose.
Intel later created a dedicated instruction, ud2, to officially provide a reliable way to trigger an invalid opcode exception. The name ud2 came about because the 0F FF variant was retroactively named ud0, and the 0F B9 variant was named ud1, leaving ud2 as the recommended undefined opcode. The choice of ud2 instead of ud1 or ud0 was due to its simplicity as a two-byte instruction without parameters, avoiding issues with decoding and potential access violations if the instruction crossed into a not-present page.
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- Why is the x86 undefined instruction called ud2? Why 2? devblogs.microsoft.com
