Xerox Alto Restoration Part 4 - first signs of processor life
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- เผยแพร่เมื่อ 15 พ.ย. 2024
- Last time we got the basic peripherals to work, but the machine seemed dead when we tried to boot. We now have to start the painstaking task of looking at the processor logic. Here we look for basic signs of processor activity: clocks and microtask switching. Ken Shirriff who researched the microcode and schematics led this session.
The Alto has a very peculiar architecture, where the processor runs every peripheral in microcode. For this, it continually switches between 16 micro-programmed tasks. Even the memory refresh is a processor driven microtask. And so is the display refresh, disk read/write, mouse input and cursor display, keyboard, Ethernet. Running the user program is the lowest level microtask, which fetches instructions in RAM and emulates a Nova minicomputer instruction set.
And for the tasks to switch, the processor must be able to process micro-instructions, as the running task has to actively relinquish the processor control. So looking at the task scheduler is a very good and simple litmus test of processor basic health, and we can do it with a 4 trace scope. Conversely, nothing will work, not even the memory, if the processor is not able to run microcode and switch tasks.
Very surprisingly, the processor seems to be switching tasks - there is a lot of things that need to be right for this to happen! This is great news. But still no boot, for the next step we will need the Logic Analyzer.
For more details, see Ken Shirriff's detailed blog post about this session:
www.righto.com/...
I love your videos! You do such a great job documenting everything - successes, failures, discoveries, joy of progress. Great stuff!
Amazing watching you guys fix an Alto, such an important piece of computing history.
Exciting! Can't wait for the followup!
Superb series, thanks for taking the time to produce these. Really satisfies my interest in the Alto.
Great progress! Better to slowly get at the faults than discover the whole backplane is smoked, etc.
I've seen other CRTs take a while "wake up" after sitting off for a long time. I've seen it attributed to buildup on the gun's emissive material getting boiled off.
Beautifull. Gran trabajo de restauración de una joya adelantada a su tiempo como fue el Xerox alto.
really enjoyed this episode
this is exactly why we developed POST
Just discovered your videos. Great stuff! Very cool computer to say the least.
The system is very similar to a Nova, but considerably more powerful. The first Nova machines had a bit-serial architecture which used a single 74181 bitslice ALU. The Alto is actually closer to the later SuperNova system.
Awesome!! Than you for making and sharing these videos!
Keep up the good work guys and thanks for sharing!
Ohhh this is good guys. Keep it up, waiting for updates!
Wonderful. Can't wait for the next part!
We were waiting for this video!!! Just about to start watching it now. Thanks!
Excellent! Keep those videos coming.
Cool, what a clever design! I'll bet that saves a *lot* of components :)
I'd still much rather have a 6085-2 which was introduced in 1989 and was approximately 40% faster than a 6085-1.
They could connect it to an emulated 360; it would involve making an FPGA-based interface and a piece of software to interface it with the Hercules emulator.
Is it possible that the boot record was written with a certain prescribed set of hardware attached, something is not present so the system does not complete the process? Perhaps a different boot record might wake it up?
From an amateur's perspective, the earliest Xerox box I ever used was the Star 8010 as there were still a few of them around USMG El Segundo.
Speaking of hardware configurations, I know that Xerox View Point (GVWIN) when configured to be connected to the Ethernet upon start up asked for the date and time from the time service, a cluster of 8000 and 8090 servers running Xerox Executive software modules.
If after a certain interval it could not get this information, it would boot no further (during pre-boot) and hang at the 0937 code (shown in the upper left corner of the monitor). Is it possible the Alto is waiting for time/date information or something similar?
So maybe a Xerox 8011/8012 AI (Artificial Intelligence) Workstation will be your next venture?
I don't remember seen in the video thay the contacts of all boards and sockets were cleaned with isopropylic alcohol or similar, maybe it is just dirty.
I heard different tasks like 'memory' and 'cursor' tasks -- what are all the tasks? Is this in a micro controller separate from the 'cpu' or did the system not have a 'cpu' or is this separate from the cpu?
The single, micro-programmable CPU does all the tasks, and has very low-level access to all the hardware cards. These are always running micro-programmed hardware tasks, in the CPU micro-control ROM, not part of the operating system. In effect this arrangement enables a single CPU to play the role of what dedicated hardware, or 16 micro-controllers spread around the machine, would have done. That is, if suitable micro-controllers had been available at the time! Very advanced way to use micro-programming to do hardware tasks and simplify the hardware design, so it can fit in a small size machine for the time. At the expense of greatly complexifying the micro-software architecture of the machine - the microcode is impossibly difficult to understand, and none of the hardware works if the CPU is not up and running the microtasks. Not even memory! Even more mind-boggling, the lowest priority of the CPU tasks, the emulator task, the only one visible to the user, emulates a proper minicomputer CPU via microcode. In this case a Data General Nova, which was popular at the time. So as far as the user is concerned, this hardware looks like a NOVA CPU. But it's not. It's a virtual, emulated NOVA CPU.
This is so amazing. Thanks for the clear answer. The Alto is a legendary computer. I hope you get it up and running! Were some of the helpers in the video involved in its design back in the day? Where are you getting the design schematics?
+CuriousMarc As a software engineer, If I had a hat it would be off to the team who programmed that. wow
There are many docs for study which you can download from here:
archive.org/search.php?query=Xerox%20alto
Thank you!
Try reinstalling windows
Any women here?
Yeah, here. It's interesting to watch this beast being nursed back to life.