Commodore Amiga A1000, Hardware Reference Manual

Page: 130 / 380

SPRITE HARDWARE DETAILS
Sprites are produced by the circuitry shown in Figure 4-13. This Figure shows in block
form how a pair of data words becomes a set of pixels displayed on the screen.
The circuitry elements for sprite display are explained below.
o Sprite data registers. The registers SPRxDATA and SPRxDATB hold the bit patterns that
describe one horizontal line of a sprite for each of the eight sprites. A line is 16 pixels
wide, and each line is defined by two words to provide election of three colors and
transparent.
o Parallel-to-serial converters. Each of the 16 bits of the sprite data bit pattern is
individually sent to the color select circuitry at the time that the pixel associated with that
bit is being displayed on-screen.
Immediately after the data is transferred from the sprite data registers, each parallel-to-
serial converter begins shifting the bits out of the converter, most significant (leftmost) bit
first. The shift occurs once during each low-resolution pixel time and continues until all 16
bits have been transferred to the display circuitry. The shifting and data output does not
begin again until the next time this converter is loaded from the data registers.
Because the video image is produced by an electron beam that is being swept from left to
right on the screen, the bit-image of the data corresponds exactly to the image that
actually appears on the screen (most significant data on the left).
o Sprite serial video data. Sprite data goes to the priority circuit to establish the priority
between sprites and playfields.
o Sprite position registers. These registers, called SPRxPOS, contain the horizontal
position value (X value) and vertical position value (Y value) for each of the eight sprites.
o Sprite control registers. These registers, called SPRxCTL, contain the stopping position
for each of the eight sprites and whether or not a sprite is attached.
o Beam counter. The beam counter tells the system the current location of the video
beam that is producing the picture.
o Comparator. This device compares the value of the beam counter to the Y value in the
position register SPRxPOS. If the beam has reached the position at which the leftmost
upper pixel of the sprite is to appear, the comparator issues a load signal to the serial-to-
parallel converter and the sprite display begins.
- Sprite Hardware 121 -