Re: Privilege Levels Below User

mitchalsup@aol.com (MitchAlsup1) writes:

EricP wrote:

Alpha had 3 levels, User, Supervisor, and a higher third mode called
PAL for Privileged Architecture Library. It was supposed to be thought
of like microcode, privileged subroutines. Then PAL mode was used to
emulate the 4 levels that VMS expected when they ported it.

>
PAL was microcode in <fast> ROM in the native ISA.

What is called when you perform a PAL call (at least on EV45, but most
likely on all Alphas) is Alpha code, and it resides in RAM and is
loaded there from the boot loader.  I know, because I enhanced the PAL
code supplied with the MILO boot loader for EV45 to activate the full
16KB of D-cache (rather than just 8KB).

It also uses less specials than I expected; e.g., on the EV45 the IMB
(instruction-memory barrier) PAL call is implemented by just executing
a big chunk of code such that the previous contents of the I-cache are
evicted, while I expected that it would set a bit in a model-specific
register.

(I think PAL mode was a way to patent a feature that made the
ISA impossible to copy without their permission,
and therefore someone can't take DEC's executables and run them
on a clone processor, like what happened to IBM with Amdahl.)

>
Worked real well for them !!

Definitely.  Note that the first Amdahl machine shipped 11 years after
the first S/360.  Alpha was canceled 9 years after the first Alpha was
shipped.

- anton
--
'Anyone trying for "industrial quality" ISA should avoid undefined behavior.'
  Mitch Alsup, <c17fcd89-f024-40e7-a594-88a85ac10d20o@googlegroups.com>