The TIA (as seen by the programmer)
The PF register is used to create a playfield of walls,
clouds, barriers, etc., that are seldom moved. This low
resolution register is written into to draw the left half
of the TV screen only. The right half of the screen is
drawn by software selection of whether a duplication or a
reflection of the right half.
The PF register is 20 bits wide, so the 20 bits are written
into 3 addresses: PF0, PF1, and PF2. PF0 is only 4 bits
wide and constructs the first 4 bits of the playfield,
starting at the left edge of the TV screen. PF1 constructs
the next 8 bits, and PF2 the last 8 bits which end at
the center of the screen. The PF register is scanned from
left to right and where a 1 is found the PF color is
drawn, and where a 0 is found, the BK color is drawn. To
clear the playfield, obviously zeros must be written into
PF0, PF1, and PF2.
To make the right half of the playfield into a duplication
or copy of the left half, a 0 is written to D0 of the
CTLPF (control playfield) register. Writing a 1 will
cause the reflection to be displayed.