Keyboard Links
The BBC Model B (and B+) features a set of eight links as part of the keyboard interface. These links control certain behaviours of the OS or DFS, described in the Advanced User Guide, page 274.
Default filing system
Bit 7 selects the default filing system, either NFS (when set) or DFS (when clear).
Disc drive timing
Bit 5 and bit 4 control disc drive timings, depending on the type of floppy disc controller. BeebEm currently ignores these settings.
| Bit 5 | Bit 4 | 8271 Timings | 1770 Timings | 1772 Timings |
|---|---|---|---|---|
| 0 | 0 | Step: 24 ms, settle: 20 ms, head load: 64 ms | Step: 30 ms, settle: 30 ms | Step: 3 ms, settle: 15 ms |
| 0 | 1 | Step: 6 ms, settle: 50 ms, head load: 32 ms | Step: 20 ms, settle: 30 ms | Step: 2 ms, settle: 15 ms |
| 1 | 0 | Step: 6 ms, settle: 16 ms, head load: 0 ms | Step: 12 ms, settle: 30 ms | Step: 12 ms, settle: 15 ms |
| 1 | 1 | Step: 4 ms, settle: 16 ms, head load: 0 ms | Step: 6 ms, settle: 30 ms | Step: 6 ms, settle: 15 ms |
Reverse Shift + Break action
Bit 3 reverses the action taken on Shift + Break, i.e., if this bit is set (ticked), pressing Break will cause DFS to boot from floppy disc.
Screen mode
Bits 0 to 2 select the default screen mode enabled on start up. Set these bits as shown below to select a given screen mode.
| Bit 2 | Bit 1 | Bit 0 | Description |
|---|---|---|---|
| 0 | 0 | 0 | Mode 7 |
| 0 | 0 | 1 | Mode 6 |
| 0 | 1 | 0 | Mode 5 |
| 0 | 1 | 1 | Mode 4 |
| 1 | 0 | 0 | Mode 3 |
| 1 | 0 | 1 | Mode 2 |
| 1 | 1 | 0 | Mode 1 |
| 1 | 1 | 1 | Mode 0 |