The Acorn 6502 Microcomputer Kit The Acorn Microcomputer (1979)

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This site has been created in order to preserve a piece of computer history – Acorn Computer’s first public offering, the Acorn Microcomputer (later known as the Acorn System 1). The one pictured below was shipped on 9 April 1979.

In addition to describing the computer, I have written an Emulator for it – so you can try it for yourself even if you don't have the original hardware. You might find the emulator useful as an educational resource, too, as it really shows how to program a computer at the hardware level (and with the mini-debugger you can watch the registers and other internal state changing as instructions are executed).

Many thanks to Fabio Arpino for improving the images here.

The Acorn Keyboard

The computer was primarily sold as a kit; two Eurocard boards (160mm x 100mm) had first to be populated and wired together. These form a self-contained and unusually compact computer (for the time), needing only a 9-volt power supply. The visible (top) board is the Acorn Keyboard:

Keyboard and display Click for a larger image

In addition to the white keyboard, this board also holds a 9-digit calculator-style 7-segment LED display (of which only 8 digits can be used) and, on the left, circuitry for the cassette recorder interface (and a scan decoder).
audio Here's a simulated recording of the first 128 bytes of the Acorn Monitor program, as would be recorded on a cassette tape (your browser may need a plug-in to play this MIDI file).

In this picture the display is showing the contents of the memory location at address (A.) FE00, namely A0. This is the first byte of the firmware monitor program.

On the keyboard, the sixteen keys on the left provide for hexadecimal input values. The eight keys to their right initiate the following command functions:

m (modify) Memory display and modification l (load) Reads a block of bytes from tape
g (go) Run program starting at an address r (return) Resume after a breakpoint
p (point) Inserts or removes breakpoint ^ (up) Increment displayed address
s (store) Writes a block of bytes to tape v (down) Decrement displayed address

The rst key resets the microprocessor, which starts the monitor program (which in turn initially displays eight dots, then waits for a command key to be pressed).

The Acorn Microcomputer

The lower board is the Acorn Microcomputer proper:

Microcomputer board Click for a larger image

It holds (from left to right, top row):

  • National Semiconductor INS8154N RAM/IO chip (128 bytes RAM plus receivers and drivers for display and keyboard)
  • MOS Technologies MPS6502 Microprocessor [0279]
  • Two 2114 static RAM chips (each 1024x4-bit, 1KiloByte total)
  • Two 74S571 PROM chips (each 512x4-bit, 512 Bytes total)
  • Sockets for additional PROM and RAM/IO chips
  • Pads for a EuroConnector.
Along the bottom are sites for interrupt switches (NMI, RST, IRQ), the 1.006MHz crystal, circuitry for address decoding and chip selection, and (at the bottom right) an LM340T5 5-Volt regulator.

The board was also marketed as a ‘Single Board Controller’ without the RAM and ROM chips.

This site was constructed by Mike Cowlishaw.
Please send me any corrections, suggestions for improvement, etc.
  Photographs and HTML text © Mike Cowlishaw 2001, 2002.