7 ms·

Restoring Y Combinator's Xerox Alto, day 4: What's running on the system

151 points by dwaxe 10y ago

sunnytherana 10y ago
Liner process with like x ray
ChuckMcM 10y ago
Wow, that is a remarkable amount of stuff that is working. My experience with wirewrapped boards is pretty mixed, some are solid and some are just flaky as heck. Really curious what they find out about the disk, nothing at all coming over the interface sounds like a big (and usually simple) issue, like card in the wrong slot or cable plugged in backwards or to the wrong connector.
- kens 10y ago
  I'm hoping for a problem that's easy to fix, but not something stupid :-) To clarify the disk issue, we're seeing sector pulses coming from the disk drive, so the drive and cabling is working. The Alto isn't sending any read requests to the drive, so it seems most likely that the microcode isn't seeing any disk request block in memory. A lot of things need to work correctly to get the request block into RAM. So it could be a problem with a chip on the ALU board, a bad memory chip, something wrong on the disk interface card, a corrupted bit somewhere, or anything. With the logic analyzer, we should be able to see at what point in the microcode things go wrong. The nice thing about the Alto is because it's all TTL chips, it's straightforward to see what's happening.
  - Lord_Nightmare 10y ago
    I noticed that 3101 SRAMs have the same pinout as the later 7400-series 7489 16x4 SRAMs; I'm not 100% sure that they are fully electrically/timing compatible, but this could end up being a way of testing using known good parts...
MrBuddyCasino 10y ago
> If backspace is pressed on the keyboard, the Alto does a Ethernet boot This sounds impressive. Was it common for machines of that era to be able to boot from the network? Also, aren't those some awfully long wires to make high-frequency measurements with an oscilloscope? Some of that ringing and overshoot in the clock signals might not actually be in the circuit that is being probed, but caused by the long leads.
- kens 10y ago
  Ethernet was invented for the Alto, so it was the first machine to boot over Ethernet. (I don't know if any other machines did network boot earlier, using other networks.) One entertaining thing about Alto boot: you could select a different boot address (i.e. boot something different) by pressing various keys at boot time. Each key controlled a different bit, so you would press many keys at once. This was called "nose boot" since you might need to use your nose to press a key if you ran out of fingers. The Alto clock is 5.88 MHz, so the signals aren't as high frequency as modern computers. We could probably get crisper signals with better probing, but since we just want to see the signals, the quality isn't too important.
  - MrBuddyCasino 10y ago
    So ctrl+alt+del did have some precedent... Good luck with the restoration effort, tracing the instructions using a logic analyzer seems like a daunting prospect, especially with such an arcane instruction set. Thanks for the detailed articles, it is much appreciated!
  - joezydeco 10y ago
    Man, doing that digital work with a scope can be painful at times. I'd really recommend a Saleae Logic here to scope out your digital lines. You probably don't need the high-level protocol decoding at this point, but for capturing longer blocks of data with a great interface you can't beat this tool. https://www.saleae.com/ https://www.saleae.com/
    - kens 10y ago
Slippery_John 10y ago
For those interested in this computer, there is a working Alto at the Living Computer Museum in Seattle that attendees are free to use.
- kogir 10y ago
  I got to play with this a few weeks ago and it was really cool. Some friends and I created and formatted a document. The whole museum is great and I highly recommend it to anyone who finds themselves in Seattle. There's something intensely satisfying about writing, compiling, and running Hello World on a teletype (paper roll!) hooked up to a PDP-7.
- tomcam 10y ago
  Holy cow! Thanks for this tip.
kixpanganiban 10y ago
This is one of my favorite ongoing stories in HN. It's like a peek into a team building a time machine, and for all intents and purposes, this IS a time machine -- one that opens a window to the past.
meatsock 10y ago
this is an excellent project that's great fun to watch the progress of. keep up the great work. i have no experience with altos but i have been wondering -- on day one of the restore, it was noted the alto in question had some modifications from stock issue[1], could the booting issues be related to the microcode or wiring changes necessary to support those modifications? is it trying to boot off of the now missing trident drive? [1] from http://www.righto.com/2016/06/restoring-y-combinators-xerox-alto-day.html http://www.righto.com/2016/06/restoring-y-combinators-xerox-... "I suspect that the Y Combinator Alto originally had both a Trident drive and a Diablo drive (as well as four Orbit boards to drive a laser printer)"
- kens 10y ago
  That's an interesting question. From reading the documentation [1], the Trident disk should co-exist with the standard disk. It uses different microcode tasks (3 and 15), requiring additional microcode. No wiring change is mentioned (or discussion of booting off the Trident disk). It seems like the Trident drive shouldn't be causing us any problems, but it's possible there's a wiring change or different PROMs. I guess we'll find out... [1] http://bitsavers.org/pdf/xerox/alto/ifs_trident/TriconDoc.pdf http://bitsavers.org/pdf/xerox/alto/ifs_trident/TriconDoc.pd...

