| ATOM mode | Address | BBC BASIC mode |
| RAM | 0000 | RAM (8K) |
| 2000 | RAM (8K OPTIONAL) | |
| 3C00 | ||
| 4000 | GRAPHICS RAM (6K) | |
| 5800 | ||
| 6000 | UTILITY ROM (4K) | |
| 7000 | I/O | |
| GRAPHICS RAM (6K) | 8000 | BBC BASIC (32K) |
| 9800 | ||
| UTILITY ROM (4K) | A000 | |
| I/O | B000 | |
| ATOM ROM (4K) | C000 | MOS EXTENSION ROM (4K) |
| EXTENSION ROM (4K) | D000 | EXTENSION ROM (4K) |
| OPTIONAL DOS (4K) | E000 | OPTIONAL DOS (4K) |
| ATOM ROM (4K) | F000 | MOS ROM (4K) |