[Simh] VAX vectors

[lists at openmailbox.org]

On Sat, 11 Jul 2015 14:42:42 +0200
Rhialto <rhialto at falu.nl> wrote:

> On Fri 10 Jul 2015 at 09:33:56 +0000, lists at openmailbox.org wrote:
> > I think you could argue at the beginning System/360 did have things in
> > common with what would later be called RISC although z/Architecture has
> > overshadowed a lot of that RISC flavor in favor of more specialized
> > instructions and addressing modes.
> 
> I recall reading that the whole RISC thing was more or less started with
> compiler writes for the 360 noticin that in practice they only used a
> rather limited subset of the available instructions. That got developed
> in hardware in the POWER cpu and later in the POWERPC (PPC).

That is not what I meant, obviously. I'm talking about the approach to the
architecture itself which was very clean, up until z/Architecture anyway.
It used very simple and few addressing modes and a reasonably compact
instruction set. On the other hand there were just the right number of
storage-to-storage instructions to be able to move data around, do
arithmetic from storage to register, and bit operations on storage without
having to load storage to a register first.

> There were no relative addressing modes with offset more than 12 bits.

And we never missed it.

> And even  in the 64-bit z/System there isn't, apparently...

There is a 20 bit signed offset in the Long Displacement Facility
instructions in z/Architecture. But we still don't use it. It is exploited
by the newer compiler versions and sometimes also by newcomers to the
system, not that there are many of those.

> and each instruction has its own different subset of addressing modes it
> works with.

More correctly each instruction may be categorized by the addressing mode
it uses. Each instruction has a proper one-to-one relationship with each
opcode. This is one of many things about that system that makes encoding
and decoding so simple. It is notable virtually all systems software and
all the compilers are still written in assembler (or PL/X, a sort of
PL/I-like high-level assembler with no runtime) on Z.

Another interesting thing is object code and virtually all application code
from the 1960s still runs on the very latest hardware and OS in 2015.

-- 
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