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.
It's like why the first (hard) drive letter is C.
A: drive is 3.5; B: drive is 5.25; C: drive is hard disk
Even further back into time, before 3.5" disks, both A: and B: were 5.25" disks. And, although my memory is hazy, 3.5's were commonly slotted into B: at first, because no one had 3.5" boot disks until the drives became somewhat common.
Yeah it's a relic from when computers booted off floppy and hard disks were rare and expensive
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?
Maybe because if the code wants to call the invalid opcode interrupt handler (INT6), it needs extra code to populate the flags and registers expected by that handler, whereas actually triggering an invalid opcode exception will get all those parameters populated automatically.
It's basically a convention. The alternative is to raise interrupts of course, but that might be application specific, or use other invalid instructions than the designated one, but they might work differently on other processor types.
x86 has...
INT Ib INT1 INT3 INTO BOUND
Is this just his speculation? Or is there evidence for it?
What the instruction does is well documented, and the history of other invalid instructions being used for the same purpose in the past I'm guessing there is also well known, though a quick search doesn't turn up any official Intel documentation on the matter.
Given who this is and the overall quality of his output over the years, I'm willing to trust it isn't pure guesswork - and anyway, I'd trust his guesswork over many other people's absolute facts.