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Acorn Atom

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Title: Acorn Atom  
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Subject: Acorn Computers, BBC Micro, ZX81, Caret notation, Acornsoft LISP
Collection: 1981 Introductions, 6502-Based Home Computers, Acorn Computers
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Acorn Atom

Acorn Atom
The Atom was Acorn's first computer to be aimed squarely at the home market.
Manufacturer Acorn Computers
Type Personal Computer
Release date 1980 (1980)
Introductory price £120 (in kit form), £170 (assembled)
Discontinued 1983
Media 100KB 5¼-inch floppy disks, Cassette tapes
CPU MOS Technology 6502 clocked at 1MHz
Memory 2 KB RAM (expandable to 12 KB), 8 KB ROM (expandable to 12 KB)
Display 64x64 (4 colors), 64x96 (4 colors), 128x96 (monochrome), 64x192 (4 colors), 128x192 (2 colors), 256x192 (monochrome)
Input Keyboard
Power 8V, 1.5A unregulated DC, 5V regulated inside.
Dimensions 381×241×64 mm
Predecessor Acorn System 3
Successor BBC Micro

The Acorn Atom was a home computer made by Acorn Computers Ltd from 1980 to 1982, when it was replaced by the BBC Micro (originally Proton).

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. In 1980 it was priced between £120 in kit form, £170 ready assembled, to over £200 for the fully expanded version with 12 KB of RAM and the floating point extension ROM.

The minimum Atom had 2 KB of RAM and 8 KB of ROM, with the maximum specification machine having 12 KB of each. An additional floating point ROM was also available. The 12 KB of RAM was divided between 1 KB for the zero page, 5 KB available for programs, and 6 KB for the high resolution graphics. The zero page was used by the CPU for stack storage, by the OS, and by the Atom BASIC for storage of the 27 variables. If high resolution graphics were not required then 5½ KB of the upper memory could be used for program storage.

It had an MC6847 Video Display Generator (VDG) video chip, allowing for both text and graphics modes. It could be connected to a TV or modified to output to a video monitor. Basic video memory was 1 KB but could be expanded to 6 KB. Since the MC6847 could only output at 60 Hz, meaning that the video could not be resolved on a large proportion of European TV sets, a 50 Hz PAL colour card was later made available. Six video modes were available, with resolutions from 64×64 in 4 colours, up to 256×192 in monochrome. At the time, 256×192 was considered to be high resolution.

It had built-in BASIC (Atom BASIC), a fast but idiosyncratic version, which included indirection operators (similar to PEEK and POKE) for bytes and words (of 4 bytes each). Assembly code could be included within a BASIC program, because the BASIC interpreter also contained an assembler for the 6502 assembly language which assembled the inline code during program execution and then executed it. This was a very unusual, but also very useful, function.

In late 1982, Acorn released an upgrade ROM chip for the Atom which allowed users to switch between Atom BASIC and the more advanced BASIC used by the BBC Micro. The upgrade was purely to the programming language; the Atom's graphics and sound capabilities remained unchanged, and hence, contrary to some pre-release beliefs, the BBC BASIC ROM did not allow Atom users to run commercial BBC Micro software, since nearly all of it took advantage of the BBC machine's advanced graphics and sound hardware.

The manual for the Atom was called Atomic Theory and Practice and was written by David Johnson-Davies, subsequently Managing Director of Acornsoft.

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

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

Memory Map

The following is the memory map for the Atom. Shaded areas indicate those present on the minimal system.
Hex Address Contents Annotations
0000 Block Zero RAM 1 KB RAM
0400 Teletext VDG RAM
0800 VDG CRT Controller
0A00 Optional FDC
1000 Peripherals space
2000 Catalogue buffer
2200 Sequential File buffers
2800 Floating point variables Internal RAM
5 KB max.
2900 Extension Text space RAM
3C00 Off-board Extension RAM
8000 8000-01FF for mode 0 (512 bytes text) Video and
6 KB max.
8000 8000-83FF for mode 1 (1 KB graphics)
8000 8000-85FF for mode 2 (1.5 KB graphics)
8000 8000-8BFF for mode 3 (3 KB graphics)
8000 8000-97FF for mode 4 (6 KB graphics)
A000 Optional Utility ROM
B000 PPIA I/O Device
B800 Optional VIA I/O Device for Printer Interface
C000 ATOM BASIC Interpreter 4 KB ROM
D000 Optional Extension ROM
E000 Optional Disk Operating System
F000 Assembler 4 KB ROM
Cassette Operating System


  • CPU: MOS Technology 6502
  • Speed: 1 MHz
  • RAM: 2 KB, expandable to 12 KB
  • ROM: 8 KB, expandable to 12 KB with various Acorn and 3rd party ROMs
  • Sound: 1 channel, integral loudspeaker
  • Size: 381×241×64 mm
  • I/O Ports: Computer Users' Tape Standard (CUTS) interface, TV connector, Centronics parallel printer
  • Storage: Kansas City standard audio cassette interface
  • Power: standard 2.1 mm power jack connector for 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 two internal LM7805 regulators (each regulating the +5V for a section of the digital logic independently) also got uncomfortably hot. Therefore some Atom enthusiasts removed and bypassed the internal regulators and powered their Atoms from an external 5V regulated power supply. Three amps were typically needed for a fully populated Atom.

There has never been a de facto standard for external 5V connections, but using the same 7-pin DIN connectors as the Atari 800XL allowed an Atari 5V linear power supply to drive an Atom, so long as the current was less than the Atari PSU rating (1 or 1.5 amps, depending on the model). These are now uncommon, but 5V wall-wart switch-mode power supplies capable of supplying several amps are a readily and cheaply available alternative.

External links

  • Yet another computer museum - Acorn Atom
  • The Acorn Atom Review
  • Atom BASIC
  • Acorn Atom in FPGA
  • The complete Atom DVD
  • YouTube page totally dedicated to the Acorn Atom
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