Different CPU ISAs Discussion

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Different CPU ISAs Discussion

Postposted on Fri Nov 02, 2012 12:32 pm

Hi gerbils. I've been thinking a lot about different ISAs (Instruction Set Architecture) lately. You know, x86, ARM, PowerPC, MIPS, SPARC, IA-64, and some other defunct ISAs such as Alpha and AMD's 29K RISC ISA. I've been thinking, if I was given a choice which ISA would've been prevalent now in x86's place, what would it be? Now x86 has a practically unmatched software and hardware installed base and infrastructure support (compilers, drivers, OSes, etc.), but what if it was SPARC that gained all that? What would have been the advantages or drawbacks? Or what if Intel decided to push the Alpha spec it got from Compaq? Feel free to spill your brains out.
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Re: Different CPU ISAs Discussion

Postposted on Fri Nov 02, 2012 1:44 pm

I think the dominant ISAs today got where they are more by twists of fate than by any sort of technical merit.

On the CISC side of the house, Motorola's 680x0 series was a much more sensible ISA than x86. But IBM went with the x86 for the first mass-market PC, and the rest is history.

With RISC I don't think there's a clear technical winner. MIPS would've probably had a better shot if they hadn't gotten sidetracked by being owned by SGI for a few years; they're currently playing second fiddle to ARM in the consumer embedded space. Alpha was a promising architecture, but Compaq effectively killed it when they sold it to Intel (some of the related tech lived on elsewhere though, e.g. the design of the original Athlon system bus). PowerPC is more of a niche product now that Apple doesn't use it any more, but it is still used in IBM servers and in high reliability military/aerospace applications. SPARC is still used by Oracle and some supercomputer vendors, but was somewhat marginalized by Sun's decline and the rise of cheap x86 servers (SPARC is interesting in that there are Open Source implementations of the ISA).

Intel even had their own RISC CPU designs for a while, the i860 and i960. The i860 was a strange hybrid of a RISC core and DSP that never really caught on in a big way. The i960 was popular as an embedded processor for devices like laser printers (same market niche that AMD's 29K dominated back in the day). Intel and AMD both eventually decided to put all of their eggs in the x86 basket, killing off their RISC lines.

So except for the Intel/AMD offerings (which were killed off intentionally) and Alpha (which you could argue was also killed intentionally), the major RISC architectures are all still around. ARM has just come to dominate in the consumer space by being in the right place at the right time - they had a near-ideal mix of performance and power utilization for the smartphone and tablet revolution.
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Re: Different CPU ISAs Discussion

Postposted on Fri Nov 02, 2012 1:52 pm

When you ask which CPU ISA can take x86's place, what it really boils down to is what CPU ISA do you feel should have been in the original IBM PC. The quality of the ISA itself really has less to do with the success of x86 than it has to do with being an off-the-shelf product, being chosen as the IBM PC CPU and the subsequent events that followed. Had Don Estridge and his team chosen, say.. Motorola's CPU at the time, I suspect that the 68k ISA would have become the x86 of today. After the IBM PC clones industry came about, it gave rise to a multitude of other off-the-shelf components, such as video cards, sound cards, motherboards, etc. which from the IBM BIOS. In other words, the success of x86 today isn't necessarily due to the ISA, but to the IBM PC and its BIOS.

Anyway, for the sake of discussion, if we could replace the ISA retroactively with hindsight being 20/20, I think MIPS would make for an interesting candidate. Like ARM, it has a core that can be licensed and I hear it has a nice assembly language to go with it. But unlike ARM, its old enough to where if it had gained prominence at its inception in the early 80s, it could have supplanted x86 in the IBM PC had it been cheap enough.
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Re: Different CPU ISAs Discussion

Postposted on Fri Nov 02, 2012 2:14 pm

bjm wrote:Anyway, for the sake of discussion, if we could replace the ISA retroactively with hindsight being 20/20, I think MIPS would make for an interesting candidate. Like ARM, it has a core that can be licensed and I hear it has a nice assembly language to go with it. But unlike ARM, its old enough to where if it had gained prominence at its inception in the early 80s, it could have supplanted x86 in the IBM PC had it been cheap enough.

