| Offset | Register |
| 00 | Data register B |
| 01 | Data register A |
| 02 | Data register B |
| 03 | Data register A |
| 04 | Timer 1 low counter / latch |
| 05 | Timer 1 high counter |
| 06 | Timer 1 low latch |
| 07 | Timer 1 high latch |
| 08 | Timer 2 low counter / latch |
| 09 | Timer 2 high counter |
| 0A | Shift register |
| 0B | Auxiliary control register |
| 0C | Peripheral control register |
| 0D | Interrupt flag register |
| 0E | Interrupt enable register |
| 0F | Data register A |
Address = Base + Offset
Base == B800 in Atom mode.
Base == 7800 in BBC Micro mode.