ACT apricot - Page 73

CRT CONTROL
The data accessed from the Screen RAM and System
RAM is dependent
on the mode of operation selected.
In
text mode, the Screen RAM is programmed with 16-bit
words, each
word consisting of a II-bit font pointer and a
5-bit
attribute code. The font pointer specifies the character
for display
and is combined with the raster address from the
CR TC to access each row of the character stored in the
System RAM. The attribute code is supplied to the video
interface
to determine the display attributes of the character
accessed from
the System RAM.
bit 15
11 10
o
l
attributes I ~ont pointer I l6-Bit Word in
. . - Screen RAM
font pointer + raster address = font cell address
The character data in the System RAM is stored in a
character font consisting of 256 characters. Each character
consists of
32 bytes in contiguous memory locations and is
organised as
16 rows of 16-bit words (defined as a font cell).
Within each row of the font cell, only the 10 LSB comprise
the actual character. The 6 MSB are not displayed on the
screen.
The 10 character bits from each row are latched into the
video shift register, converted into serial format and then
shifted out to the video interface circuitry at the dot clock
frequency.
The video interface modifies the serial data according to
the attribute code (e.g. inverts the serial data stream, if
reverse video is selected),
and then supplies the video signal
to the Display Unit.
In graphics niode, the Screen RAM is programmed with
16-bit words, of which the 11 LSB are a graphics pointer.
The remaining 5-bits of each word are redundant, since
attributes are not used in the graphics mode. The graphics
pointer is combined with the raster address from the CR TC
to access each row of the 16-bi t graphics cell image written
into the System RAM.