MODE 5
MODE 5 is a low-resolution screen MODE, introduced on the BBC Micro Model A and supported by the entire BBC series including the Archimedes. It allows colour graphics to be shown while leaving 15 KB or more free for programs; however to achieve this there are only 20 character columns on screen.
In Elite and Revs a custom interrupt routine splits the screen in two; the bottom half is configured as MODE 5 to show the dashboard and add colour.
The MOS provided features that made it easy to simulate double buffering on the Model B. While one bitplane was being displayed, the other could be hidden (by setting it to the background colour), undrawn and redrawn. Battle Tank (Superior, 198?) is an example of a game that used this technique. Acorn went on to implement true double buffering in the Master and Archimedes.
| Type | Graphical ('ASCII') | |
| Resolution | Text | 20 × 32 characters |
| Graphics | 160 × 256 pixels | |
| Character size | 8 × 8 pixels, user definable | |
| Pixel aspect | 2:1 (wide) | |
| Bits per pixel | 2 | |
| Default palette | Logical colour | Actual colour |
|---|---|---|
| 0 | 0 (black) | |
| 1 | 1 (red) | |
| 2 | 3 (yellow) | |
| 3 | 7 (white) | |
| Memory | allocated | 10240 bytes |
| on screen | 10240 bytes | |
| Address range | Model B etc. | &5800..&7FFF |
| Configuration | Video ULA control register | &C4 |
| Video ULA palette register file | &07 &17 &26 &36 &47 &57 &66 &76 &84 &94 &A0 &B0 &C4 &D4 &E0 &F0 | |
| CRTC register file | &3F &28 &31 &24 &26 &00 &20 &22 &01 &07 &67 &08 (from table in MOS 1.20 at &C486..&C491) | |
-- beardo 14:39, 3 May 2007 (BST)
