Restoring YC's Xerox Alto day 9: tracing a crash through software and hardware

Aww fun fun. The offspring of SWAT was the the debugger in the D-systems. They had a 3 digit LED display on the front of the machine and when you crashed that display would show 918 (if I recall correctly). At which point you could connect over ethernet to the machine and be inside the debugger which felt a lot like your typical JTAG station does today. The D-machines also shared the fungible microcode and it was always very important to have the right version of the microcode for the version of the MESA compiler or you ended up with really crazy bugs.<p>That said, you could iteratively design both the language code generator and the microcode to optimize for speed or code density or low interrupt latency, what ever. As a young engineer at Intel (my wife was an engineer at Xerox) I felt like that sort of architecture was 20 years ahead of its time, and eventually the nearly all hard coded instructions of microprocessors would be considered quaint. Watching these concepts come in and go out of favor several times, reading about the Alto feels like looking at the embryo of the future.

Now struggling to resist the urge to upgrade my sad old analog oscilloscope. Thanks, guys!<p>Seriously, though, this is by far the most interesting series of articles I&#x27;ve read in at least 10 years. Digging in and understanding equipment from decades ago, where <i>everything</i> is potentially questionable is a heroic effort. It&#x27;s like all the good parts of Radio Electronics &amp; Byte magazine combined. A mystery who-done-it for nerds.

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I find it hard enough to debug something when it is at the software level, and here we are looking at individual on&#x2F;off signals between chips.

Is the Alto simple enough to simulate in toto on an FPGA, microcode and all?<p>It sounds like it would be a fascinating system to play with.

Old hardware is so simple it is possible to trace, understand and fix.<p>What era computer will we not be able do this anymore ? For example if you have a multilayer PCB with damages it is very hard to fix. Also components become so small it is hard to physically exchange them. Will all newer than round 1987 hardware become nonworking museum pieces in a few years ?

God, I miss being able to put clips directly on a chip.

I need to remind myself to not read these for a while. I find myself disappointed when the post ends in a cliffhanger. I want to just binge read these after its all said and done. Really great reads!

<i>Thus, the cause of our crashes was these two wires that someone had added to the board! Could we simply cut these wires and have the system work correctly? We figured we should understand why the wires were there, rather than randomly ripping them out. Maybe our control board and CRAM board were incompatible? Maybe these wires were to support the Trident disk drive we aren&#x27;t using? It was the end of the day by this point, so further investigation will wait until next time.</i><p>Cool story that ultimately doesn&#x27;t go anywhere, bummer.