
Restoring Y Combinator's Xerox Alto, day 4: What's running on the system - dwaxe
http://www.righto.com/2016/07/restoring-y-combinators-xerox-alto-day_31.html
======
MrBuddyCasino
> 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.

~~~
Animats
_" Was it common for machines of that era to be able to boot from the
network?"_

What network? This was the first machine to _have_ a local area network.

A full Alto network included workstations, a file server, a laser printer, and
a gateway to other Xerox networks using Parc Universal Protocol over 3Mb/s
Ethernet. They had a complete all-Xerox vision.

~~~
greenyoda
_" This was the first machine to have a local area network."_

There were other networking implementations being developed around the same
time as Ethernet (1973-74), for example:

\- DECnet (1974):
[https://en.wikipedia.org/wiki/DECnet](https://en.wikipedia.org/wiki/DECnet)

\- Token Ring networks (1974):
[https://en.wikipedia.org/wiki/Token_ring](https://en.wikipedia.org/wiki/Token_ring)

The earliest reference I could find to a LAN was the "Octopus" network from
1970, which predates Ethernet:

[https://en.wikipedia.org/wiki/Local_area_network](https://en.wikipedia.org/wiki/Local_area_network)

~~~
Animats
Octopus was what we would today call a "Storage Area Network". Around 1968,
IBM built the IBM 1360 Photo-Digital Storage System [1][2] for several of the
big atomic energy labs. This was 1.2GB of storage. On microfilm. The whole
thing was automatic, with hardware to write digital data on microfilm, develop
the film, store and retrieve individual filmstrips, and read them. The film
was directly written with an electron beam in vacuum. IBM electromechanical
technology at its height. The Computer History Museum in Mountain View has one
of the seven units built.

Since this was a big, but slow, storage device, it had to be front-ended by a
computer with disks, and made accessible to other computers. That's most of
what Octopus did. It wasn't peer to peer, like Ethernet.

[1] [http://computer-
history.info/Page4.dir/pages/Photostore.dir/...](http://computer-
history.info/Page4.dir/pages/Photostore.dir/PhotostoreManual.pdf) [2]
[http://computer-
history.info/Page4.dir/pages/Photostore.dir/...](http://computer-
history.info/Page4.dir/pages/Photostore.dir/index.html)

~~~
agumonkey
Surprising to see how sophisticated IBM hardware division was at the time
compared to what it is to them now.

A PhotoStore "chip"
[https://www.youtube.com/watch?v=4-Jvd7lOjWA](https://www.youtube.com/watch?v=4-Jvd7lOjWA)

------
ChuckMcM
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
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
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...

------
Slippery_John
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
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.

------
meatsock
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-...](http://www.righto.com/2016/06/restoring-y-combinators-xerox-alto-
day.html) "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
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.pd...](http://bitsavers.org/pdf/xerox/alto/ifs_trident/TriconDoc.pdf)

------
kixpanganiban
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.