Design work started in 1981, but the first actual MIPS CPU wasn't available until '85, the same year Windows 1.0 was released. x86 was already well on its way to being entrenched by then.
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Re: Different CPU ISAs Discussion

Postposted on Fri Nov 02, 2012 2:26 pm

Ah, true. With the prominence of assembly during those days, there wasn't going to be any "just re-compile and you're done" type of ports either. So even if there was an attempt to move to MIPS in '85, it would've failed.
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Re: Different CPU ISAs Discussion

Postposted on Fri Nov 02, 2012 2:32 pm

just brew it! wrote:PowerPC is more of a niche product now that Apple doesn't use it any more, but it is still used in IBM servers and in high reliability military/aerospace applications.

And keeps several thousand employed about 3 miles down the road from my house.
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Re: Different CPU ISAs Discussion

Postposted on Fri Nov 02, 2012 2:36 pm

just brew it! wrote:On the CISC side of the house, Motorola's 680x0 series was a much more sensible ISA than x86. But IBM went with the x86 for the first mass-market PC, and the rest is history.
Agreed. What was interesting was that Wozinak came very close to using the 6800 in what became the Apple ][, before settling on the 6502 (for cost reasons, IIRC). Had he gone with the 6800, ancestor to the 68000 found in the original Macintosh, Apple would have had a much cleaner upgrade path for all the Apple ][s that were out in the world (and particularly in education), and the Apple III might have been less of a failure and more of a bridge.

What is rather hilarious (in a poetic justice / turning wheel kind of way) is that the original ARM design was in many ways inspired by the 6502, as that was the processor the design team was most familiar with and it had several features they thought worth keeping even as they took a RISC approach.
With RISC I don't think there's a clear technical winner. MIPS would've probably had a better shot if they hadn't gotten sidetracked by being owned by SGI for a few years; they're currently playing second fiddle to ARM in the consumer embedded space. Alpha was a promising architecture, but Compaq effectively killed it when they sold it to Intel (some of the related tech lived on elsewhere though, e.g. the design of the original Athlon system bus). PowerPC is more of a niche product now that Apple doesn't use it any more, but it is still used in IBM servers and in high reliability military/aerospace applications. SPARC is still used by Oracle and some supercomputer vendors, but was somewhat marginalized by Sun's decline and the rise of cheap x86 servers (SPARC is interesting in that there are Open Source implementations of the ISA).
Well, in its day Alpha was widely regarded as the clear technical winner, as it pioneered a lot of innovations that were later used by x86. And, like MIPS, there was a Windows implementation for it (and DEC even had a fancy JIT cross-compiler that enabled x86 binaries to run fairly well on it). But as you say, it was a victim of larger forces and bad management/vision -- which is pretty much part of the eulogy for any good but commercially-doomed processor design you can think of. And, speaking of DEC, they had their own ARM implementation too, which ended up going to Intel in a lawsuit. Intel called it XScale and was using it to replace their own ix60 RISC designs, before they decided to get out of that business entirely and passed it on to Marvell.
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Re: Different CPU ISAs Discussion

Postposted on Sat Nov 03, 2012 6:45 am

JBI wrote:

PowerPC is more of a niche product now that Apple doesn't use it any more, but it is still used in IBM servers and in high reliability military/aerospace applications.


Well, not to mention all three of today's major game consoles (PS3, X360 and Wii) use the PowerPC architecture. I think the majority of the computer boxes used in cars also use PowerPC, as well as, as you've mentioned, aerospace, which includes jet fighters and such.

SPARC is still used by Oracle and some supercomputer vendors, but was somewhat marginalized by Sun's decline and the rise of cheap x86 servers (SPARC is interesting in that there are Open Source implementations of the ISA).


In fact, SPARC is the ISA of choice for Japan's K computer, today's 2nd fastest supercomputer. Not bad, but Fujitsu (and others) need to keep moving this ISA forward.

MIPS has also been quite popular in the video game console market during the past few generations. I think the Playstation 1 and 2 as well as the Nintendo 64 all use MIPS.

BJM wrote:

Had Don Estridge and his team chosen, say.. Motorola's CPU at the time, I suspect that the 68k ISA would have become the x86 of today


Very good point, bjm. And Intel wouldn't be the giant it is today. Imagine if Motorola is what Intel is today.

Personally, if the world is going to do everything again I would've chosen SPARC. The spec is open and, as I understand it, is free for anyone to use. That would at least make sure that the industry can continue without fear of one company trying to rake it all in for itself. But I wouldn't have liked it to turn out like ARM or MIPS, where the innovator company would design the cores for licensing to other companies. Instead, I would have liked the SPARC standards body to just put up the spec out there, with Microsoft supporting the OS perhaps, and companies can just create their own core designs as well as system architectures (such as something like Hypertransport) to support the dominant OSes.
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