Moving Objects Graphics Registers
There are five graphics registers for moving objects on this chip. These graphics registers are loaded (written) in parallel by the microprocessor and like the playfield register are scanned and converted to serial output. Unlike the playfield register, which is always scanned beginning at the left side of each horizontal line, moving object graphics registers are scanned only when triggered by a start decode from their horizontal position counter.
Missile Graphics
The graphics registers for both missiles are identical and very simple. They each consist of a one bit register called missile enable (ENAM0, ENAM1). This graphics bit is scanned (outputted) only when triggered by its corresponding position counter. There are control bits (bits D4, D5, of NUSIZ0, NUSIZ1) that can stretch this single graphics bit out over widths of 1, 2, 4, or 8 clocks of horizontal line time. (A full line is 160 clocks).
Player Graphics
The graphics registers for both players are identical and are rather complex. They each consist of eight bit parallel registers (GRP0, GRP1) and a bidirectional parallel to serial scan counter that converts the parallel data into serial output. A one bit control register (REFP0, REFP1) determines the direction (reflection) of the parallel to serial scan, outputting either D7 through D0, or D0 though D7. This allows reflection (horizontal flipping) of player serial graphics data without having to flip the microprocessor data. The clock into the scan counter can be controlled (three bits of NUSIZ0 and NUSIZ1) to slow the scan rate and stretch the eight bits of serial graphics out over widths of 8, 16, or 32 clocks of horizontal line time. These same control bits are used in the player-missile motion counters to control multiple copies, so only three player widths (scan rates) are available.
Vertical Delay
Each of the player graphics registers actually consists of two 8 bit parallel registers. The first (GRP0, GRP1) is loaded (written) from the microprocessor 8 bit data bus. The second is automatically loaded from the output of the first. The reason for this is a complex subject called vertical delay. A large amount of microprocessor time is required to generate player, missile and playfield graphics (table look up, masking, comparisons, etc.) and load these into this chip's registers. For most game programs this time is just too large to fit into one horizontal line time. In fact for most games it will barely fit into two line times (127 microseconds). Therefore, individual graphics registers are loaded (written) every two lines, and used twice for serial output between loads. This type of programming will obviously limit the vertical height resolution of objects to multiples of two lines. It also will limit the resolution of vertical motion to two lines jumps.
Nothing can be done about the vertical height resolution; however, vertical motion can be resolved to a single line by addition of a second graphics register that is automatically parallel loaded from the output of the first, one line time after the first was loaded from the data bus. This second graphics register output is therefore always delayed vertically by one line. A control bit called vertical delay (VDEL0, VDEL1) selects which of these two registers is to be used for serial output. If this control bit is set by the microprocessor between picture frames, the object will be moved down (delayed) by one line during the next frame.
In most programming applications player 0 graphics and player 1 graphics are loaded (written) alternately, during the blank time just prior to each line. Since GRP0 and GRP1 addresses from the microprocessor alternate, they are delayed by one line from each other. The GRP0 address decode can therefore be used to load the delayed graphics register for player 1, and GRP1 likewise to load the delayed graphics register for player 0. The two vertical delay bits (VDEL0, VDEL1 then select delayed or un-delayed registers for player 0 and player 1 as serial outputs.
Ball Graphics
The ball graphics register is almost identical to the missile graphics register. It also consists of a single enable bit ENABL whose output is triggered by the ball position counter. It also has two control bits (bits D4, D5 of CTRLPF that can stretch this single graphics bit out over widths of 1, 2, 4, or 8 clocks of horizontal line time. Unlike the missile graphics; however, the ball graphics register has capability for vertical delay similar to the player graphics. A second graphics (enable) bit is alternately loaded from the output of the first, one line after the first was loaded from the data bus.
A ball vertical delay bit VDELBL selects which of these two graphics bits is used for the ball serial output. The first graphics bit ENABL should be loaded during the same horizontal blank time as player 0 GRP0, because GRP1 is used to load the second enable bit from the output of the first on alternate lines.
Collision Detection Latches
Definitions
The serial outputs from all the graphics registers represent real time horizontal location of objects on the television screen. If any of these outputs occur at the same time, they will overlap (collide) on the screen. There are six objects generated on this chip (five moving and playfield) allowing fifteen possible two object collisions. These overlaps (collisions) are detected by fifteen "and" gates whenever they occur, and are stored in fifteen individual latch register bits. For details see Collision Registers.
Related
see TIA