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Tracing the History of the Computer - Acorn Atom


The Acorn Atom was a home computer made by Acorn Computers Ltd between 1981 and 1983, when it was replaced by the BBC Micro (which was originally called the Proton) and later the Acorn Electron.

Acorn Atom

Acorn Atom, 1981 - 1983

The Atom was a progression of the MOS Technology 6502 based machines that the company had been making from 1979. The Atom was a cut-down Acorn System 3 without a disk drive but with an integral keyboard and cassette tape interface, sold in either kit or complete form. It was priced at around 175.

It had a MC6847 VDU video chip, allowing for text or two-colour graphics modes. It could be connected to a TV or modified to output to a video monitor. Basic video memory was 1 kbyte but could be expanded to 6 kbyte. A PAL colour card was also available.

It had built-in BASIC (Atom BASIC), although in an idiosyncratic version, which included poke and peek operators for bytes and quadruple bytes. It also included an assembler allowing you to produce machine code as output of a program.

The Acorn LAN, Econet, was first configured on the Atom.

The computer case was designed by industrial designer Allen Boothroyd of Cambridge Product Design Ltd.



CPU: MOS Technology 6502

Speed: 1 MHz

RAM: 2 kB, expandable to 12 kB

ROM: 8 kB, expandable to 12kB with various Acorn and 3rd party ROMs

Sound: 1 channel, integral loudspeaker

Size: 381 x 241 x 64 mm

I/O Ports: Computer Users Tape Standard (CUTS) interface, TV connector, Centronics parallel printer

Storage: Kansas City standard audio cassette interface

Power: 8 volts unregulated DC, providing 5 volts regulated inside the Atom

Note: the Acorn 8V power supply was only rated to 1.5 amps, which was not enough for an atom with fully-populated RAM sockets. The Atom's internal regulators also got uncomfortably hot. Therefore most Atom enthusiasts removed and by-passed the internal regulators and powered their Atoms from an external 5V regulated power supply. 3 amps were typically needed for a fully-populated Atom.

There was no de-facto standard for external 5V connection, but using the same 7-pin DIN connectors as the Atari 800XL allowed the Atari power supply to drive low-power (up to 1.5A) Atoms.


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