bbc frame buffer

[rogerjowett]

40/80 column text mode and teletext
couldnt they be tricked into doing hi rez like the 81 was able?

[Robert Newson]

On 22/12/11 16:23, rogerjowett wrote:
> 40/80 column text mode and teletext
> couldnt they be tricked into doing hi rez like the 81 was able?

In Mode 7 (Teletext), the 1K RAM was scanned and sent to a Teletext
character generator chip which created the output

In the other two "text" modes (Mode 6 & 3) the memory was sent
"directly" to the output device by the 6845 - so they were hi-res.
However, the 6845 was set up so that after 8 lines of memory scan being
sent two blank lines were sent (on which the 6845 provided the blinking
cursor).

The 6845 could be tweaked once in MODE 6 & 3 so that it didn't send the
two blank lines and the effect was that the screen would be squashed
vertically, but all the memory locations would then be next to each
other and lines that were drawn would be continuous - I can't say if the
normal PLOT, etc commands would work, but by directly writing to the
screen memory lines, etc could be drawn.

Similarly it is possible to set up the 6845 to do odd modes, like MODE 2
using 10K of memory by making the screen only half the width (in terms
of characters) - I'm not sure (ie I can't remember) if you can set it up
fully so that PLOT, etc work properly with the new mode.

[jgharston]

rogerjowett wrote:
> 40/80 column text mode and teletext
> couldnt they be tricked into doing hi rez like the 81 was able?

80-column mode (MODE 0 and MODE 3) *is* high-res.

JGH