CHAPTER 4
SPRITE HARDWARE
INTRODUCTION
Sprites are hardware objects that are created and moved independently of the
playfield display and independently of each other. Together with playfields,
sprites form the graphics display of the Amiga. You can create more complex
animation effects by using the blitter, which is described in the chapter
called "Blitter Hardware." Sprites are produced on-screen by eight special-
purpose sprite DMA channels. Basic sprites are 16 pixels wide and any number
of lines high You can choose from three colors for a sprite's pixels, and a
pixel may also be transparent,
Showing any object behind the sprite. For larger or more complex objects, or
for more color choices, you can combine sprites.
- Sprite Hardware 93 -
Sprite DMA channels can be reused several times within the same display
field. Thus, you are not limited to having only eight sprites on the
screen at the same time.
ABOUT THIS CHAPTER
This chapter discusses the following topics:
o Defining the size, shape, color, and screen position of sprites.
o Displaying and moving sprites.
o Combining sprites for more complex images, additional width, or
additional colors.
o Reusing a sprite DMA channel multiple times within a display field to
create more than eight sprites on the screen at one time.
FORMING A SPRITE
To form a sprite, you must first define it and then create a formal data
structure in memory. You define a sprite by specifying its
characteristics:
o On-screen width of up to 16 pixels.
o Unlimited height.
o Any shape.
o A combinalion of three colors, plus transparent.
o Any position on the screen.
SCREEN POSITION
A sprite's screen position is defined as a set of X,Y coordinates.
Position (0,0), where X = 0 and Y = 0, is the upper left-hand corner of
the display. You define a sprite's location by specifying the coordinates
of its upper left-hand pixel. Sprite position is always defined as though
the display modes werc low-resolution and non-interlaced. The X,Y
coordinate system and definition of a sprite's position are graphically
represented in Figure 4-1. Notice that because of display overscan,
position (0,0) (that is, X = 0, Y = 0) is not normally in a viewable
region of the screen.
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(0,0)
\ Visible screen area
\_______________________ /
| | /
| ____________|____________/
| | Y |
| | | |
| | |______ |
|-------X--------| /\ | |
| | |__\/__| |
| | |
| | |
FIGURE 4-1: (Defining Sprite On-screen Position)
The amount of viewable area is also affected by the size of the playfield
display window (defined by the values in DDFSTRT, DDFSTOP, DIWSTRT, DIWSTOP,
etc.). See the "Playfield Hardware" chapter for more information about
overscan and display windows.
HORIZONTAL POSITION
A sprite's horizontal position (X value) can be at any pixel on the screen
from 0 to 447. To be visible, however, an object must be within the
boundaries of the playfield display window. In the examples in this chapter,
a window with horizontal positions from pixel 64 to pixel 383 is used (that
is, each line is 320 pixels long). Larger or smaller windows can be defined
as required, but it is recommended that you read the "Playfield
Hardware" chapter before attempting to do so. A larger area is actually
scanned by the video beam but is not usually visible on the screen.
If you specify an X value for a sprite that takes it outside the display
window, then part or all of the sprite may not appear on the screen. This is
sometimes desirable; such a sprite is said to be "clipped. "
To make a sprite appear in its correct on-screen horizontal position in the
display window, simply add its left offset to the desired X value. In the
example given above, this would involve adding 64 to the X value. For example,
to make the upper leftmost pixel of a sprite appear at a position 94 pixels
from the left edge of the screen, you would perform this calculation:
Desired X position + horizonal-offset of display window = 94 + 64 = 158
- Sprite Hardware 95 -
Thus, 158 becomes the X value, which will be written into the data structure.
NOTE
The X position represents the location of the very first (leftmost) pixel in
the full 16-bit-wide sprite. This is always the case, even if the leftmost
pixels are specified as transparent and do not appear on the screen.
If the sprite shown in Figure 4-2 were located at an X value of 158, the
actual image would begin on-screen four pixels later at 162. The first four
pixels in this sprite are transparent and allow the background to show
through.
