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Mandelbrot for Xerox Alto, written in BCPL
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Mandelbrot for the Xerox Alto

To build and run:

  • Copy mandel.bcpl and pressml.asm to the Alto (e.g. with FTP)
  • asm pressml.asm
  • bcpl mandel.bcpl
  • bldr/d/l/v mandel pressml
  • mandel

This program takes about an hour to run, since the Xerox Alto is not a fast computer.

You can run this using thie Contralto emulator if you don't have an Alto available.

The Alto inconveniently uses \r instead of \n as a line separator, so I've included mandel.txt as a more readable version of mandel.bcpl.

How it works

The Xerox Alto uses 16-bit integers, and no floating point. Since the Mandelbrot computation uses real numbers, I represent them in fixed point: 4 bits to the left of the decimal point and 12 bits to the right. (Fixed-point arithmetic is a common trick for fast Mandelbrot calculation.)

To multiply two 16-bit numbers, I use the double precision MulFull function (written in Nova assembler) in PressML.asm, part of the Computer History Museum's archived Alto software. The result must be right-shifted 12 bits to restore the fixed-point offset.

Setting the pixels is a bit tricky. A block of memory is allocated for the display and then the appropriate bit in the appropriate word is set to get a black pixel. The gory details are in chapter 4 of the Xerox Alto Hardware Manual.

Some notes on BCPL

The code is written in BCPL, the main langauge used on the Alto. BCPL is a precursor to C and many features of C are clearly visible in BCPL.

BCPL has some trivial syntax differences compared to C. Keeping the following in mind, you should be able to understand the BCPL code:

  • Blocks indicated with [ and ] instead of { and }.
  • Indexing with a!1 instead of a[0].
  • And, Or, and Shift bit operations are &, %, and lshift/rshift.
  • Variable definitions with let.
  • Headers included with get.
  • Comparison with eq instead of ==.
  • Arrays defined with vec.

More information on BCPL is in the BCPL Reference Manual.


Output from the mandel program

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