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The Acorn Atom was a home computer
Home computer

A home computer was a class of personal computer entering the market in 1977 and becoming common during the 1980s. They were marketed to consumers as accessible personal computers, more capable than video game consoles....
 made by Acorn Computers Ltd from 1980 to 1981 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 Random Access Memory, 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 integrated circuit design and Semiconductor device fabrication company based in Norristown, Pennsylvania, in the United States....
 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 central processing unit on the market by a considerable margin, costing less than one-sixth the price of competing designs from larger companies such...
 based machines that the company had been making from 1979.






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Acorn Atom Zx1
The Acorn Atom was a home computer
Home computer

A home computer was a class of personal computer entering the market in 1977 and becoming common during the 1980s. They were marketed to consumers as accessible personal computers, more capable than video game consoles....
 made by Acorn Computers Ltd from 1980 to 1981 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 Random Access Memory, 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 integrated circuit design and Semiconductor device fabrication company based in Norristown, Pennsylvania, in the United States....
 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 central processing unit on the market by a considerable margin, costing less than one-sixth the price of competing designs from larger companies such...
 based machines that the company had been making from 1979. The Atom was a cut-down Acorn System 3
Acorn System 3

The System 3 was a home computer produced by Acorn Computers Ltd from 1980. It was the successor to the Acorn System 2. It featured a disk drive....
 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

Kilobyte is a unit of Computer data storage equal to either 1,024 bytes or 1,000 bytes , depending on context.It is abbreviated in a number of ways: KB, kB, K and Kbyte....
 of RAM
Ram

Ram, ram, or RAM as a non-acronymic wordAs a non-acronymic word Ram, ram, or RAM may refer to:...
 and the floating point extension ROM
Read-only memory

Read-only memory is a class of computer storage media used in computers and other electronic devices. Because data stored in ROM cannot be modified , it is mainly used to distribute firmware ....
.

The minimum Atom had 2 KB of RAM
Ram

Ram, ram, or RAM as a non-acronymic wordAs a non-acronymic word Ram, ram, or RAM may refer to:...
 and 8 KB of ROM
Read-only memory

Read-only memory is a class of computer storage media used in computers and other electronic devices. Because data stored in ROM cannot be modified , it is mainly used to distribute firmware ....
, with a fully loaded machine having 12 KB of each. An additional floating point
Floating point

In computing, floating point describes a system for numerical representation in which a String of digits represents a rational number.The term floating point refers to the fact that the radix point can "float": that is, it can be placed anywhere relative to the Significant figures of the number....
 ROM
Read-only memory

Read-only memory is a class of computer storage media used in computers and other electronic devices. Because data stored in ROM cannot be modified , it is mainly used to distribute firmware ....
 was also available. The 12 KB of RAM
Ram

Ram, ram, or RAM as a non-acronymic wordAs a non-acronymic word Ram, ram, or RAM may refer to:...
 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.

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....
 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 Television Composite video 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 a color-encoding system used in broadcast television systems in large parts of the world. Other common analog television systems are SECAM and NTSC....
 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 function used for reading the contents of a memory cell at a specified memory address. The corresponding command to set the contents of a memory cell is POKE....
) for bytes and words (4 bytes). Assembly code
Assembly language

An assembly language is a low-level language for programming computers. It implements a symbolic representation of the numeric machine codes and other constants needed to program a particular CPU architecture....
 could be included within a BASIC program, because the BASIC interpreter also contained an Assembler
Assembly language

An assembly language is a low-level language for programming computers. It implements a symbolic representation of the numeric machine codes and other constants needed to program a particular CPU architecture....
 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 covering a small physical area, like a home, office, or small group of buildings, such as a school, or an airport....
, Econet
Econet

Econet was Acorn Computers Ltd'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 (from 1
Acorn Atom

The Acorn Atom was a home computer made by Acorn Computers Ltd from 1980 to 1981 when it was replaced by the BBC Micro and later the Acorn Electron....
). Shaded areas indicate those present on the minimal system.





























Hex
addr		Contents
0000Block Zero RAM
0400Teletext VDG RAM
0800VDG CRT Controller
0900 
0A00Optional FDC
0A80 
1000Peripherals space
2000Catalogue buffer
2200Sequential File buffers
2800Floating point variables
2900Extension Text space RAM
3C00Off-board Extension RAM
80008000-01FF for mode 0 (512 bytes text)Video RAM
8000-83FF for mode 1 (1 KB graphics)
8000-85FF for mode 2 (1.5 KB graphics)
8000-8BFF for mode 3 (3 KB graphics)
8000-97FF for mode 4 (6 KB graphics)
9800 
A000Optional Utility ROM
B000PPIA I/O Device
B800Optional VIA I/O Device for Printer Interface
C000ATOM BASIC Interpreter
D000Optional Extension ROM
E000Optional Disk Operating System
F000Assembler
Cassette Operating System


Specifications


  • CPU
    Central processing unit

    A central processing unit is an electronic circuit that can execute computer programs. This broad definition can easily be applied to many early computers that existed long before the term "CPU" ever came into widespread usage....
    : MOS Technology 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 central processing unit on the market by a considerable margin, costing less than one-sixth the price of competing designs from larger companies such...
  • Speed: 1 MHz
  • RAM: 2 KB, expandable to 12 KB
  • ROM
    Read-only memory

    Read-only memory is a class of computer storage media used in computers and other electronic devices. Because data stored in ROM cannot be modified , it is mainly used to distribute firmware ....
    : 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
    Cuts

    Cuts is an United States sitcom that aired on the United Paramount Network network from February 14, 2005, to May 11, 2006, and is a spin-off of another UPN series, One on One ....
    ) interface, TV connector, Centronics
    Centronics

    Centronics Data Computer Corporation was a pioneering American manufacturer of computer printers, now remembered primarily for the Centronics printer port that bears its name....
     parallel printer
  • Storage: Kansas City standard
    Kansas City standard

    The Kansas City standard , or Byte standard, is a digital data format for compact audio cassette drives. Byte Magazine sponsored a symposium in November 1975 in Kansas City, Missouri to develop a standard for storage of digital microcomputercomputer data on inexpensive consumer quality Compact Cassette, at a time when floppy dis...
     audio cassette interface
  • Power: 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 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 was no de-facto standard for external 5 V connection, but using the same 7-pin DIN connectors as the Atari 800XL allowed the Atari power supply to drive low-power (up to 1.5 A) Atoms.

External links