Motorola 68K TRAP instruction as a bridge to OS

Question

I'm not an expert, but just a hobbyist. I was playing with 68000 architecture in the past and I've been always thinking of its TRAP instruction. This instruction is always described as a "bridge" to an OS (in some systems however it's not used in this regard, but that's a different story). How this is achieved? TRAP itself is a privileged instruction, so how this OS invoking mechanism works in user mode? My guess is that the privilege violation exception is triggered and the exception handler checks what particular instruction has caused the exception. If it's a TRAP instruction then the instruction is simply executed (maybe TRAP's operand i.e. TRAP vector number is checked as well), of course now in the supervisor mode. Am I right?

Answer 1

The TRAP instruction is not privileged, you can call it from either user mode or supervisor mode.

It's the TRAP instruction itself that will force the CPU to supervisor mode, and then depending of the #xx number you used will jump to any of the 16 possible callbacks from the memory area $80 to $BC.

TRAP also pushes to the stack the PC and SR values, so when the last function call returns it goes back to whatever mode was setup before you called TRAP.