SEE FIGURE 4-2: (Position of Sprites)
VERTICAL POSITION
You can select any position from line 0 to line 262 for the topmost edge of
the sprite. In the examples in this chapter, an NTSC window with vertical
positions from line 44 to line 243 is used. This allows the normal display
height of 200 lines in non-interlaced mode. If you specify a vertical position
(Y value) of less than 44 (i.e, above the top of the display window) the top
edge of the sprite may not appear on screen.
To make a sprite appear in its correct on-screen vertical position, add the Y
value to the desired position. Using the above numbers, add 44 to the desired
Y position. For example, to make the upper leftmost pixel appear 25 lines
below the top edge of the screen, perform this calculation:
Desired Y position + vertical-offset of the display window = 25 + 44 = 69
Thus, 69 is the Y value you will write into the data structure.
- 96 Sprite Hardware -
CLIPPED SPRITES
As noted above, sprites will be partially or totally clipped if they pass
across or beyond the boundaries of the display window. The values of 64
(horizontal) and 44 (vertical) are "normal" for a centered display on a
standard NTSC video monitor. See Chapter 3, "Playfield Hardware", for
more information on display offsets. Information on PAL displays will be
found there. If you choose other values to establish your display window,
your sprites will be clipped accordingly.
SIZE OF SPRITES
Sprites are 16 pixels wide and can be almost any height you wish... as
short as one line or taller than the screen. You would probably move a
very tall sprite vertically to display a portion of it at a time.
Sprite size is based on a pixel that is 1/320th of a screen's width,
1/200th of a NTSC screen's height, or 1/256 of a PAL screen's height.
This pixel size corresponds to the low-resolution and non-interlaced
modes of the normal full-size playfield. Sprites, however, are
independent of playfield modes of display, so changing the resolution or
interlace mode of the playfield has no effect on the size or resolution
of a sprite.
SHAPE OF SPRITES
A sprite can have any shape that will fit within the 16-pixel width. You
define a sprite's shape by specifying which pixels actually appear in
each of the sprite's locations. For example, Figures 4-3 and 4-4 show a
spaceship whose shape is marked by Xs. The first figure shows only the
spaceship as you might sketch it out on graph paper. The second figure
shows the spaeeship within the 16-pixel width. The 0s around The
spaceship mark the part of the sprite not covered by the spaceship and
transparent when displayed.
x x
x x x x x
x x x x x x x x x x
x x x x x x x x x x
x x x x x x
x x
FIGURE 4-3: Shape of Spaceship
- Sprite Hardware 97 -
o o o o x x o o o o o o o o o o
o o x x x x x x o o o o o o o o
x x x x x x x x x x o o o o o o
x x x x x x x x x x o o o o o o
o o x x x x x x o o o o o o o o
o o o o x x o o o o o o o o o o
FIGURE 4-4: Sprite with Spaceship Shape Defined
In this example, the widest part of the shape is ten pixels and the shape is
shifted to the left of the sprite. Whenever the shape is narrower than the
sprite, you can control which part of the sprite is used to define the shape.
This particular shape could also start at any of the pixels from 2-7 instead
of pixel 1.
SPRITE COLOR
When sprites are used individually (that is, not "attached" as described
under "Attached Sprites" later), each pixel can be one of three colors or
transparent. Colors are selected in much the same manner as playfield colors.
Figure 4-5 shows how the color of each pixel in a sprite is determined.
- 98 Sprite Hardware -
SEE FIGURE 4-5: (Sprite Color Definition)
The 0s and 1s in the two data words that define each line of a sprite in the
data structure form a binary number. This binary number points to one of the
four color registers assigned to that particular sprite DMA channel. The eight
sprites use system color registers 16 - 31. For purposes of color selection,
the eight sprites are organized into pairs and each pair uses four of the
color registers as shown in Figure 4-6.
NOTE
The color value of the first register in each group of four registers is
ignored by sprites. When the sprite bits select this register, the
"transparent" value is used.
- Sprite Hardware 99 -
Codes 01,10,or 11 select one of three possible registers from the normal
color register from the normal color register group, from which the
actual color data is taken.
