Re: Privilege Levels Below User

Lawrence D'Oliveiro wrote:

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.

Both have the property where the PC is pointing at the first
instruction
not executed.

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