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From: mitchalsup@aol.com (MitchAlsup1)
Newsgroups: comp.arch
Subject: Re: The Design of Design
Date: Tue, 30 Apr 2024 20:59:47 +0000
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Scott Lurndal wrote:

> mitchalsup@aol.com (MitchAlsup1) writes:
>>Scott Lurndal wrote:
>>
>>> John Levine <johnl@taugh.com> writes:
>>>>According to Thomas Koenig  <tkoenig@netcologne.de>:
>>>>>> S/360 invented eight bit byte addressed memory with larger power of 2
>>>>
>>>>>Brooks wrote that the design was supposed to have been 32-bit
>>>>>clean from the start, but that the people who implemented the BALR
>>>>>instruction (which puts some bits of the PSW into the high-value
>>>>>byte) didn't follow that guideline.  He blamed himself for not making
>>>>>that sufficiently clear to all the design team.
>>>>
>>>>Yup.  Even worse, the OS programmers were under extreme pressure
>>>>to save memory so in every data structure with address words,
>>>>they used the high byte for flags or other stuff.  So when they
>>>>went to 31 bit addressing, they needed new versions of all of
>>>>the control blocks.
>>
>>> The B300 had fixed instruction format that included three
>>> operand fields.  For instructions that didn't use all three
>>> operands, the programmer was encouraged to use the unused
>>> operand fields as scratch fields.
>>
>>
>>For a modern ISA, the architect should specify that various bits

> The B300 was an extension of the 1950's Electrodata 220, and had
> very little total memory.

> In modern systems with several orders of magnitude more memory, the
> more useful restriction is to make the text section read-only
> via the MMU.

Necessary but insufficient. You not only want malicious programs
from being prevented from altering .text you also need software 
generating programs (compilers, linkers, JITs) from creating BAD
bit-patterns that could be mistaken as an instruction.

> Yes, for extensibility, the hardware should, generally, fault
> on unused instruction encodings (having a NOP space that can be
> extended with 'hint' instructions in future versions of the
> instruction space maintains backwards compatability with software
> built for later generations when run on earlier generations which
> treat the encoding as a NOP, viz. ARM64).