Acorn Atom
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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 12kB 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 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 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.
| #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 |
Specifications
- CPU: MOS Technology 6502
- Speed: 1 MHz
- RAM: 2 kB, expandable to 12 kB
- ROM: 8 kB, expandable to 12kB with various Acorn and 3rd party ROMs
- Sound: 1 channel, integral loudspeaker
- Size: 381 x 241 x 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
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
- 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
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