Displaying A Sprite - Commodore Amiga Hardware Reference Manual

End-or-data Words
When the vertical position of the beam counter is equal to the VSTOP value in the
sprite control words, the next two words fetched from the sprite data structure are writ-
ten into the sprite control registers instead of being sent to the color registers. These
two words are interpreted by the hardware in the same manner as the original words
that were first loaded into the control registers. If the VSTART value contained in
these words is lower than the current beam position, this sprite will not be reused in this
display field. For consistency, the value 0 should be used for both words when ending
the usage of a sprite. Sprite reuse is discussed later.
The following data structure is for the spaceship sprite. It will be located at V
=
65
and H
= 128
on the screen.
SPRITE:
DC.W
DC.W
DC.'V
DC.W
DC.W
DC.W
DC.W
$6D60,$7200
$0990,$07EO
$13C8,$OFFO
$23C4,$lFF8
$13C8,$OFFO
$0990,$07EO
$0000,$0000
Displaying a Sprite
;VSTART, lISTART, VSTOP
;First pair of descriptor words
;End of sprite data
After building the data structure, you need to tell the system to display it. This section
describes the display of sprites in "automatic" mode. In this mode, once the sprite DMA
channel begins to retrieve and display the data, the display continues until the VSTOP
position is reached. Manual mode is described later on in this chapter.
The following steps are used in displaying the sprite:
1. Decide which of the eight sprite DMA channels to use.
2. Set the sprite pointers to tell the system where to find the sprite data.
3. Turn on sprite direct memory access if it is not already on.
104 Sprite Hardware