The
Acorn Atom was a home computer made by
Acorn Computers Ltd from 1980 to 1981 it was the forerunner to the hugely popular BBC Computer System, it was then 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 had 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 basic 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.
The video chip, allowed for text or two-colour graphics modes. It could be connected to a TV or modified to connect and output to a video monitor.The Basic video memory was 1 KB but could be expanded to 6 KB. 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 came built-in with Atom BASIC, a fast but idiosyncratic version, which included indirection operators 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 case was designed by industrial designer Allen Boothroyd of Cambridge Product Design Ltd.