151 points by dwaxe 10y ago

Liner process with like x ray

Wow, that is a remarkable amount of stuff that is working. My experience with wirewrapped boards is pretty mixed, some are solid and some are just flaky as heck. Really curious what they find out about the disk, nothing at all coming over the interface sounds like a big (and usually simple) issue, like card in the wrong slot or cable plugged in backwards or to the wrong connector.

kens 10y ago
I'm hoping for a problem that's easy to fix, but not something stupid :-) To clarify the disk issue, we're seeing sector pulses coming from the disk drive, so the drive and cabling is working. The Alto isn't sending any read requests to the drive, so it seems most likely that the microcode isn't seeing any disk request block in memory. A lot of things need to work correctly to get the request block into RAM. So it could be a problem with a chip on the ALU board, a bad memory chip, something wrong on the disk interface card, a corrupted bit somewhere, or anything. With the logic analyzer, we should be able to see at what point in the microcode things go wrong. The nice thing about the Alto is because it's all TTL chips, it's straightforward to see what's happening.
- Lord_Nightmare 10y ago
  I noticed that 3101 SRAMs have the same pinout as the later 7400-series 7489 16x4 SRAMs; I'm not 100% sure that they are fully electrically/timing compatible, but this could end up being a way of testing using known good parts...

MrBuddyCasino 10y ago

> If backspace is pressed on the keyboard, the Alto does a Ethernet boot This sounds impressive. Was it common for machines of that era to be able to boot from the network? Also, aren't those some awfully long wires to make high-frequency measurements with an oscilloscope? Some of that ringing and overshoot in the clock signals might not actually be in the circuit that is being probed, but caused by the long leads.

kens 10y ago
Ethernet was invented for the Alto, so it was the first machine to boot over Ethernet. (I don't know if any other machines did network boot earlier, using other networks.) One entertaining thing about Alto boot: you could select a different boot address (i.e. boot something different) by pressing various keys at boot time. Each key controlled a different bit, so you would press many keys at once. This was called "nose boot" since you might need to use your nose to press a key if you ran out of fingers. The Alto clock is 5.88 MHz, so the signals aren't as high frequency as modern computers. We could probably get crisper signals with better probing, but since we just want to see the signals, the quality isn't too important.
- MrBuddyCasino 10y ago
  So ctrl+alt+del did have some precedent... Good luck with the restoration effort, tracing the instructions using a logic analyzer seems like a daunting prospect, especially with such an arcane instruction set. Thanks for the detailed articles, it is much appreciated!
- joezydeco 10y ago
  Man, doing that digital work with a scope can be painful at times. I'd really recommend a Saleae Logic here to scope out your digital lines. You probably don't need the high-level protocol decoding at this point, but for capturing longer blocks of data with a great interface you can't beat this tool. https://www.saleae.com/ https://www.saleae.com/
  - kens 10y ago

Slippery_John 10y ago

For those interested in this computer, there is a working Alto at the Living Computer Museum in Seattle that attendees are free to use.

kogir 10y ago
I got to play with this a few weeks ago and it was really cool. Some friends and I created and formatted a document. The whole museum is great and I highly recommend it to anyone who finds themselves in Seattle. There's something intensely satisfying about writing, compiling, and running Hello World on a teletype (paper roll!) hooked up to a PDP-7.
tomcam 10y ago
Holy cow! Thanks for this tip.

kixpanganiban 10y ago

This is one of my favorite ongoing stories in HN. It's like a peek into a team building a time machine, and for all intents and purposes, this IS a time machine -- one that opens a window to the past.

meatsock 10y ago

this is an excellent project that's great fun to watch the progress of. keep up the great work. i have no experience with altos but i have been wondering -- on day one of the restore, it was noted the alto in question had some modifications from stock issue[1], could the booting issues be related to the microcode or wiring changes necessary to support those modifications? is it trying to boot off of the now missing trident drive? [1] from http://www.righto.com/2016/06/restoring-y-combinators-xerox-alto-day.html http://www.righto.com/2016/06/restoring-y-combinators-xerox-... "I suspect that the Y Combinator Alto originally had both a Trident drive and a Diablo drive (as well as four Orbit boards to drive a laser printer)"

kens 10y ago
That's an interesting question. From reading the documentation [1], the Trident disk should co-exist with the standard disk. It uses different microcode tasks (3 and 15), requiring additional microcode. No wiring change is mentioned (or discussion of booting off the Trident disk). It seems like the Trident drive shouldn't be causing us any problems, but it's possible there's a wiring change or different PROMs. I guess we'll find out... [1] http://bitsavers.org/pdf/xerox/alto/ifs_trident/TriconDoc.pdf http://bitsavers.org/pdf/xerox/alto/ifs_trident/TriconDoc.pd...