Running a 68060 CPU in Quadra 650
github.com74 points by zdw 3 days ago
74 points by zdw 3 days ago
I appreciate how in the YouTube video on the repo's README, his cat walks right on top of his project, and he doesn't even react at all. The cat just gets out of the way all by itself. But I guess it _had_ to be part of the clip, heh.
Related message board thread: https://68kmla.org/bb/index.php?threads/macintosh-68060-redu...
Was the 040/060 always supposed to be the end of the line, or might we have seen more advanced "68k" chips if sales had continued?
If there was demand for more high performance 68k processors, presumably Motorola would have made more. But by the time of the 68060 customers for high end 68k had either already moved onto various RISC architectures (Mac, UNIX workstations) or were dead / dying (ST, Amiga).
A related question would be if Motorola could pull off what Intel did with x86, and drag the architecture to be competitive with RISC. Part of Intel's trick was that x86 isn't the most CISCy of chips, so it was easy to implement a processor that takes a "RISCy subset" of instructions and translate them into one or two µops. 68k is perhaps more on the VAX side of things, so whether they'd be able to pull off the same trick is a bit unclear. Clearly we need to check the parallel universes where IBM chose the 68008 for Chess instead of the 8088...
I checked one of the adjacent ones, and there Motorola used traps and microcode internally for emulation of some of the CISCier instructions and a fast RISCy core for the rest.
Everyone was encouraged to move on to the fast subset and the new multimedia instructions.
The problem was mainly in x86, existing code already mostly used instructions that belonged to the fast subset, and on 68k, it didn't. So while you could evolve the instruction set towards a saner design (and the 68060 was well on the way towards that), only new code would benefit from it.
And of course that's exactly what they did with Coldfire - rounding off the inconvenient corners of the ISA to produce CPUs with lower power requirements and able to run at higher clock speeds.
They did it with the 68030 before Coldfire. They discarded a number of things (e.g. addressing modes) that seemed like good ideas for the <=68020 but didn't end up being used in practice.
on the m68k, the "cisc-y-ness" is in the many many addressing modes, whereas x86 in that particular aspect of the architecture has always been rather "risc-y" (read: rather limited compared to other CISC architectures, including m68k).
The core instruction set of the m68k, as far as ALU/FPU is concerned, is simple enough. But converting the addressing modes to "risc building blocks" (μops or whatever term you like to use) is harder.
> only new code would benefit from it.
Not only new code. Old code in an high-level language would benefit, too, if the language compiler was updated and the code recompiled.
Not everyone has that sort of luxury of access to source etc., people want their existing binaries to run faster.
Also, if you need to recompile to get a performance boost, why not recompile for a cleaner modern architecture? You can always use an emulator for legacy code, if it isn't going to run fast on a modern CPU either way...
Or sometimes, binary pached.
This. Don't underestimate the amount of M68K code written in assembly language.
Motorola wanted to replace the 68000 series with their RISC design, the 88000, so it was probably always going to end there. Their customers were also all in full RISC hype fever and were looking for RISC chips to move to.
In Apple's search for a RISC chip, didn't want to be stuck being single-sourced again so they had IBM and Motorola work together to launch PowerPC instead, so Motorola gave up on the 88k.
I have seen mentioned a couple of times that the 88K would have had some inherent design flaw that made the architecture a dead end. I have never found an explanation what that would have been though.
Perhaps this had just been a misunderstanding of a second-hand statement. Its first iteration was flawed in that it was an expensive multi-chip solution with few buyers. The second was more integrated though.
We did, sort of: https://en.wikipedia.org/wiki/NXP_ColdFire
Coldfire V4/V5 are significantly faster than 060.
Can someone with a bit more chip-level knowledge explain to me why nobody has put a PiStorm into a classic Mac yet?
https://github.com/captain-amygdala/pistorm
If there are problems with the ISA or something, they're much much easier to tweak here. It already works in multiple models of Amiga, including both 68000 and 68020, and there's at least one effort to get one working in an Atari ST...
someone did https://tinkerdifferent.com/threads/pistorm-on-the-macintosh...
personally I think it's a bit too close to just emulating the whole thing
I didn't know about that! :-o Thanks!
> personally I think it's a bit too close to just emulating the whole thing
Yes, I do know what you mean.
Although it's functionally more limited, there is also Emu68, which removes the native PiOS from the setup completely.
inb4 the fastest 68k-based Mac is an Apollo Vampire running a Macintosh emulator
Vampire is not really a 68k, it's an FPGA based system. A kind of fantasy could-have-been system.
I've always wondered if someone could build a new FPGA based 68k mac, I know this isnt that, but it brings the thought to mind.
Isn't the 68060 backward compatible? Why wouldn't it run the same binaries compiled for other 68k CPUs?
68060 has a reduced integer and floating point instruction set ( Motorola cut down some of the lesser used instructions in order to keep the die size under control. However, to remedy this they made available an ISP (Integer Support package) and FPSP (Floating point support package))
Bus error stack frames have been changed significantly
(per the bulletin board discussion linked below)
https://www.cpu-world.com/CPUs/68060/index.html:
“Supervisor mode of the Motorola 68060 CPU differs from the 68040 due to changes in exception processing. User mode of the Motorola 68060 is object-compatible with MC68040, assuming that the CPU uses special software to simulate a few instructions that were present in 68040 CPU and are missing in MC68060.”
