Acorn Atom
Encyclopedia
The Acorn Atom was a home computer
made by Acorn Computers Ltd from 1980 to 1982 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 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 Acorn LAN
, Econet
, was first configured on the Atom.
The case was designed by industrial designer Allen Boothroyd
of Cambridge Product Design Ltd.
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.
Home computer
Home computers were a class of microcomputers entering the market in 1977, and becoming increasingly common during the 1980s. They were marketed to consumers as affordable and accessible computers that, for the first time, were intended for the use of a single nontechnical user...
made by Acorn Computers Ltd from 1980 to 1982 when it was replaced by the BBC Micro
BBC Micro
The BBC Microcomputer System, or BBC Micro, was a series of microcomputers and associated peripherals designed and built by Acorn Computers for the BBC Computer Literacy Project, operated by the British Broadcasting Corporation...
(originally Proton) and later the Acorn Electron
Acorn Electron
The Acorn Electron is a budget version of the BBC Micro educational/home computer made by Acorn Computers Ltd. It has 32 kilobytes of RAM, and its ROM includes BBC BASIC along with its operating system....
.
The Atom was a progression of the MOS Technology
MOS Technology
MOS Technology, Inc., also known as CSG , was a semiconductor design and fabrication company based in Norristown, Pennsylvania, in the United States. It is most famous for its 6502 microprocessor, and various designs for Commodore International's range of home computers.-History:MOS Technology, Inc...
6502
MOS Technology 6502
The MOS Technology 6502 is an 8-bit microprocessor that was designed by Chuck Peddle and Bill Mensch for MOS Technology in 1975. When it was introduced, it was the least expensive full-featured microprocessor on the market by a considerable margin, costing less than one-sixth the price of...
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
Kilobyte
The kilobyte is a multiple of the unit byte for digital information. Although the prefix kilo- means 1000, the term kilobyte and symbol KB have historically been used to refer to either 1024 bytes or 1000 bytes, dependent upon context, in the fields of computer science and information...
of RAM
Ram
-Animals:*Ram, an uncastrated male sheep*Ram cichlid, a species of freshwater fish endemic to Colombia and Venezuela-Military:*Battering ram*Ramming, a military tactic in which one vehicle runs into another...
and the floating point extension ROM
Read-only memory
Read-only memory is a class of storage medium used in computers and other electronic devices. Data stored in ROM cannot be modified, or can be modified only slowly or with difficulty, so it is mainly used to distribute firmware .In its strictest sense, ROM refers only...
.
The minimum Atom had 2 KB of RAM and 8 KB of ROM
Read-only memory
Read-only memory is a class of storage medium used in computers and other electronic devices. Data stored in ROM cannot be modified, or can be modified only slowly or with difficulty, so it is mainly used to distribute firmware .In its strictest sense, ROM refers only...
, with a fully loaded machine having 12 KB of each. An additional floating point
Floating point
In computing, floating point describes a method of representing real numbers in a way that can support a wide range of values. Numbers are, in general, represented approximately to a fixed number of significant digits and scaled using an exponent. The base for the scaling is normally 2, 10 or 16...
ROM
Read-only memory
Read-only memory is a class of storage medium used in computers and other electronic devices. Data stored in ROM cannot be modified, or can be modified only slowly or with difficulty, so it is mainly used to distribute firmware .In its strictest sense, ROM refers only...
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
MC6847
The MC6847 is a video display generator first introduced by Motorola and used in the TRS-80 Color Computer, Dragon 32/64, Laser 200 and Acorn Atom among others. It is a relatively simple display generator compared to other display chips of the time. It is capable of displaying text and graphics...
VDG video chip
Video Display Controller
A Video Display Controller or VDC is an integrated circuit which is the main component in a video signal generator, a device responsible for the production of a TV video signal in a computing or game system...
(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
PAL
PAL, short for Phase Alternating Line, is an analogue television colour encoding system used in broadcast television systems in many countries. Other common analogue television systems are NTSC and SECAM. This page primarily discusses the PAL colour encoding system...
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
PEEK and POKE
In computing, PEEK is a BASIC programming language extension used for reading the contents of a memory cell at a specified address. The corresponding command to set the contents of a memory cell is POKE.-Statement syntax:...
) for bytes and words (4 bytes). Assembly code
Assembly language
An assembly language is a low-level programming language for computers, microprocessors, microcontrollers, and other programmable devices. It implements a symbolic representation of the machine codes and other constants needed to program a given CPU architecture...
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
Local area network
A local area network is a computer network that interconnects computers in a limited area such as a home, school, computer laboratory, or office building...
, Econet
Econet
Econet was Acorn's low-cost local area network system, intended for use by schools and small businesses. Econet is rumoured to be an abbreviation of Economy Network, but Acorn were always careful to stress the Greek root, oikos, meaning "house"....
, was first configured on the Atom.
The case was designed by industrial designer Allen Boothroyd
Allen Boothroyd
Allen Boothroyd is an industrial designer. He trained as a mechanical engineer and went on to study industrial design at the Royal College of Art....
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.| Hex Address | Contents | Annotations |
|---|---|---|
| 0000 | Block Zero RAM | 1KB 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 | Internal RAM 5KB max. |
| 2900 | Extension Text space RAM | |
| 3C00 | Off-board Extension RAM | |
| 8000 | 8000-01FF for mode 0 (512 bytes text) | Video and BASIC RAM 6KB max. |
| 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) | |
| 9800 | ||
| 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 |
Specifications
- CPUCentral processing unitThe central processing unit is the portion of a computer system that carries out the instructions of a computer program, to perform the basic arithmetical, logical, and input/output operations of the system. The CPU plays a role somewhat analogous to the brain in the computer. The term has been in...
: MOS Technology 6502MOS Technology 6502The MOS Technology 6502 is an 8-bit microprocessor that was designed by Chuck Peddle and Bill Mensch for MOS Technology in 1975. When it was introduced, it was the least expensive full-featured microprocessor on the market by a considerable margin, costing less than one-sixth the price of... - Speed: 1 MHz
- RAM: 2 KB, expandable to 12 KB
- ROMRead-only memoryRead-only memory is a class of storage medium used in computers and other electronic devices. Data stored in ROM cannot be modified, or can be modified only slowly or with difficulty, so it is mainly used to distribute firmware .In its strictest sense, ROM refers only...
: 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 (CUTSCutsCuts is an American sitcom that aired on the UPN network from February 14, 2005, to May 11, 2006, and is a spin-off of another UPN series, One on One...
) interface, TV connector, CentronicsCentronicsCentronics Data Computer Corporation was a pioneering American manufacturer of computer printers, now remembered primarily for the parallel interface that bears its name.-The beginning:Centronics began as a division of Wang Laboratories...
parallel printer - Storage: Kansas City standardKansas City standardThe Kansas City Standard , or Byte standard, is a digital data format for audio cassette drives. Byte magazine sponsored a symposium in November 1975 in Kansas City, Missouri to develop a standard for storage of digital computer data on inexpensive consumer quality cassettes, at a time when...
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.