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From: John Levine <johnl@taugh.com>
Newsgroups: comp.arch
Subject: Re: Fun with a Vax, Cost of handling misaligned access
Date: Sun, 2 Feb 2025 20:35:30 -0000 (UTC)
Organization: Taughannock Networks
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According to Thomas Koenig  <tkoenig@netcologne.de>:
>Anton Ertl <anton@mips.complang.tuwien.ac.at> schrieb:
>> problem these days, but the 48 pages or so potentially needed by VAX
>> complicated the OS.
>
>48 pages?  What instruction would need that?

I think it was actually 50.

The MOVTC and MOVTUC instructions had six operands, five
of which were multibyte, and one of which was one byte.
Each of those multibyte operands could cross a page
boundary, so that's 11 pages.

But all of the operands could use indirect addressing, each of which
could cross a page boundary, so that's 12 more pages.

The instruction itself could cross a page boundary, two more pages,
for a total of 25.

The user mode page tables on a Vax were in kernel virtual memory,
so by carefully pessimized memory allocation, each of those 25
pages could need a separate page table page, for another
25, total of 50.

I am not sure how far along the Vax's design was when they noticed this.
-- 
Regards,
John Levine, johnl@taugh.com, Primary Perpetrator of "The Internet for Dummies",
Please consider the environment before reading this e-mail. https://jl.ly