| Range | Size | In Atom Mode |
In BBC mode |
|
| 0000- 1000-1FFF |
8K |
The Atom has |
RAM | The BBC mode only has room for |
| 2000 3000 |
8K | RAM | Optional RAM |
|
| 4000 5000 |
8K | RAM | Video RAM |
|
| 6000-6FFF | 4K | ROM | Atom Utility (IC24) |
|
| 7000-7FFF | 4K | I/O | 8255 and 6522 I/O chips |
|
| 8000 9000-9FFF |
8K | Video RAM |
BBC BASIC (16K) |
|
| A000-AFFF | 4K | Atom Utility ROM (IC24) |
||
| B000-BFFF | 4K | 8255 and 6522 I/O chips |
||
| C000-CFFF | 4K | Atom BASIC ROM (Bottom of IC20) |
BBC MOS extension |
|
| D000-DFFF | 4K | Floating Point Extension ROM (IC21, optional) |
Floating Point Extension ROM |
|
| E000-EFFF | 4K | Atom DOS ROM (external, optional) |
Atom DOS ROM |
|
| 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.