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

> It’s called ud2 because the 0F FF variant was retroactively named ud0, and the 0F B9 variant was retroactively named ud1, leaving ud2 as the recommended undefined opcode.

It was a surprise for me as a reader. When I came to this sentence I assumed that 0f ff would become #1 and 0fb9 -- #2. But no, Intel counts from zero, so there is a third ud.

Nowadays UD0 UD1 UD2 are in the SDM and APM.

We also got UDB (D6), the one-byte variant that arrived with x86-64 for 64-bit mode.

And we have always had UDW (FF FF), aka group #5 (1st FF) with a modrm byte of mod=11b r/m=111b (/7) reg=111b (2nd FF) -- that one matters for memory with all bits set to 1, or for buses terminated to all 1 when no device claims an access.

I'm not much of an x86 person but on other architectures you can raise software interrupts/exceptions. Does x86 not have this or did those facilities not cover enough use cases?

It's like why the first (hard) drive letter is C.

Is this just his speculation? Or is there evidence for it?