Why are there only i686 and i586 Version of glibc and kernel? -- not "older systems," "older ISAs"

[Bryan J. Smith]

Matthew Miller wrote:  
> Enough people seem to care about keeping older-system support in Core,
> and I don't care enouigh about moving it out, that I going to just
> drop this for now. No point in arguing.

Just FYI, it's not about "older systems."  It's about "older ISAs."

Some of these embedded x86 processors are very fast, have all the latest
"toys" around them, but are very low power, with everything in a single
chip.  The way they do this is with an older ISA.

IDT-Centaur's original WinChip, which ran well over 200MHz (up to
300MHz?), was actually a non-pipelined 486 ISA, with a massive (at the
time) amount of cache.  It was not only cheaper to make, but took less
than 18 months to design, and worked with standard 3.3V signaling.

A lot of companies licensed that design for use in their own products
(STMicro?).  IDT-Centaur is also a major MIPS partner, so doing this was
nothing new to them.  In fact, I wouldn't be surprised to find out a lot
of companies did, and have 6th-Gen equivalent performing x86 cores that
are only 486 ISA compatible.

The rule of thumb is, everything that can be built for 386 ISA, should
be built for it.  Optimize for 686 architectures (PPro/II+, Athlon,
etc...), but make it 386 ISA compatible.  If that is not possible (e.g.,
NPTL), then 486 ISA.


-- 
Bryan J. Smith, E.I. -- b.j.smith at ieee.org