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• Download the complete
  Blinky archive
  
  Note 20260315 on zip file:
  from PiDP-8/10 simh scripts,
  remove all 'realcons' lines
  before running.

Hill's Blinky

'Demo code' that deserves a closer look

Mike Hill wrote Blinky programs for the PDP-8, 11, 10 and 1; they often grace the PiDP replica series at their default boot mode. It lights up the front panels in a pretty way without having to boot an operating system. However, their real purpose is educational: to show in a very concise way how you program these machines.

This page gives an overview, to show why studying Blinkies is worth some time. There is much more to Blinkies than meets the eye. To the right, we show the Minsky Chaos easter egg hidden in Blinky-1. But all the Blinkies have interactive settings and non-obvious features.

That's only one reason to have a look at Blinky code and documentation. More important is that they are written to give a quick flavour of the machine's instruction set, the programming style, and some hacker style coding tricks that might surprise you.

For people who are completely new to assembly programming, this might be a steep Hill (pun intended) to climb. Read the DEC CPU handbooks a little bit first. But with the smallest amount of background, you will easily read Mike's commented code, provided in the source code boxes below.

PDP-1 Blinky: DPkaos graphical easter egg
(variation on Minsky, only in recent version!)

So, even if you have no intention to do any machine language programming on your machine, it is still fun to look. There are 'Hill's Blinky' boot modes on all the PiDPs, but this page contains them all - and might have the latest updated versions too.

PDP-1 Blinky

Mike's latest and greatest Blinky - so we start here. Blinky-1 is supplied as a ready-to-run paper tape. The code runs to over 6000(!) lines, partly because Blinky-1 is not a small program, but mostly because the source code is written as a tutorial. It assumes some basic knowledge of assembly language in general, if you know even the littlest about assembly code, it is easy reading and will teach you some elegant tricks.

How to use
This is the Blinky-1 help page. It will be typed out on the PDP-1 typewriter if you set Sense Switch 4 when you load the paper tape. You might have to scale the font size (Ctrl-+/- on the GUI to read it all. Beware, it takes a while to type out, before the machine continues with running Blinky itself. On a real PDP-1, of course, you would print it out once, tear off the paper and keep it... If you want to skip the printout and 'get to Blinky', just set Sense Switch 4 to off again.
Blinky is controlled through the SENSE and TEST WORD switches.

• Assuming all your TW switches are off, the program shows the Big Fast Loop light pattern. Many other patterns are possible, experiment with the TW switches. The main 4 patterns are shown in the help text: Big Fast Loop, Reversal, Fast Rotation and Bouncing.
• Speed: TW15-17 (rightmost 3 switches) regulate the main software 'clock' that Blinky is ticking on. To the left, TW12-14 add a delay to whatever is flowing through the IO register lights, and to the left of that, TW9-11 add delays on flow through the accumulator lights.
• The program flags are animated as well. And of course, audio output is connected to the PF1-4 lights. Thus, if you enable the audio on your machine, you'll hear the pops every time the PF1-4 lights are (un)set. That's how audio on the PDP-1 works, there's nothing more to it. And still it can play music :-). Play with the clock (TW15-17) and the PF pattern (TW3) to experiment.

On to the graphic 'easter eggs'. Assuming all TW switches are off, then:

• SS6 enables the display mode. Then, Set ADDRESS switches to 4 and set SS2 because things get confusing and this will let you use STOP/START to properly reset Blinky.
  (To explain, setting ADDRESS to 4 then pressing START will restart BLINKY-1 (better to use ADDRESS 3000). Changing SS2 from OFF to ON or ON to OFF -then- changing the TW will also restart BLINKY-1)
• you'll now see random noise on the display. Blinky is just outputting whatever is in the AC and IO registers on the display. Press STOP, then START. You see the diagonal flash, which you should see as Blinky's reset flash.
• set TW to 135000. This drops you in the easter eggs, and you get the 'lone satellite' first. Set to 135007, you'll change its direction. Then set to 131007: multiple rockets. See the other effects in the help file. Leave the easter eggs by toggling switch TW0.

The last mode is the easter egg inside the easter eggs: DYkaos.

• Set Blinky to display the 'lone satellite'. Make sure you see it (SS6 is set). Now change the TW to 134 and DYkaos comes up.
• You can use the light pen to drag the pattern.
• See the source code (BLINKY-1c.lst, line 2677 and on) for more nifty tricks. It is a little algorithm with a big history: invented by Marvin Minsky, and documented in ITEM 149 of MIT's AIM 239 "HAKMEM". All of HAKMEM.HAKMEM is worth looking at, if you have not heard of it.

Code
Download the full code package through the orange button at the top left of the page, including the latest paper tape image file. You can have a quick view to the Blinky-1 code below. It comes in three parts:

• 1c - Blinky proper (start here)

  Click to load...