Big picture: Motorola's 68K architecture was a case study in Death by Feature Creep.
Short-term: The 68K was loved by assembly language programmers - a big thing, in the 80's. And it felt and sounded so cool for Motorola to add even more great features to each generation of the architecture.
Long-term: If you want your microprocessor architecture to stay a thing in higher-performance desktop/server use (vs. toaster ovens & thermostats & such) - then you need to have a few Senior Implementation Engineers looking a decade or so ahead, and saying "NO" to cool features that could turn into implementation hell.
The 68060 was where the long-term issues really caught up with the 68K architecture, and it hit the brick wall / grave stone. Which is why Motorola pretty much dumped their 68K for IBM's Power architecture, in the PowerPC.
Motorola added too many complex addressing modes in the '020
Edit: This was wrong ", that they dropped already with the next CPU, the '030."
They dropped them only much later, in Coldfire, not in 68030, which was completely compatible with 68020. MC68040 dropped some features, then MC68060 dropped more features, but for user programs they remained binary compatible with MC68020.
They have probably realized quickly that adding memory indirect addressing modes was a mistake, but they could not drop them because that would have broken all programs ported to MC68020.
With Coldfire, they were able to prune the unwanted addressing modes, because Coldfire was intended for embedded computers, where the software is recompiled for every new project and binary compatibility with legacy programs is not necessary.
Both the addressing modes of Intel 80386 (1985) and of Motorola MC68020 (1984) were inspired by the addressing modes of DEC VAX, but in a very rare event in the history of Intel processors the Intel designers have done the right thing by choosing the useful subset of the DEC VAX addressing modes, while the Motorola designers have made a mistake by choosing to implement an even more complex set of addressing modes than that of DEC VAX.
I double-checked, and you're right. I stand corrected.
Weird, I had believed for a long time that the MC68030 had dropped the memory-indirect address modes.
The 68030 really only dropped the Call Module and Return from Module instructions, which nobody used anyway since relatively few developers wanted to write code only for 68020 and higher around the time the 68020 and then 68030 shipped.
So nothing like x86 then?
x86 is not nearly as bad as 68k, more by lucky accident than design. As it was a stop-gap project designed to hastily extend an earlier design to 16 bit and 20-bit addressing while the proper cpu of the future (iAPX432) needed a few extra months to bake, the designers didn't have enough time to properly fubar the core of the instruction set. This made x86 assembly much less nice to write than the nearly fully orthogonal 68k, but also made it much easier to make the subset of it that was in actual use faster later.
68k designers were not being dumb when they designed it. At that time pretty much the entire industry was deep in the weeds of "closing the semantic gap", or making CPUs directly run the operations that would be encoded in high-level languages. All CPUs designed to this paradigm were doomed, and how doomed they ended up being depended mainly on how well they managed to implement it.
IBM's 801 and Patterson's RISC would blow it all up in the early 80's.
A shame, really. 68k was (and is) much more approachable for those learning assembly. No need to deal with 64k segmented memory, for instance.
As an aside… National Semiconductor also had an ill-fated architecture in the NS32000, which I also wish took off. On paper, it really did a lot right (VAX-like design, flat memory model, 32-bit almost immediately out of the gate) yet NS was woefully incapable of producing masks without bugs. It took them many tries to get it right, and before then, they already were being beat to market by their competition.
Then to add insult to injury, NS’ own compiler for NS32000 chips was producing rather unoptimized code. It took GNU porting GCC to the platform in 1987 for them to fully realize their potential, years after they missed their chance.
If NS did have their act together… dare I say an IBM PC built around their CPU would have been possible and more interesting than the 8088 they ultimately went with.
AFAIK, NS used the Green Hills compiler; at least, my ns32532 dev system comes with it. It's not great, but not terrible. I personally don't remember the compiler being in the top 5 show-stopper issues with the 32k (the first 3 were 'cpu bugs', 'mmu bugs' and 'fpu bugs'). And it was slow, particularly if you used the MMU.
The 32000 line (like the 68000) found a very long life as an embedded processor, particularly in the printer/fax space (ns32cg16 and followons, ns32gx32).
The 32332 was a nice processor. The 32532 was very, very nice. Both way too late.
Given what IBM was trying to deliver with the PC, I doubt they'd have looked at the 32000. Single source, few i/o support chips, relatively expensive, etc., etc. Way more likely that a non-Intel IBM PC would have had a Z8000 inside (and not a 68k, for mostly IBM political reasons).
Good points all around.
That said, I’d possibly contest you on the single source issue you brought up. IBM likely would have told NS… much like they told Intel back in the day… that if they wanted to do business with them, that they needed to ensure second sourcing was possible.
Judging by how desperate NS was willing to make deals, I’m quite sure that hurdle would have been overcome quite easily, with AMD or even MOS Technology stepping up to fill the void.
If we want to pick nits, NS had a second source: TI. But that was, afaik, just paperwork at that point (and I honestly don't know if TI ever produced a 32k processor). It takes time to bring a second source on-line. And given the trouble NS had building the chips themselves, if I was IBM, I'd have Questions.
That said...even if NS could wave a magic wand and produce a second source, there were plenty of other reasons to discount the 32k, and I've never seen the slightest evidence that IBM ever considered it.