The Worst CPUs Ever Made (2021)

The ISA wasn't that bad, but the weak memory-ordering model was a huge pain in the ass. I worked for a while with some of the Alpha folks years later, and they did a lot of really great work, but they did bring that weak memory model along with them. It allowed us to find many Linux kernel bugs that had lain dormant since Alpha because nothing since had repeated the mistake. Fun times ... not.

It didn't die.

Intel bought it from HP, stripped it for parts, then killed it.

HyperTransport and a few other things were essentially just copies of Alpha's stuff cleanroom implemented by ex-Alpha employees. Designs like Sandy Bridge look quite similar to EV8. QuickPath is just Alpha's interconnect with some updates (HyperTransport was also a cleanroom copy from ex-Alpha employees). Even AVX seems inspired by the 1048-bit SIMD planned for EV9/10.

I've owned 4 or 5 of the CPUs on that list over the years. I'm sure there are worse.

Intel was too expensive for me, so I ended up buying a Cyrix => performance (floating point) was terrible in Falcon 3, I was sooo sad - but on the other hand that gave me until today the push to really focus on details before taking a decision => thank you Cyrix for having changed my life hehe.

I'm currently building a homebrew system built on the TMS99105A CPU, one of the final descendants of the TMS9900.

It's a nifty little CPU. There's a lot of hidden little features once you dig in. It can actually address multiple separate 64k memory namespaces: data memory, instruction memory, macroinstruction memory, and mapped memory with the assistance of a then-standard chip. Normally these are all the same space and just need external logic to differentiate them. There's also a completely separate serial and parallel hardware interface bus.

The macroinstruction ("Macrostore") feature is pretty fun. There's sets of opcodes that will decode into illegal instructions that, instead of immediately erroring out, will go looking for a PC and workspace pointer (the "registers") in memory and jump there. Their commercial systems like the 990/12 used this feature to add floating point and other features like stack operations.

Yup, there's no stack. Just the 16 "registers," which live in main memory. There are specific branch and return instructions that store the previous PC and register pointer in the top registers of the new "workspace," allowing you direct access to the context of the caller. The assembly language is simple and straightforward with few surprises, but it's also clearly an abstraction over the underlying mechanisms of the CPU. I believe this then classifies this CPU as CISC incarnate.

There are some brilliant and insane people on the Atari Age forums! One of them managed to extract and post the data for a subset of those floating point instructions, and then broke it all down and how it all worked. Some are building new generations of previous TMS9900 systems. One of them is replicating the CPU in an FPGA. A few others are building things like a full-featured text editor and, of course, an operating system.

I've learned a hell of a lot during this project. I've been documenting what I'm doing and am planning to eventually make it into a pretty build log. I think this is a beautiful dead platform that deserved better.