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From: anton@mips.complang.tuwien.ac.at (Anton Ertl)
Newsgroups: comp.arch
Subject: Re: Privilege Levels Below User
Date: Sun, 09 Jun 2024 12:25:44 GMT
Organization: Institut fuer Computersprachen, Technische Universitaet Wien
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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>