Acorn Atom

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 KB of RAM and the floating point extension ROM.

The minimum Atom had 2 KB of RAM and 8 KB of ROM, with a "fully loaded" machine having 12 KB of each. An additional floating point ROM was also available. The 12 KB of RAM was divided between 5 KB available for programs, 1 KB for the page zero and 6 KB 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 KB 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 KB but could be expanded to 6 KB. 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), a fast but 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.

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

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 (from 1). Shaded areas indicate those present on the minimal system.

Hex addr	Contents
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	8000-01FF for mode 0 (512 bytes text)	Video RAM
	8000-83FF for mode 1 (1 KB graphics)
	8000-85FF for mode 2 (1.5 KB graphics)
	8000-8BFF for mode 3 (3 KB graphics)
	8000-97FF for mode 4 (6 KB graphics)
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

Specifications

* 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: 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.

References

# Atomic Theory and Practice

External links

*http://www.xs4all.nl/~fjkraan/comp/atom/index.html
*http://www.stairwaytohell.com/atom/wouterras/
*http://home.casema.nl/hhaydn/index-uk.html
* [http://flickr.com/photos/91764962@N00/sets/1336039 Bug Byte magazine]
* [http://flickr.com/photos/91764962@N00/sets/1335936/ Acorn Atom UK Brochure]
* [http://flickr.com/photos/91764962@N00/sets/1335961/ Acorn Atom flyer UK]