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What I find most interesting is the "social history" of RISC vs CISC: how did a computer architecture issue from the 1980s turn into something that people vigor

by kens 3y ago

What I find most interesting is the "social history" of RISC vs CISC: how did a computer architecture issue from the 1980s turn into something that people vigorously debate 40 years later? I have several theories: 1. x86 refused to die as expected, so the RISC vs CISC debate doesn't have a clear winner. There are reasonable arguments that RISC won, CISC won, or it no longer matters. 2. RISC vs CISC has clear teams: now Apple vs Intel, previously DEC, Sun, etc vs Intel. So you can tie the debate into your "personal identity" more than most topics. The debate also has an underdog vs entrenched monopoly vibe that makes it more interesting. 3 RISC vs CISC is a simple enough topic for everyone to have an opinion (unlike, say, caching policies). But on the other hand, it's vague enough that nobody can agree on anything. 4. RISC exists on three levels: First, a design philosophy / ideology. Second, a style of instruction set architecture that results from this philosophy. Finally, a hardware implementation style (deep pipelines, etc) that results. With three levels for discussion, there's lots of room for debate. 5. RISC vs CISC has a large real-world impact, not just for computer architects but for developers and users. E.g. Apple switching to ARM affects the user but changing the internal bus architecture does not. (I've been trying to make a blog post on this subject, but it keeps spiraling off in random directions.)

phire 3y ago
> 4. RISC exists on three levels... Finally, a hardware implementation style (deep pipelines, etc) that results. I agree with the philosophy and ISA, but I don't think RISC is actually counts as a hardware architecture. Yes, there is a certain style of architecture strongly associated with RISC, the "classic RISC pipeline" that a lot of early RISC implementations share. But RISC can't claim ownership over the concept of a pipelined CPUs in general and designers following the RISC philosophy almost immediately branched out into other hardware architectures directions like superscalar and out-of-order execution (some also branched into VLIW). Today, the "class RISC pipeline" is almost entirely abandoned outside of very low-power and low-gate count embedded cores. The primary advantage of the RISC philosophy was that it allowed them to experiment with new hardware architecture ideas several years early than those competitors who were stuck supporting legacy CISC instruction sets. Especially when they could just dump their previous ISA and create a new one hyper-specialised for that exact hardware architecture. Those CISC designers also followed the same path in the 80s and 90s, implementing pipelined architectures, and then superscalar and then out-of-order, but their designs always had to dedicate more gates to adapting their legacy ISAs to an appropriate internal representation. ---- But eventually silicon processes got dense enough for this inherent advantage of the RISC philosophy to fade away. The overhead of supporting those legacy CISC ISAs got smaller and smaller. All high-performance CPUs these days seem to have settled on a common hardware architecture, doesn't matter if they use CISC or RISC ISAs, the diagrams all seem to look more or less the same. This architecture doesn't really have a name (which might be part of the reason why everyone is stuck arguing RISC vs CISC), but it's the out-of-order beast with absolutely massive reorder buffers, wide-decoders, physical register files, long-pipelines, good branch predictors, lots of execution units and (often) an uOP cache. Intel's Sandybridge is the first example of this exact hardware architecture (though that design linage starts all the way back with the Pentium pro, and you also have AMD examples that get close), but Apple quickly follows up with Cyclone and then AMD with Zen. Finally ARM starts rapidly catching up from about the Cortex A76 onwards.
wk_end 3y ago
I think it’s a legacy of the enormous marketing push around RISC. Marketers at Apple, IBM, SGI, ARM, Sun, even Nintendo drilled into the collective computer industry’s head that the distinction was extremely important, and we’ve never shaken it, even as the distinction’s importance has faded. (And to be fair to those marketers, for a time the microarchitectural improvements that the RISC movement wrought really were enormous.)

by kens 3y ago

phire 3y ago

> 4. RISC exists on three levels... Finally, a hardware implementation style (deep pipelines, etc) that results. I agree with the philosophy and ISA, but I don't think RISC is actually counts as a hardware architecture. Yes, there is a certain style of architecture strongly associated with RISC, the "classic RISC pipeline" that a lot of early RISC implementations share. But RISC can't claim ownership over the concept of a pipelined CPUs in general and designers following the RISC philosophy almost immediately branched out into other hardware architectures directions like superscalar and out-of-order execution (some also branched into VLIW). Today, the "class RISC pipeline" is almost entirely abandoned outside of very low-power and low-gate count embedded cores. The primary advantage of the RISC philosophy was that it allowed them to experiment with new hardware architecture ideas several years early than those competitors who were stuck supporting legacy CISC instruction sets. Especially when they could just dump their previous ISA and create a new one hyper-specialised for that exact hardware architecture. Those CISC designers also followed the same path in the 80s and 90s, implementing pipelined architectures, and then superscalar and then out-of-order, but their designs always had to dedicate more gates to adapting their legacy ISAs to an appropriate internal representation. ---- But eventually silicon processes got dense enough for this inherent advantage of the RISC philosophy to fade away. The overhead of supporting those legacy CISC ISAs got smaller and smaller. All high-performance CPUs these days seem to have settled on a common hardware architecture, doesn't matter if they use CISC or RISC ISAs, the diagrams all seem to look more or less the same. This architecture doesn't really have a name (which might be part of the reason why everyone is stuck arguing RISC vs CISC), but it's the out-of-order beast with absolutely massive reorder buffers, wide-decoders, physical register files, long-pipelines, good branch predictors, lots of execution units and (often) an uOP cache. Intel's Sandybridge is the first example of this exact hardware architecture (though that design linage starts all the way back with the Pentium pro, and you also have AMD examples that get close), but Apple quickly follows up with Cyclone and then AMD with Zen. Finally ARM starts rapidly catching up from about the Cortex A76 onwards.

wk_end 3y ago

I think it’s a legacy of the enormous marketing push around RISC. Marketers at Apple, IBM, SGI, ARM, Sun, even Nintendo drilled into the collective computer industry’s head that the distinction was extremely important, and we’ve never shaken it, even as the distinction’s importance has faded. (And to be fair to those marketers, for a time the microarchitectural improvements that the RISC movement wrought really were enormous.)