COLOR REGISTER SET
_________________________
__ | Unused | 16
| 00 |_________________________| \
Sprite 0 or 1 | 01 |_________________________| \
| 10 |_________________________| \
|__ 11 |_________________________| \
__ | Unused | 20 \
| 00 |_________________________| \ \
Sprite 2 or 3 | 01 |_________________________| \ \
| 10 |_________________________| \ \
|__ 11 |_________________________| \ \
__ | Unused | 24 ---------> Yeilds
| 00 |_________________________| / / Trans-
Sprite 4 or 5 | 01 |_________________________| / / parent
| 10 |_________________________| / /
|__ 11 |_________________________| / /
__ | Unused | 28 /
| 00 |_________________________| /
Sprite 6 or 7 | 01 |_________________________| /
| 10 |_________________________| /
|__ 11 |_________________________| 31 /
FIGURE 4-6: (Color Register Assignments)
If you require certain colors in a sprite, you will want to load the sprite's
color registers with those colors. The "Playfield Hardware" chapter contains
instructions on loading color registers.
The binary number 00 is special in this color scheme. A pixel whose value is
00 becomes transparent and shows the color of any other sprite or playfield
that has lower video priority. An obJect with low priority appears "behind"
an object with higher priority. Each sprite has a fixed video priority with
respect to all the other sprites. You can vary the priority between sprites
and playfields. (See Chapter 7, "System Control Hardware," for more
information about sprite priority.)
- 100 Sprite Hardware -
DESIGNING A SPRITE
For design purposes, it is convenient to lay out the sprite on paper first.
You can show the desired colors as numbers from 0 to 3. For example, the
spaceship shown above might look like this:
0000122332210000
0001223333221000
0012223333222100
0001223333221000
0000122332210000
The next step is to convert the numbers 0-3 into binary numbers, which will
be used to build the color descriptor words of the sprite data structure.
The section below shows how to do this.
BUILDING THE DATA STRUCTURE
After defining the sprite, you need to build its data structure, which is a
series of 16-bit words in a contiguous memory area. Some of the words contain
position and control inforsnation and some contain color descriptions. To
create a sprite's data structure, you need to:
o Write the horizontal and vertical position of the sprite into the first
control word.
o Write the vertical stopping position into the second control word.
o Translate the decimal color numbers 0 - 3 in your sprite grid picture into
binary color numbers. Use the binary values to build color descriptor (data)
words and write these words into the data structure.
o Write the control words that indicate the end of the sprite data structure.
NOTE
Sprite data, like all other data accessed by the custom chips, must be loaded
into Chip RAM. Be sure all of your sprite data structures are word aligned in
Chip Memory.
Table 4-1 shows a sprite data structure with the memory location and function
of each word:
- Sprite Hardware 101 -
TABLE 4-1: Sprite Data Structure
Memory
Location 16-bit Word Function
N Sprite control word 1 Vertical and horizontal start
position
N+1 Sprite control word 2 Vertical stop position
N+2 Color descriptor low word Color bits for line 1
N+3 Color descriptor high word Color bits for line 1
N+4 Color descriptor low word Color bits for line 2
N+5 Color descriptor high word Color bits for line 2
.
.
.
End-of-data words Two words indicating
the next usage of this sprite
All memory addresses for sprites are word addresses. You will need enough
contiguous memory to provide room for two words for the control information,
two words for each horizontal line in the sprite, and two end-of-data words.
Because this data structure must be accessible by the special-purpose chips,
you must ensure that this data is located within chip memory.
Figure 4-7 shows how the data structure relates to the sprite.
- 102 Sprite Hardware -
/|\ <------------- 16 Bits ----------->
| _________________________________----\ _ Each group of words
| | | | / defines one vertical
| | VSTART, HSTART | | / usage of a sprite.
| |_________________________________| | / Contains starting
| | |/ location & physical
I | VSTOP, control bits | | appearance of this
N |_________________________________| | sprite image.
