---
title: Computer Graphics on the PDP-1
author:
- Leah Neukirchen
- blinry's birthday barcamp 2021
date: 2021-07-07
x-compile-with: pandoc -t revealjs -s -o pdp-1-computer-graphics.html pdp-1-computer-graphics.md --variable theme=moon
---

# The DEC Programmable Data Processor-1

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![](https://2k8fuf10pw8w25f6ce428ify-wpengine.netdna-ssl.com/ModernComputer/Electronic/Images/PDP-1.jpg)

- available as of late 1959, only ~60 were built total
- 18-bit words, 5MHz clock
- default memory: 4kw = 9kb

# Precision CRT Display Type 30

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![](https://assets.fontsinuse.com/static/use-media-items/10/9963/upto-700xauto/56702262/tumblr_mhi3ahYxQ61rm93g3o1_1280.jpeg){height=500px}
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- 16-inch random-position, point-plotting cathode ray tube
- 1024x1024 resolution
- no frame buffer!
- single instruction API

```
dpy  720007  Display One Point

             Clears the light pen status bit and
             displays one point using bits 0
             through 9 of of the AC to represent
             the (signed) X coordinate of the
             point and bits 0 through 9 of the
             IO as the (signed) Y coordinate.
             
             The third lowest nibble (bits 9-11)
             encodes the intensity (brightness)
             in 1's complement, where 3 is the
             brightest, 0 the default intensity,
             and 7 (-0) barely visible.
```
- Spacewar! used this display, but we'll consider a much simpler program
::::
:::

# Munching Squares

[Demo at Masswerk](https://www.masswerk.at/minskytron/?version=munchingsquares)

# The Code: Jackson Wright, 1962

```
msq, lat           ; Load AC from Test Word switches
     sas msv       ; Skip next instruction if C(AC) = C(Y), msv = 777777
     sza i         ; Skip on ZERO (+0) AC
     law 2345      ; Load the AC with the number N
     add 7         ; Add C(Y) to C(AC)
     dac 7         ; Deposit C(AC) in Y
     rcl 9s        ; Rotate combined AC & IO left  9s = 777
     xor 7         ; Exclusive OR C(Y) with C(AC)
     dpy-i+300     ; Display One Point
     jmp msq       ; Take next instruction from Y
msv, 777777
```

# Later versions for the PDP-10

[HAKMEM - Artificial Intelligence Memo No. 239](https://interglacial.com/~sburke/stuff/hakmem.html)

<section style="text-align: left;">
HAKMEM Item 2:
```
     DATAI 2
     ADDB 1,2
     ROTC 2,-22
     XOR 1,2
     JRST .-4
```

HAKMEM Item 3 (Schroeppel):<br>
Munching squares is just views of the graph Y = X XOR T for consecutive values for T = time. 
</section>

# Munching squares in tixy

<iframe src="https://tixy.land/?code=%28x%5Ey%29%2F4%3C%28t%256%29" width="800" height="500">
</iframe>

# Munching squares on your Linux computer

![](https://l2.re/-k-tNh)

```
% /usr/libexec/xscreensaver/munch
```

# Thank you!

Sources:

- <https://www.masswerk.at/minskytron/>
- <http://bitsavers.informatik.uni-stuttgart.de/bits/DEC/pdp1/from_peter_samson/dpys5.mac>
- <https://www.masswerk.at/spacewar/inside/pdp1-instructions.html>
- <https://interglacial.com/~sburke/stuff/hakmem.html>