TIA 1A - TELEVISION INTERFACE ADAPTOR (MODEL 1A)
8.5 Priority Encoder
A. Purpose
As discussed in the section on collisions, simultaneous
serial outputs from the graphics registers represent
overlap on the television screen. In order to have color-
luminosity values assigned to individual objects it is
necessary to establish priorities between objects when
overlapped. The priority encoder is shown in figure 3.
The lack of any objects results in a color-lum value called
the background. The background (BK) has lowest priority and
only appears when no objects are outputing. In order to
simplify the logic, each missile is given the same color-
lum value and priority as it's corresponding player (P0,
M0) and the ball is given the same color-lum value and
priority as the playfield (PF, BL).
The following table illustrates the normal priority
assignment:
Highest Priority P0, M0
Second Highest P1, M1
Third Highest PF, BL
Lowest Priority BK
Objects with higher priority will appear to move in front
of objects with lower priority. Players will therefore move
in front of playfield (clouds, walls, etc.).
There are two playfield control bits that affect priority,
one called playfield priority (PFP) (bit 2 of CTRLPF) and
one called score (bit 1 of CTRLPF). When a one is written
into the PFP bit the priority assignment is modified as
shown below.
Highest Priority PF, BL
Second Highest P0, M0
Third Highest P1, M1
Lowest Priority BK
Players will then move behind playfield (clouds, wall,
etc.). When a one is written into the score control bit,
the playfield is forced to take the color-lum of player 0
in the left half of the screen and player 1 in the right
half of the screen. This is used when displaying score and
identifies the score with the correct player. The priority
encoder produces 4 register select lines shown in figure 3)
that are mutually exclusive. These 4 lines select either
background, player 0, player 1 or playfield, and only one
of them can be true at a time.