Re: The Design of Design

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
of the general format "must be zero"* when those bits are not used
in the instruction. We do this (and we have verification guys check that the HW raises an exception when those bits are non-zero. This
is the only sane way to prevent malicious use of the ISA by erroneous
compilers or from malicious SW with writeable access to code memory.
(*) must be zero can also mean must be one or must be this pattern
should 1 or "this pattern" be meaningful.
What you want is for the ISA document to specify everything every
instruction can do, and also to specify that all encodings not
leading to well defined calculations and accesses fail. This preserves the encoding space for the future.
{{And in a strange way: this makes verification easier, too}}