ACT apricot - Mode Selection

CRT CONTROL
bit 15 1110 o
I not used I graphics pointer I
graphics + raster
pointer address
16-Bi t
Word in
Screen RAM
graphics cell
image address
Each graphics cell image consists of 32 bytes of data in
contiguous memory locations and is organised as 16 rows of
16-bit words. Each pixel
on the screen is mapped directly by
a corresponding
bit within each 16-bit row of the graphics
cell image.
The full 16-bits from each row are latched into the video
shift register,
converted into serial format and then shifted
out to the video interface at the dot clock frequency. The
video interface then supplies the video signal to the Display
Unit.
Unlike the text mode, where the Screen RAM has to be
updated
with a new font pointer and attribute bits to specify
a different
character for display on the screen, the graphics
pointers
remain unchanged once programmed into the
Screen RAM. Changing the display on the screen in graphics
mode is achieved
by updating the image of the graphics cell
stored
in the System RAM.
Mode Selection
Whether the circuitry operates in the text
(alphanumerics) or graphics mode depends on:
(a) The logic state on the mode select line alpha/
graphics from the Parallel Interface, which is directly
under software control.
(b) The initialisation of the CRTC.
(e) The data programmed into the Screen RAM.
(d) The data programmed into the System RAM.