Atom Memory Map

Range Size

In Atom Mode

  

In BBC mode
0000-
1000-1FFF
8K

The Atom has
the bottom
32K free for
on-board RAM
and external expansion
(details)

 RAM

The BBC mode only has room for
16K RAM, due to the re-mapping.
2000
3000		 8K RAM

Optional RAM
4000
5000		 8K RAM

Video RAM
Only the bottom 6K is used
6000-6FFF 4K ROM

Atom Utility (IC24)
7000-7FFF 4K I/O

I/O Block
8000
9000-9FFF		 8K

Video RAM
Only the bottom 6K is used

  

BBC BASIC (16K)
A000-AFFF 4K

Atom Utility ROM (IC24)

  
B000-BFFF 4K

I/O Block

  
C000-CFFF 4K

Atom BASIC ROM (Bottom of IC20)

  

BBC MOS extension
D000-DFFF 4K

Floating Point Extension ROM (IC21, optional)
E000-EFFF 4K

Atom DOS ROM (external, optional)
F000-FFFF 4K

Assembler and COS ROM (Top of IC20)

  

Atom to BBC MOS

In the BBC Micro, the BBC BASIC interpreter resides in the third 16K block, and the display data resides in the BBC's RAM in the bottom 32K. The BBC Basic conversion board effectively swaps the Atom's middle two 16K blocks to look more like the BBC.

The BBC Expansion board plugged in to chip sockets on the Atom PCB, which was upside down. The sockets did not grip well and the weight of the expansion board tended to pull it out, and reduce reliability.

Atom I/O Block

Base+000 1K 8255 for system
Base+400 1K Good place to map a second 8255
Base+800 1K 6522 for Atom printer or User I/O
Base+C00 1K Good place to map a second 6522