
Xerox Alto Restoration Part 5 [video] - kens
https://www.youtube.com/watch?v=Wr7vDZpniNI&feature=youtu.be
======
Animats
Logic analyzers connected to pins on a CPU. You don't see that much any more.
The connections they're looking at are inside the chip today. Of course, now
we often have JTAG access to the innards.

~~~
kens
Let me emphasize that there's no CPU chip in the Alto - the CPU is made up of
dozens of TTL chips on three different boards. We're tracing the individual
microcode instructions that the Alto is running. As Animats points out, this
would be deep inside a modern chip.

~~~
cnvogel
There's a brilliant talk, given in 2009 at HOPE09, NYC titled
"Indistinguishable From Magic: Manufacturing Modern Computer Chips".

At roughly 36 minutes, there's a cross-section of a (in 2009) modern chip. The
topmost layer is the shiny rainbowy topmost metal-layer you see on typical
(from the top) die photographs. The very, very small comb-looking bottommost-
layer is the individual transistors, and structures there are of the size that
give a "xxx nm" process its name.

[https://youtu.be/NGFhc8R_uO4?t=36m8s](https://youtu.be/NGFhc8R_uO4?t=36m8s)

I very much recommend everyone to watch this video, to get a sense for the
complexity that goes into producing a modern CPU.

To come back to the thread I'm answering here: The logic analyzer on the Alto
(probing microcode) connected to interconnects of individual gates probably
would be connected to signals somewhere in the lower middle of the stack in a
modern CPU, I guess.

------
Lord_Nightmare
What I'm personally afraid of is that, due to the bad power supply which was
repaired in restoration part 1, the wrong voltage was applied to one of the
power pins of several boards full of DRAM chips, and this may have fried the
entire lot of them.

If the Alto2 has 128KiW of dram, and the drams used are old tms4108 chips(I'm
not actually sure about this, they may be even older), and there are 18 or 19
bits of ram per word (16 bits plus 3 parity/ECC? Again, not sure.), then there
could be as many as 304 chips to replace (or at the very least, to pull and
test each one and replace the bad ones).

Fortunately, if they used triple-voltage (5v, 12v, -5v) 4108 or 4116 drams,
you can replace those chips with the 5v-only 4164 chip by bending 2 pins
upward on the 4164 and soldering a wire to another pin. This isn't "period
accurate" but should work to replace a few bad chips until working ICs can be
found.

See [http://picmania.garcia-
cuervo.net/recursos/redpicdatasheets/...](http://picmania.garcia-
cuervo.net/recursos/redpicdatasheets/memorias_ram/4116_to_4164.htm)

LN

~~~
kens
We tested and repaired the power supplies _before_ powering up the circuit
boards, to make sure we didn't fry anything.

Each of the four boards has 80 memory chips. These are 4116 DRAM chips, each
holding 16 kilobits. Each board holds 128K 10-bit-wide "chunks". Data is
stored as 32 bits + 8 bits (actually 7) of Hamming error correction and
parity.

~~~
Lord_Nightmare
The issue is not when _you_ powered up the machine and the ram boards, its
more whoever powered them up _before you got the machine_.

That power supply may have failed decades ago. Was the machine known working
when it was last powered up?

Is there any way you could build a rig to test each 4116 chip, or a rig you
could plug each board into which would supply proper voltages (in the correct
bring-up and bring-down order, because 4116 drams are weird like that) for the
dram, and do a pattern test on the entire board all at once?

Personally I don't like 4116 drams at all because they're very unreliable.
(4164s are far superior in this regard.) See the PARC notes on bitsavers re:
the notetaker project, the notetaker test systems were frequently out of
commission due to failed 4116 dram chips, or running with one or more chips
dead and the ECC constantly correcting the errors.

LN

------
irq
Where's the text writeup? The previous 4 parts had one.

~~~
kens
As with the previous parts, the text writeup will be a few days after the
video. I'm not as fast at writing as Marc is with video editing :-)

------
kombucha2
Very cool.

------
gus_massa
[The [http://youtu.be](http://youtu.be) submissions are autokilled. Remember
to submit the [http://youtube.com](http://youtube.com) variant next time.]

~~~
JoshTriplett
Was the shortener used for spam submissions at some point?

