In the Acorn Atom, the MC6847 CRTC (VDG) is fed with only eight bits of data, but to do full justice to the MC6847 four extra bits might be used for the mode control lines INT/EXT, INV, A/S and CSS , which than can be changed on character-by-character basis.
The pin numbers and functions of these control lines are:
- Pin 31, INT/EXT = Internal/External character generator (also used for Four or Six block Semi-graphic mode)
Pin 32, INV = Inverting Alphanumerics
Pin 34, A/S = Alphanumeric/Semi-graphic mode
Pin 39, CSS = Colour Set Select (all modes except SG4)
Nowhere in this universe can I find any information about the use of such a twelve-bit (three nibble) input to the MC6847, other than in the MC6847 datasheet.
Does anyone know of any application (schematic) where these control lines of the MC6847 are connected to some extra data lines ?
Does anyone have any idea how (in what order) these inputs best be connected to an extra (high) nibble (D8, D9, D10, D11) ?
(There are 4!=24 possible ways to connect them.)
If there is/was no official application using these, what would be a favourable way/order to connect them ?
Information/documentation and ideas/suggestions about such a twelve-bit input to the MC6847 (in any system, not only Acorn) are more than welcome