C _________________________________ ___|___
R | | | |
E | Low word of data, line 1 | | |
A |_________________________________| | |
S | | | |\
I | High word of data, line 1 | | | \
N |_________________________________| | | \_ Pairs of words
G _____ | | containing color
_____ Data describing central | | information for
A _____ lines of this sprite. | | pixel lines.
D _________________________________ | |
D | | | |
R | low word of data, last line | | |
E |_________________________________| | |
S | | | |
S | High word of data, last line | | |
E |_________________________________|___|___|
S ____|
_________________________________
| | |\
| | 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 | \
| |_________________________________| \_ Last word pair contains
| _________________________________ all zeros if this sprite
| | | processor is to be used
| | 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 | only once vertically in
\|/ |_________________________________| the display frame.
V
FIGURE 4-7 PART ONE: (Data Structure Layout)
SEE FIGURE 4-7 PART TWO: (Data Structure Layout)
- Sprite Hardware 103 -
SPRITE CONTROL WORD 1: SPRxPOS
This word contains the vertical (VSTART) and horizontal (HSTART) starting
position for the sprite. This is where the topmost line of the sprite will
be positioned.
Bits 15-8 contain the low 8 bits of VSTART
Bits 7-0 contain the high 8 bits of HSTART
SPRITE CONTROL WORD 2: SPRxCTL
This word contains the vertical stopping position of the sprite on the screen
(i.e., the line AFTER the last displayed row of the sprite). It also contains
some data having to do with sprite attachment, which is described later on.
SPRxCTL
Bits 15-8 The low eight bits of VSTOP
Bit 7 (Used in attachment)
Bits 6-3 Unused (make zero)
Bit 2 The VSTART high bit
Bit 1 The VSTOP high bit
Bit 0 The HSTART low bit
The value (VSTOP - VSTART) defines how many scan lines high the sprite will
be when it is displayed.
SPRITE COLOR DESCRIPTOR WORDS
It takes two color descriptor words to describe each horizontal line of a
sprite; the high-order word and the low-order word. To calculate how many
color descriptor words you need, multiply the height of the sprite in lines
by 2. The bits in the high-order color descriptor word contribute the left
most digit of the binary color selector number for each pixel; the low-order
word contributes the rightmost digit.
- 104 Sprite Hardware -
To form the color descriptor words, you first need to form a picture of the
sprite, showing the color of each pixel as a number from 0 - 3. Each number
represents one of the colors in the sprite's color registers. For example,
here is the spaceship sprite again:
0000122332210000
0001223333221000
0012223333222100
0001223333221000
0000122332210000
Next, you translate each of the numbers in this picture into a binary number.
The first line in binary is shown below. The binary numbers are represented
vertically with the low digit in the top line and the high digit right below
it. This is how the two color descriptor words for each sprite line are
written in memory.
0000100110010000 <---LowSpriteWord
0000011111100000 <---HighSpriteWord
The first line above becomes the color descriptor low word for line 1 of the
sprite. The second line becomes the color descriptor high word. In this
fashion, you translate each line in the sprite into binary 0s and 1s. See
Figure 4-7.
Each of the binary numbers formed by the combination of the two data words for
each line refers to a specific color register in that particular sprite
channel's segment of the color table. Sprite channel 0, for example, takes its
colors from registers 17 - 19. The binary numbers corresponding to the color
registers for sprite DMA channel 0� are shown in Table 4-2.
Table 4-2: Sprite Color Registers
BINARY NUMBER COLOR REGISTER NUMBER
00 Transparent
01 17
10 18
11 19
�Recall that binary 00 always means transparent and never refers to a color
except background.
END-OF-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 written into the sprite control registers instead of being sent
to the color registers. These two words are interpreted by the
- Sprite Hardware 105 -
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 normally visible part of the screen.
SPRITE:
DC.W $6D60, $7200 ;VSTART, HSTART, VSTOP
DC.W $0990, $07E0 ;First pair of descriptor words
DC.W $13C8, $0FF0
DC.W $23C4, $1FF8
DC.W $13C8, $0FF0
DC.W $0990, $07E0
DC.W $0000, $0000 ;End of sprite data
DISPLAYING A SPRITE
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 (making certain that
the chosen channel is available).
