Bring back Blinkenlights!!! 3272

On Sun, 23 Apr 2006, Brian Inglis

Do you want to have a useful system, or do you want a plaything?

Do you want something that you look at briefly while playing, or do you want something 24-7?

Those are the questions that you have to answer in order to guide your decisions; and you have to understand that others may not have the same answers to those questions.

With my answers, an emulated display is worse than useless.

There is a reason why the KL and KS didn't have lights. Most of the things shown on lights on the old machines (PDP-6, KA10, KI10) would be quite expensive to display in any microcoded system. On the old machines, the light was driven by voltage level on a wire that carried the signal. In microcode, it's just some bits somewhere in RAM.

The usual answer, even in hardware implementions, is to sample the various registers at a much slower interval. The problem is how much slower is "much slower". It doesn't make much sense to drive the lights much faster than every 50-100ms. But that gets maybe one out of a million states. Lights that show 1-1,000,000th of the status are not useful to me.

Another matter: even if you solve the performance issues, most of what was shown in the old machines' lights is not interesting while the system is running. It'll all be various shades of "pink" instead of blinking. The old machines were slow enough, and certain operations were costly and narrow enough (TOPS-10 shuffling on PDP-6s and KAs being a good example) that it made for the pretty patterns.

Now, if you are going to do a PDP-8, then you don't have as much of a problem. In hardware (such as an SBC6120), it's slow enough so that it isn't all "pink"; and in emulation it's quite alright to slow it down since it'll still be a lot faster than any real PDP-8. A PDP-10 is a different matter.

The Panda display does something much more modest than the register display on a PDP-6-KA-KI; it was carefully designed to be both useful and to minimize the cost to the PDP-10 system. What it shows: . 36 bit display register, updated on demand by user LITES% JSYS . 250ms heartbeat, indicating: . system load in a 3-bit register, with 7 meaning a load average .ge. 1 . CPU state (a toggle), very much like the KS10's RUN light. . disk activity during last interval . tape activity during last interval . network activity during last interval

The machine is up 24-7 and runs headless. Those lights are used for the original purpose of lights: to monitor system status. I can look at it across the room and immediately know if the system is healthy and if something is going on that I might care to look at.

-- Mark --

Democracy is two wolves and a sheep deciding what to eat for lunch. Liberty is a well-armed sheep contesting the vote.