Re: FSF drops Apple boycott, opens gate fo

[kenner@lab.ultra.nyu.edu (Richard Kenner)]

In article <1995Feb8.114712.3756@driftwood.cray.com> brh@cray.com writes:
>There is also an rs6000 version of gcc/gnat.  The rs6000 uses the power
>chip set which is fairly close to the power pc instruction set.

Though this is off-topic, I think I should say something about this
families of architectures, since there is a lot of confusion out there
(not necessarily in this thread).

The POWER architecture first appeared a number of years ago in the IBM
product known as the RS/6000.  The processor itself didn't have a
public name, but most people referered to it by its last IBM code name
(RIOS).

The PowerPC architecture is neither a superset nor a subset of POWER.
However, the instructions present in both is a very large subset of
both; i.e., the differences between the two are a relatively small
number of instructions.  One other difference is that the mnemonics
used in the documentation of both architectures differ, even for the
instructions in both; GCC has a option that says which mnemonics to
use which is orthogonal (except in its default) to what architecture
you're compiling for.

The PowerPC architecture is a 64-bit architecture with defined
subsets.  All announced implementations omit the 64-bit instructions
and some omit a few of floating-point instructions (that are not in
the POWER architecture).  You also view it as having optional
supersets consisting of those instructions.

What is popularly known as the "PowerPC" chip is more properly known
by its Motorola part number, MPC601.  This is *not* a pure PowerPC
processor.  In fact, it implements the *union* of the POWER
architecture and the smallest defined subset of PowerPC (less the
64-bit and the extra FP instructions).  GCC knows this and when
compiling for the 601 (the default when you say "powerpc") it will use
either POWER or PowerPC instructions, depending on which are more
efficient for a particular expression.  The MPC603 and MPC604 do not
implement the POWER-only instructions.

To make life even *more* interesting, IBM recently announced products
based on the POWER2 architecture.  This architecture is a superset of
the POWER architecture.  Some, but not all, of these added
instructions are also in the optional part of the PowerPC
architecture.  GCC also supports POWER2.

This is probably much more than most people want to know on this
topic, but I thought I should straighten out the confusion.