| Bits | Description |
|---|
| 7 | nextreg 0x44 second byte indicator |
| 6:4 | Display timing |
| 3 | User lock on display timing applied |
| 2-0 | Machine type |
| Bits | Description |
|---|
| 7 | 1 to allow changes to bits 6:4 |
| 6:4 | Selects display timing
affects port decoding and contention
000 = Internal Use
001 = ZX 48K display timing
010 = ZX 128K/+2 display timing
011 = ZX +2A/+2B/+3 display timing
100 = Pentagon display timing (changes to 50 Hz) |
| 3 | 1 to toggle user lock on display timing (hard reset = 0) |
| 2:0 | Selects machine type (config mode only)
determines roms loaded
000 = Configuration mode
001 = ZX 48K
010 = ZX 128K/+2
011 = ZX +2A/+2B/+3
100 = Pentagon |
Notes:
A write to this register disables the bootrom