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The Acorn Atom was a home computer made by Acorn Computers Ltd from 1980 to 1981 when it was replaced by the BBC Micro (originally Proton) and later the Acorn Electron.

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 KiB of RAM and the floating point extension ROM.

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

It had a MC6847 VDG video chip (Video Display Generator), 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 KiB but could be expanded to 6 KiB. A PAL colour card was also 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), although in an idiosyncratic version, which included indirection operators (similar to PEEK and POKE) for bytes and words (4 bytes). 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.

The manual for the Atom was called Atomic theory and practice

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

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

Contents 1 Memory Map 2 Specifications 3 Trivia 4 References 5 External links

The following is the memory map for the Atom (from 1). Shaded areas indicate those present on the minimal system. Graphics Mode 1 was not accessible, as that RAM was used for code.

#0000	Block Zero RAM
#0400	Teletext VDG RAM
#0800	VDG CRT Controller
#0900
#0A00	Optional FDC
#0A80
#1000	Peripherals space
#2000	Catalogue buffer
#2200	Sequential File buffers
#2800	Floating point variables
#2900	Extension Text space RAM
#3C00	Off-board Extension RAM
#8000	VDG Screen RAM
#8200	Graphics Mode 1
#8400	Graphics Mode 2
#8600	Graphics Mode 3
#8C00	Graphics Mode 4
#9800
#A000	Optional Utility ROM
#B000	PPIA I/O Device
#B800	Optional VIA I/O Device for Printer Interface
#C000	ATOM BASIC Interpreter
#D000	Optional Extension ROM
#E000	Optional Disk Operating System
#F000	Assembler
	Cassette Operating System

• CPU: MOS Technology 6502
• Speed: 1 MHz
• RAM: 2 KiB, expandable to 12 KiB
• ROM: 8 KiB, expandable to 12 KiB 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: 8 volts unregulated DC, providing 5 volts regulated inside the Atom

Note the Acorn 8 V 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 also got uncomfortably hot. Therefore some Atom enthusiasts removed and by-passed the internal regulators and powered their Atoms from an external 5 V regulated power supply. Three amps were typically needed for a fully populated Atom.

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

The kits of the Atom could be problematic for Acorn if the customer didn't have the relevant assembly skills - according to this article [1] one customer assembled his Atom with glue, logic dictating that the heat from the soldering iron would damage the kit .....

Another problem was the big chips falling out of their upside down sockets, using the keyboard shook them loose. A fix was to spot solder the two corner legs of each chip to their socket.

1. Atomic Theory and Practice

• http://www.xs4all.nl/~fjkraan/comp/atom/index.html
• http://www.howell1964.freeserve.co.uk/Acorn/Atom/Atom.htm
• http://home.wanadoo.nl/hhaydn/index-uk.html

List of Acorn Computers microcomputers
Microcomputer System • Atom • BBC Micro (aka Proton) • Electron • Communicator • Business Computer • BBC Master • Archimedes range • Risc PC • Network Computer • Set-Top Box • Phoebe