3. Playfield graphics Register

A. Description

Objects such as walls, clouds, and score) which are not required to move, are written into a 20 bit register called the playfield register. This register (Figure 5) is loaded from the data bus by three separate write addresses (PF0, PFl, PF2). Playfield may be loaded at any time. To clear the playfield, zeros must be written into all three addresses.

B. Normal Serial Output

The playfield register is automatically scanned (and converted to serial output) by a bi-directional shift register clocked at a rate which spreads the twenty (20) bits out over the left half of a horizontal line. This scanning is initiated by the end of horizontal blank (left edge of television screen). Normally the same scan is then repeated, duplicating the same twenty (20) bit sequence over the right half of the horizontal line.

C. Reflected Serial Output

A reflected playfield may be requested by writing a one into bit zero of the playfield control register (CTRLPF). When this bit is true the scanning shift register will scan the opposite direction during the right half of the horizontal line, reversing the twenty (20) bit sequence.

D. Timing Constraints

Even though the playfield bytes (PF0, PFl, PF2) may be written to any time, if one of them is changed while being serially scanned, part of the new value may both show up on the television horizontal line.

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