Sprite Hardware Details - Commodore Amiga Hardware Reference Manual

You display sprites manually by writing to the sprite data registers SPRxDATB and
SPRxDATA, in that order. You write to SPRxDATA last because that address "arms"
the sprite to be output at the next horizontal comparison. The data written will then be
displayed on every line, at the horizontal position given in the "H" portion of the posi-
tion registers SPRxPOS and SPRxCTL. If the data is unchanged, the result will be a
vertical bar. If the data is reloaded for every line, a complex sprite can be produced.
The sprite can be terminated ("disarmed") by writing to the SPRxCTL register. If you
write to the SPRxPOS register, you can manually move the sprite horizontally at any
time, even during normal sprite usage.
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 pat-
terns 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 selection 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.
120 Sprite Hardware