Acorn Atom

Acorn Atom
	The Atom was Acorn's first computer to be aimed squarely at the home market.
Manufacturer	Acorn Computers
Type	Personal Computer
Release date	1980
Introductory price	£120 (in kit form), £170 (assembled)
Discontinued	1983
Media	100KB 5¼-inch floppy disks, Cassette tapes
CPU	MOS Technology 6502 clocked at 1MHz
Memory	2 KB RAM (expandable to 12 KB), 8 KB ROM (expandable to 12 KB)
Display	64×64 (4 colours), 64×96 (4 colours), 128×96 (monochrome), 64×192 (4 colours), 128×192 (2 colours), 256×192 (monochrome)
Input	Keyboard
Power	8V, 1.5A unregulated DC, 5V regulated inside.
Dimensions	381×241×64 mm
Predecessor	Acorn System 3
Successor	BBC Micro

The Acorn Atom is a home computer made by Acorn Computers Ltd from 1980 to 1982, when it was replaced by the BBC Micro. The Micro began life as an upgrade to the Atom, originally known as the Proton.

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 (equivalent to £734 in 2019) 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 the maximum specification machine having 12 KB of each. An additional floating point ROM was also available. The 12 KB of RAM was divided between 1 KB for the zero page, 5 KB available for programs, and 6 KB for the high resolution graphics. The zero page was used by the CPU for stack storage, by the OS, and by the Atom BASIC for 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 an MC6847 Video Display Generator (VDG) video chip, allowing for both text and 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. Since the MC6847 could only output at 60 Hz, meaning that the video could not be resolved on a large proportion of European TV sets, a 50 Hz PAL colour card was later made 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.

The manual for the Atom was called Atomic Theory and Practice and was written by David Johnson-Davies, subsequently Managing Director of Acornsoft. (The manual used the jargon 'pling' for exclamation mark, a term which may have originated at Acorn, and of which this may have been the first published usage.)

The Acorn LAN, Econet, was first configured on the Atom.

The case was designed by industrial designer Allen Boothroyd of Cambridge Product Design Ltd.

Atom BASIC

It had built-in BASIC, a fast but idiosyncratic version developed by Sophie Wilson, which included indirection operators (similar to PEEK and POKE) for bytes and words (of 4 bytes each). 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 unusual.

String handling was unique. A byte vector A() to Z() could be DIMensioned and then referred to with the string operator $A to be treated as a string. This sample program, adapted from Atomic Theory and Practice, demonstrates some of Atom BASIC's peculiarities:

    1 REM Encoder/Decoder
   10 S=TOP; ?12=0
   20 INPUT'"CODE NUMBER"T; REM Use code number to seed random number generator
   30 !8=ABS(T)
   40 INPUT'$S
   50 FOR P=S TO S+LEN(S); REM For each character, if it is a letter add the next random number to it, modulo 26.
   60 IF ?P<#41 GOTO 100
   70 R=ABS(RND)%26
   80 IF T<0 THEN R=26-R
   90 ?P=(?P-#41+R)%26+#41
  100 NEXT P
  110 PRINT $S
  120 GOTO 40

In late 1982, Acorn released an upgrade board 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 hardware 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 much more advanced graphics and sound hardware and greater RAM capacity. Commercial BBC Micro cassettes could not have been loaded anyway, as they ran at a transfer rate of 1200 baud and the Atom's cassette interface only supported 300 baud.

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	1 KB 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 5 KB max.
2900	Extension Text space RAM	Internal RAM 5 KB max.
3C00	Off-board Extension RAM
8000	8000-81FF for mode 0 (512 bytes text)	Video and BASIC RAM 6 KB max.
	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
A000	Optional Utility ROM
B000	PPIA I/O Device
B800	Optional VIA I/O Device for Printer Interface
C000	ATOM BASIC Interpreter	4 KB ROM
D000	Optional Extension ROM
E000	Optional Disk Operating System
F000	Assembler	4 KB ROM
F000	Cassette Operating System	4 KB ROM

Specifications

CPU: MOS Technology 6502
Speed: 1 MHz
RAM: 2 KB, expandable to 12 KB
ROM: 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) interface, TV connector, Centronics parallel printer
Storage: Kansas City standard audio cassette interface
Power: standard 2.1 mm power jack connector for 8 volts unregulated DC, providing 5 volts regulated inside the Atom

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 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 bypassed 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 has never been a de facto standard for external 5V 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 the model). These are now uncommon, but 5V wall-wart switch-mode power supplies capable of supplying several amps are a readily and cheaply available alternative.

External links

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