TIA 1A - TELEVISION INTERFACE ADAPTOR (MODEL 1A)
4. Horizontal Position Counters
A. Description
The playfield is a fixed graphics register, always starting
its serial output when triggered by the beginning of each
television line. This chip also includes five "moveable"
graphics registers, whose serial outputs are triggered by
five separate horizontal position counters every time these
counters pass through zero count. These position counters
are clocked continuously during the unblanked portion of
every horizontal line and their count length is exactly
equal to the normal number of clocks supplied to them
during this time. They will therefore pass through zero at
the same time during each horizontal television line and
the triggered outputs will have no horizontal motion. A
typical horizontal counter is shown in figure 4.
If extra clocks are supplied to these counters (or normal
clocks suppressed) the zero crossing time will shift and
the object will have moved left (extra clocks) or right
(fewer clocks). Some position counters have extra decoders
(in addition to a zero decode) to trigger multiple copies
of the same object across a horizontal line.
All position counters can be reset to zero count by the
microprocessor at any time, by a write instruction to the
reset addresses (RESBL, RESM0, RESMl, RESP0, RESPl). If
reset occurs during horizontal blank, the object will
appear at the left side of the television screen. Properly
timed resets may position an object at any horizontal
location consistent with the microprocessor cycle time.
B. Ball position Counter
The ball position counter has only the zero crossing decode
and therefore cannot trigger multiple copies of the ball
graphics.
C. Player Position Counters
Each player position counter has three decodes in addition
to the zero crossing decode. These decodes are controlled
by bits 0,1,2 of the number size control registers (NUSIZ0,
NUSIZ1), and trigger 1,2, or 3 copies of the player (at
various spacings) across a horizontal line as shown on page
___. These same control bits are used for the decodes on
the missile position counter, insuring an equal number of
players and missiles.
D. Missile Position Counters
Missile position counters are identical to player position
counters except that they have another type of reset in
addition to the previously discussed horizontal position
reset. These extra reset addresses (RESMP0, RESMP1) write
data bit 1 into a one bit register whose output is used to
position the missile (horizontally) directly on top of its
corresponding player, and to disable the missile serial
output.