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From: "Paul A. Clayton" <paaronclayton@gmail.com>
Newsgroups: comp.arch
Subject: Re: 80286 protected mode
Date: Sun, 13 Oct 2024 13:32:32 -0400
Organization: A noiseless patient Spider
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On 10/13/24 3:56 AM, Michael S wrote:
> On Sat, 12 Oct 2024 18:32:48 +0000
> mitchalsup@aol.com (MitchAlsup1) wrote:
[snip memory copy instruction]
>> The 3rd Operand can, indeed, be a constant.
>> That causes no restartability problem when you have a place to
>> store the current count==index, so that when control returns
>> and you re-execute MM, it sees that x amount has already been
>> done, and C-X is left.
> 
> I don't understand this paragraph.
> Does constant as a 3rd operand cause restartability problem?
> Or does it not?
> If it does not, then how?
> Do you have a private field in thread state? Saved on stack by by
> interrupt uCode ?

The extra state is saved in the context save area (like
for My 66000's extra state for the PREDicate instruction
modifier).

(Of course, restartability could also be provided by using
an ordinary register for the in-progress count even for
immediate counts. The instruction would effectively become a
load immediate and memory copy. Implicit/extra state has
some benefits.)

> OS people would not like it. They prefer to have full control even when
> they don't use it 99.999% of the time.

On the other hand, isolating some state and functionality might
facilitate less trust requirements? Some OS people might not like
having the OS be less than fully trusted.