GlobalFoundries to Acquire MIPS

Whoa, had no idea this existed. Wild stuff. Might be "somewhat" confusing to read assembler code like that without knowing about this particular technique..

Allow me to introduce you to register windows.

https://www.jwhitham.org/2016/02/risc-instruction-sets-i-hav...

Both register windows and the delay slot exist on SPARC processors, which you’re much more likely to run into in a data center (running open-source software).

Itanium was the really odd one — it not only used register windows but could offload some of the prior windows onto the heap. Most people would probably never notice… unless you’re trying to get a conservative scanning GC working and are stumped why values in some registers seem to not be traced…

I was going to make a reference to Patterson & Hennessy, but it's too bad that the 5th and later editions are hidden behind a DRM paywall. You don't "own" books anymore.

Pour one out for Itanium. It tried to make the panacea of VLIW and branch hints work, but it didn't pan out.

From an interview with Bob Colwell:

'Anyway this chip architect guy is standing up in front of this group promising the moon and stars. And I finally put my hand up and said I just could not see how you're proposing to get to those kind of performance levels. And he said well we've got a simulation, and I thought Ah, ok. That shut me up for a little bit, but then something occurred to me and I interrupted him again. I said, wait I am sorry to derail this meeting. But how would you use a simulator if you don't have a compiler? He said, well that's true we don't have a compiler yet, so I hand assembled my simulations. I asked "How did you do thousands of line of code that way?" He said “No, I did 30 lines of code”. Flabbergasted, I said, "You're predicting the entire future of this architecture on 30 lines of hand generated code?" [chuckle], I said it just like that, I did not mean to be insulting but I was just thunderstruck. Andy Grove piped up and said "we are not here right now to reconsider the future of this effort, so let’s move on".'

https://www.sigmicro.org/media/oralhistories/colwell.pdf

VLIW is maybe cool, but people will be relieving themselves on EPIC's grave for the pain that it inflicted on them.

Like if you tried to debug a software crash on Itanium. The customer provided core dump was useless as you could not see what was going on. Intel added a debug mode to their compilers which disabled all that EPIC so hopefully you could reproduce the crash there, or on other CPU architectures. Otherwise you were basically screwed.

EPIC :nauseous face emoji:

That HP-Intel arrangement was weird. One time, an Intel-badged employee came out to change a tape drive on a (Compaq->HP->HPE) Compaq SSL2020 tape robot. Okay, I guess they shared employees. ¯\_(ツ)_/¯

Sun had some funny stories around this too. When they came up with their multi-core system, and they used code from 10-15 years earlier for traces. And then said 'well, nobody actually uses floating code' so we don't need it. Of course over those 10 years Floating point became much more common and stand. Leading to a chip that had one FPU for 8 cores, basically meaning, even minimal floating point would destroy concurrency. Arguably Sun had already lose the chip war and this was just making them fall behind further. They did market it in quite well.

And a lesser known thing that I couldn't find much information on is that Sun also worked on VLIW chip during the 90s. Apparently Bill Joy was convinced that VLIW was the future so they did a VLIW chip, and the project was lead by David Ditzel. As far as I am aware this was never released. If any Sun veterans have any idea about this, I would love to know.

As far as the single FPU that you mention, the T1 is an open-source CPU.

https://www.oracle.com/servers/technologies/opensparc-t1-pag...

The T2 is also open, and places an FPU in each core.

https://www.oracle.com/servers/technologies/opensparc-t2-pag...

When there is such complaint about closed firmware in the Raspberry Pi, and the risk of the Intel ME and other closed CPU features, I wonder why these open designs are ignored. Yes, the performance and power consumption would be poor by modern standards.

These designed are not ignored. They were used for a few things here and there. But the usefulness of 'over the wall' open code without backing is always a bit limited and for processors that cost 100k to tap out, even more so.

By now there are much better more modern design out-there and for RISC-V.

Many assemblers had an option to reorder on assembly so you could write it normally, while only taking care to avoid hazards near branches.

At least one toolchain would just pad the slots with nops