ACT apricot - Page 70

CRT CONTROL
As the electron beam scans across the screen (i.e. during
the active line period), the CRT circuit supplies a serial
video signal
to the Display Unit. The video signal
modulates the electron beam to produce the display
information on the screen.
The method of generating the video signal is detailed in
the following paragraphs.
The CRTC generates addresses (refresh addresses and
raster addresses) during the active scan line period, which
access data stored in the two areas of RAM. The same
sequence of addresses are automatically repeated at the
screen refresh rate (i.e. frame rate).
The CR TC addresses also map the text character/graphics
cell
position on the screen. Each text character on the
screen is bounded wi thin a 10 by 16 pixel character cell. A
graphics cell is defined as
an area 16 pixels wide by 16 pixels
high
on the screen.
The first refresh address generated by the CR TC at the
start of each new frame period (i.e. after the vertical retrace
period), corresponds
to the first character/graphics cell
position
on the screen; the second refresh address
corresponds to
the second cell position on the screen, etc.,
as
illustrated in Figures 2 and 3.
The refresh address changes every cell period during the
active scan line period, but repeats the same sequence of
addresses over 16 adjacent scan lines.
The raster address
changes every scan
line but repeats the same sequence every
16
scan lines and defines the row of each character/graphics
cell displayed
on the screen.