# 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