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.

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