The Acorn Atom

The Acorn Atom

The Atom was Acorn's first foray into the new consumer computer market.  It was heavily based on the System 2/3 computers but with everything on a single board, integrating at least five Eurocards and a keyboard.  The architecture was very similar to the System 2/3, a significant deviation was with the video which used the cheaper 6847 CRTC and shared memory with the processor, the keyboard was scanned by software rather than using a dedicated interface IC.  The OS and BASIC software were derived from System COS and BASIC with changes to accommodate the hardware (video) differences.  A disk interface with DOS, Atom Disk Pack, and an Econet interface with NFS were available again derived from the System versions.

My Atom was purchased back in the early 1980's and at that time I disassembled the ROMs, more recently I have converted them to .ca65 format (updated December 2020).

Atom COS


Atom BASIC is essentially System BASIC with an additional ~2K of code which includes the disassembler.  I have produced a single ca65 source code which covers both the System and Atom variants.

The Atom has an extension ROM socket (IC21) which can accept either a BASIC Floating Point Extension or ONLIBASIC Extension both of which are similar to the original System versions. 


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