Re: Privilege Levels Below User

On Mon, 10 Jun 2024 20:41:31 +0300, Michael S wrote:

Intel's official terminology makes distinction between interrupts and
exceptions. The former are external/asynchronous, the later are
internal/synchronous. Exceptions are further sub-divided into faults,
traps and aborts.

That all sounds very DEC-like.

In particular, the DEC definition of a “fault” is that the saved PC on the
stack still points at the instruction that caused the exception, so a
return-from-exception will attempt to re-execute the same instruction.
This is exactly what you want for page faults, for example, but also for
long-running interruptible instructions that haven’t finished yet.

Whereas a “trap” left the PC pointing at the following instruction. So a
return from the exception handler will simply resume execution there.

Over the evolution of the VAX architecture, some exceptions which
initially were “traps” became “faults” instead.