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.
4. For each subsequent display field, during the vertical blanking interval,
rewrite the sprite pointers.
CAUTION
If sprite DMA is tumed off while a sprite is being displayed (that is, after
VSTART but before VSTOP), the system will continue to display the line of
sprite data that was most recently fetched. This causes a vertical bar to
appear on the screen. It is recommended that sprite DMA be tumed off only
during vertical blanking or during some portion of the display where you are
sure that no sprite is being displayed.
- 106 Sprite hardware -
SELECTING A DMA CHANNEL AND SETTING THE POINTERS
In deciding which DMA channel to use, you should take into consideration the
colors assigned to the sprite and the sprite's video priority.
The sprite DMA channel uscs two pointers to read in sprite data and control
words. During the vertical blanking interval before the first display of the
sprite, you need to write the sprite's memory address into these pointers.
The pointers for each sprite are called SPRxPTH and SPRxPTL, where "x" is the
number of the sprite DMA channel. SPRxPTH contains the high three bits of the
memory address of the first word in ,the sprite and SPRxPTL contains the low
sixteen bits. Thc least significant bit of SPRxPTL is ignored, as sprite data
must be word aligned. Thus, only fifteen bits of SPRxPTL are used. As usual,
you can write a long word into SPRxPTH.
In the following example the processor initializes the data pointers for
sprite 0. Normally, this is done by the Copper. The sprite is at address
$20000.
MOVE.L #$20000,SPR0PTH+CUSTOM ;Write S20000 to sprite 0 pointer...
These pointers are dynamic; they are incremented by the sprite DMA channel
to point first to the control words, thcn to the data words, and finally
to the end-of-data words. After reading in the sprite control information
and storing it in other registers, they proceed to read in the color
descriptor words. The color descriptor words are stored in sprite data
registers, which are used by the sprite DMA channel to display the data
on screen. For more information about how the sprite DMA channels handle
the display, see the "Hardware Details" section below.
RESETTING THE ADDRESS POINTERS
For one single display field, the system will automatically read the data
structure and produce the sprite on-screen in the colors that are specificd
in the sprite's color registers. If you want the sprite to be displayed in
subsequent display fields, you must rewrite the contents of the sprite
pointers during each vertical blanking interval. This is necessary because
during the display field, the pointers are incremented to point to the data
which is being fetched as the screen display progresses.
The rewrite becomes part of the vertical blanking routine, which can be
handled by instructions in the Copper lists.
- Sprite Hardware 107 -
SPRITE DISPLAY EXAMPLE
This example displays the spaceship sprite at location V = 65, H = 128.
Remember to include the file "hw_examples.i", located in Appendix J.
; First, we set up a single bit-plane.
;
LEA CUSTOM,a0 ; Point a0 at custom chips
MOVE.W #$1200,BPLCON0(a0) ; 1 bit-plane color is on
MOVE.W #$0000,BPL1MOD(a0) ; Modulo = 0
MOVE.W #$0000,BPLCON1(a0) ; Horizontal scroll value = 0
MOVE.W #$0024,BPLCON2(a0) ; Sprites have priority over playfields
MOVE.W #$0038,DDFSTRT(a0) ; Set data-fetch atart
MOVE.W #$00D0,DDFSTOP(a0) ; Set data-fetch stop
; Display window definitions.
MOVE.W #$2C81,DIWSTRT(a0) ; Set display window start
; Vertical start in high byte.
; Horizontal start * 2 in low byte.
MOVE.W #$F4C1,DIWSTOP(a0) ; Set display window stop
; Vertical stop in high byte.
; Horizontal stop * 2 in low byte.
;
; Set up color registers.
