Mandelbrot on a vintage IBM1401

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Ken Shirriff hat im Computer History Museum in San Francisco einen ollen, aber funktionierenden IBM1401-Rechner gefunden. Also hat er dafür ein Mandelbrot-Apfelmännchen in Assembler programmiert, das Programm auf ebenso olle Lochkarten gestanzt, das Teil einmal durchrattern lassen – „The cards fly through the reader at the remarkable speed of 800 cards per minute so the Mandelbrot program loads in just over a second“ – und das ganze schließlich auf ’nem Zeilendrucker ausgegeben. Nice! (via Visualizing Math)

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The program fits onto just 16 cards, but the card format is a bit unusual. The machine code for the Mandelbrot program is punched into the left half of the each card, with code such as M384417A395417. An interesting thing about the 1401 is the machine code is almost human-readable. M384417 means Move field from address 384 to address 417. A395417 means Add the number at address 395 to the number at address 417. The text on these cards is the actual machine code that gets executed, not the assembly code. Since the machine is character-based, not binary, there's no difference between the characters "428" and the address 428.

If you look at the right half of the cards, there's something totally different going on, with text like L033540,515522,5259534. There's no operating system, so, incredibly, each card has code to copy its contents into the right place in memory (L instruction), add the word marks (, instruction), and load the next card. In other words, the right hand side of each card is a program that runs card-by-card to load into memory the program on the left hand side of the card deck, which is executed after the last card is loaded.

To run the program, first you hit the "Power On" button on the IBM 1401 console. Relays clunk for a moment to power up the system and then the computer is ready to go (unlike modern computers that take so long to boot). You put the cards into the card reader and hit the "Load" button. The cards fly through the reader at the remarkable speed of 800 cards per minute so the Mandelbrot program loads in just over a second. The console starts flickering as the program runs, and every few seconds the line printer hammers out another line of the fractal. After 12 minutes of execution, the fractal is done.

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