• 1b - the help text printer

  Click to load...

• 1a - the loader

  Click to load...

There are more PDP-1 goodies in the download package - Mike's radically upgraded version of the macro1 cross-assembler. It is fully compatible, but offers a lot more features (do not forget, there's also the AM1 macro-assembler from Bill Ezell and Angelo Papenhoff's. I thought that is useful to mention here). Rimmer, to make efficient paper tapes. Also, Mike developed Blinky-1 before the PiDP-1 itself was finished, and thus developed his own version of the first Javascript PDP-1 simulator from Barry Silverman, Brian Silverman, and Vadim Gerasimov, building on the sgeo.github.io/jspdp1 version.

In the ZIP file, you will find a BLINKY-1_lem variant. Use this one, it disables Extend on the front panel (leaving the Extend switch set is a common user mistake on the PDP-1, this version prevents that from disrupting anything).

PDP-1 PUNCHY

This little demo types out any typewriter input on the paper tape punch. In that sense, it is a successor to the ribbon demo already in the PiDP-1 takes archive. But Punchy has some additional hacks built in. Download here (link), and read the .lst file in the ZIP archive for details. Punchy's code is very straight and simple PDP-1 assembly, so a good place to start out from.

PDP-11 IDLED

On the PDP-11, we have IDLED. It, too, comes as a heavily commented simh boot script, and pulls out all the PDP-11 programming tricks to make the front panel do interesting things. It was called a poem in 64 words, trying to use as many non-standard coding techniques as possible. So where PDP-10 Blinky is straight-up programming, PDP-11 Blinky can best be seen as a puzzle - it is hard to figure out some of the tricks. Use simh's built-in debugger! If you completely understand this 'poem', you may well call yourself an accomplished PDP-11 hacker.
Why 64 words? It was designed to fit in the boot block of any supported PDP-11 device, which means it needed to fit into 64 words. The first word has to be NOP for this purpose, so Mike was left with 63 of them.

IDLED is actually two parts. The original IDLED, which is a front panel Blinkenlights demo. IDLED+ is a plug-in for IDLED. It is enabled through setting SR6. Stored in locations 000176-000575, it adds switch selection (SR15-SR8) of the LED patterns including different ones for the Display Register. The 1-page cheat sheet with code & summary of features (click twice to zoom): Code
Download the full code package through the orange button at the top left of the page. Or you can read the IDLED code below:

• Commentary to solve the puzzles

  Click to load...

• Simh script with inline assembly - the way to run IDLED on the PiDP-11

  Click to load...

• When running IDLED, stop (Ctrl-E) to run this Simh script:
  disassembles IDLED live on the PiDP-11

  Click to load...

Mike also wrote an improved version of rhialto's macro11 assembler, which is included in the download file.

PDP-8 Blinky

PDP-8's Blinky shows off the PDP-8 front-panel by using the switch register to display varying patterns in the lamps. Like IDLED on the 11, it comes as a self-contained and self-documenting simh boot script. Because it makes a point of using most of the PDP-8 instruction-set and programming methods (e.g. subroutines, interrupts, and auto-indexing), it is a very good first or second PDP-8 program to study. If you want to catch a flavour of what programming the Eight is like!

Operation through the SR switches: There are three switches per setting. The left-hand switch selects the pattern and the right-hand two switches select the speed. 00,01-02: PC & MA patterns, 03,04-05: MB pattern, 06,07-08: AC pattern, 09,10-11: MQ pattern. The 1-pager with code & summary of features (click twice to zoom): Code
Download the full code package through the orange button at the top left of the page. Or you can read the code below:

• Simh script with inline assembly - the way to run Blinky on the PiDP-8

  Click to load...

You normally run a simh script by hitting Ctrl-E to go to the simh> command line, and type "do BLINKY8.txt" However, if you add LISTor DATA to the command, Blinky will list its code and data tables for you. Add EXEC to the end to make the script auto-start itself (so you do not have to type 'continue').

EZBlinky: 4 instructions to toggle in on the PDP-8 front panel (and amaze the audience)

A minimum BLINKY for the PDP-5, PDP-8, and PDP-12 that you can just toggle in:

PDP-10 Blinky

PDP-10 Blinky is a much smaller, traditional, Blinkenlights demo. It is supplied as a stand-alone simh boot script. The code fits on a single page. It shows some neat tricks of how to animate register lights whilst they are in actual use, running code. The DATA switches are used to play with the demo. The cheat sheet:


Code
Download the full code package through the orange button at the top left of the page. Or you can read the 10-Blinky code below:

• Source code as a one-pager (click twice to zoom)

  
• Assembled, with lots of commentary, as a simh/PiDP-10 boot script

  Click to load...

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