Re: The Design of Design

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).