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The Atom was Acorn's first computer to be aimed squarely at the home market.
|Introductory price||£120 (in kit form), £170 (assembled)|
|Media||100KB 5¼-inch floppy disks, Cassette tapes|
|Power||8V, 1.5A unregulated DC, 5V regulated inside.|
|CPU||MOS Technology 6502 clocked at 1MHz|
|Memory||2KB RAM (expandable to 12 KB), 8KB ROM (expandable to 12 KB)|
|Display||64x64 (4 colors), 64x96 (4 colors), 128x96 (monochrome), 64x192 (4 colors), 128x192 (2 colors), 256x192 (monochrome)|
|Predecessor||Acorn System 3|
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 bottom kilobyte of 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 5½ 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 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.
|0000||Block Zero RAM||1KB RAM|
|0400||Teletext VDG RAM|
|0800||VDG CRT Controller|
|2200||Sequential File buffers|
|2800||Floating point variables||Internal RAM
|2900||Extension Text space RAM|
|3C00||Off-board Extension RAM|
|8000||8000-01FF for mode 0 (512 bytes text)||Video and
|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||4KB 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 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 (each regulating the + 5V for a section of the digital logic independently) 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 has never been a de facto standard for external 5 V 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 model). These are now uncommon, but 5V wall-wart switch-mode power supplies (e.g. for powering USB hubs) capable of supplying several amps are a readily and cheaply available alternative.
- Yet another computer museum - Acorn Atom
- The Acorn Atom Review
- Atom BASIC
- Acorn Atom in FPGA
- The complete Atom DVD