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The Acorn Atom was a Home Computer made by Acorn Computers Ltd from 1981 to 1983 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 1982 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 2kB of RAM and 8kB of ROM , with a ''fully loaded'' machine having 12kB of each. An additional Floating Point ROM was also available. The 12kB of RAM was divided between 6kB available for programs and 6kb for the high resolution graphics. In practice around 1kB of the lower memory was lost to variable storage of the 27 variables, and zero page memory used by the operating system. If high resolution graphics were not required then 5 1/2kB of the upper memory could be used for program storage. It had a MC6847 VDU video chip, 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 kbyte but could be expanded to 6 kbyte. A PAL colour card was also available. Six video modes were available, with resolutions from 64x64 in 4 colours, up to 256 x 192 in monochrome. At the time 256 x 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, assembled during program execution and then executed. 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.
SPECIFICATIONS
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 internal regulators also got uncomfortably hot. Therefore some Atom enthusiasts removed and by-passed 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 was no de-facto standard for external 5V connection, but using the same 7-pin DIN connectors as the Atari 800XL allowed the Atari power supply to drive low-power (up to 1.5A) Atoms. REFERENCES # Atomic Theory and Practice EXTERNAL LINKS
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