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How the Atari ST almost had Real Unix

posted Jan 27, 2011, 2:44 PM by Troy Cheek   [ updated Jan 27, 2011, 2:49 PM ]
The Dadhacker talks about how the Atari ST almost had Unix as an operating system:

The latest problem I was working out was how to run Unix on the Atari ST. The Tramiels had somehow wrangled a license for AT&T’s SVR-something-or-other version of Unix (might have been SVR3, but this was in the bad old days when AT&T was actively fucking up Unix, and it could have been just about any version, including SVR666). The license was for a mind boggling, nay, jaw-dropping ten bucks a seat. The problem was that the ST didn’t have any kind of memory management hardware, just a raw CPU flinging real addresses at naked DRAM, and the machine’s cheap-ass vanilla 68000 was incapable of recovering from a fault unless you cheated.

[What's that about Linux? Dear child, Linus was probably not out of whatever they use for high school in Finland. All we had in the market was 4.2bsd running on Vaxen, 68K-based Suns with a ton of hardware to work around the 68000 limitation re faulting, and a whole running field of wannabes that would sink without a trace in five years. Oh, and some screwed up AT&T workstations with monochrome graphics and UIs that curdled your eyeballs and left you wishing AT&T had simply stuck with making phones.]

The hardware folks were convinced that grafting an MMU into the ST was impossible; in theory you could still run something like Unix, but with no memory protection and no way to easily grow and shrink a process’s address space a straight-forward port of Unix would be glacial and prone to crashing really badly. The hardware guys were mostly right; the 68000 wasn’t capable of handling a page fault (it didn’t save enough information on its exception frames to restart all cases of a faulted instruction). Motorola didn’t offer an MMU chip anyway (the 68020 didn’t exist yet, and the sticker shock of its optional external MMU meant that only Apple folks could afford it, and it was still optional on most Macs for several years). Furthermore, the memory system of the ST wouldn’t tolerate the delays that a traditional MMU would incur; the ST’s DRAMs were being thrashed ten or fifteeen nanoseconds under spec (“You have to understand,” said our hardware guys, “DRAMs are really analog devices,” and I’m sure a DRAM designer somewhere felt cold and shivery all of a sudden, and didn’t know why).

To run Unix effectively we needed some hardware that was very fast, that was simple enough to put into a minor spin of the ST’s memory controller with little project risk, and that would still provide some kind of memory relocation and protection. The ability to have separate address spaces to isolate processes would be good, too.

“If you can come up with something that takes about a gate delay, I’ll put it in,” said John, the memory controller guy. He seemed dubious, but willing to listen...
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