ACT apricot - Graphics Mode

CRT CONTROL
The position of the selected character on the screen is
determined by two factors, the address location of the I6-bit
word in the Screen RAM and the start address programmed
into the CRTC. If the CRTC start address is programmed to
OOOOH, the first character displayed on the screen is the one
selected by
the font pointer stored at word address location
FOOOOH in the Screen RAM; the second character displayed
on the screen, the one selected by the font pointet: stored at
word address location F0002H; the third character, the one
selected by
the font pointer at F0004H, etc.
If the CRTC start address registers are programmed to a
different
value than OOOOH, the first character displayed on
the screen is selected by the pointer stored at the address in
the Screen RAM corresponding to FOOOOH + twice the
CR TC start address; the second character displayed,
selected
by pointer stored at the Screen address FOOOO +
twice the CRTC start address + 2, etc.
For example,
if the start address is programmed to 0050H
(decimal 80), the first character displayed on the screen is
specified by
the pointer, stored at Screen RAM address
FOOAOH, the second character is specified by the pointer
stored at Screen RAM address FOOA2H, etc. Due to the
cyclical nature of the generation of the refresh addresses
from
the CR TC, the last character (the 2000th.) displayed
on the screen, is specified by the pointer stored at Screen
RAM address
F009EH.
This example illustrates how scrolling the screen
upwards
by one text line is achieved. If the CR TC start
address is incremented by SOH, the text line originally the
second line is moved to the top of the screen, and the text
line originally the first text line is moved to the bottom of
the screen. New parameters (pointers and attribute bits)
then can be issued to the original first text line address
locations, as required.
Graphics Mode
In the graphics mode the first 1250 word address locations
in the Screen RAM are initialised with 16 bit.words, of
which the 11 least significant bits (LSB) are the II-bit
graphics pointer. The 5 most significant bits (MSB) of the