;
MOVE.W #$0008,COLOR00(a0) ; Background color = dark blue
MOVE.W #$0000,COLOR01(a0) ; Foreground color = black
MOVE.W #$0FF0,COLOR17(a0) ; Color 17 = yellow
MOVE.W #$00FF,COLOR18(a0) ; Color 18 = cyan
MOVE.W #$0FOF,COLORl9(a0) ; Color 19 = magenta
;
; Move Copper list to $20000.
;
MOVE.L #$20000,a1 ; Point A1 at Copper list destination
LEA COPPERL(pc),a2 ; Point A2 at Copper list source
CLOOP:
MOVE.L (a2),(a1)+ ; Move a long word
CMP.L #$FFFFFFFE,(a2)+ ; Check for end of list
BNE CLOOP ; Loop until entire list is moved
;
; Move sprite to $25000.
;
MOVE.L #$25000,a1 ; Point A1 at sprite destination
LEA SPRITE(pc),a2 ; Point A2 at sprite source
SPRLOOP:
MOVE.L (a2),(a1)+ ; Move a long word
CMP.L #$00000000,(a2)+ ; Check for end of sprite
BNE SPRLOOP ; Loop until entire sprite is moved
;
; Now we write a dummy sprite to $30000, since all eight sprites are activated
; at the same time and we're only going to use one. The remaining sprites
; will point to this dummy sprite data.
;
MOVE.L #$00000000,$30000 ; Write it
;
; Point Copper at Copper list.
- 108 Sprite Hardware -
;
MOVE.L #$20000,COP1LC(a0)
;
; Fill bit-plane with $FFFFFFFF.
;
MOVE.L #$21000,a1 ; Point A1 at bit-plane
MOVE.W #l999,d0 ; 2000-1(for dbf) long words = 8000 bytes
FLOOP
MOVE.L #$FFFFFFFF,(al)+ ; Move a long word of $FFFFFFFF
DBF d0,FLOOP ; Decrement, repeat until false.
;
; Start DMA.
;
MOVE.W d0,COPJMP1(a0) ; Force load into Copper
; program counter
MOVE.W #$83A0,DMACON(a0) ; Bit-plane, Copper, and aprite DMA
RTS ; ..return to rest of program
;
; This is a Copper list for one bit-plane, and 8 sprites.
; The bit-plane lives at $21000.
; Sprite 0 lives at $25000; all others live at $30000 (the dummy sprite).
;
COPPERL:
DC.W BPL1PTH,$0002 ; Bit plane 1 pointer = $21000
DC.W BPL1PTL,$1000
DC.W SPR0PTH,$0002 ; Sprite 0 pointer = $25000
DC.W SPR0PTL,$5000
DC.W SPR1PTH,$0003 ; Sprite 1 pointer = $30000
DC.W SPR1PTL,$0000
DC.W SPR2PTH,$0003 ; Sprite 2 pointer = $30000
DC.W SPR2PTL,$0000
DC.W SPR3PTH,$0003 ; Sprite 3 pointer = $30000
DC.W SPR3PTL,$0000
DC.W SPR4PTH,$0003 ; Sprite 4 pointer�$30000
DC.W SPR4PTL,$0000
DC.W SPR5PTH,$0003 ; Sprite 5 pointer = $30000
DC.W SPR5PTL,$0000
DC.W SPR6PTH,$0003 ; Sprite 6 pointer - S30000
DC.W SPR6PTL,$0000
DC.W SPR7PTH,$0003 ; Sprite 7 pointer = $30000
DC.W SPR7PTL,$0000
DC.W $FFFF,$FFFE ; End of Copper liat
;
; Sprite data for spaceship sprite. It appears on the screen at V-65 and
; H-128.
;
SPRITE:
DC.W $6D60,$7200 ; VSTART, HSTART, VSTOP
DC.W $0990,$07E0 ; First pair of descriptor words
DC.W $13C8,$0FF0
DC.W $23C4,$1FF8
DC.W $13C8,$0FF0
DC.W $0990,$07E0
DC.W $0000,$0000 ; End of sprite data
- Sprite Hardware 109 -
MOVING A SPRITE
A sprite generated in automatic mode can be moved by specifying a different
position in the data structure. For each display field, the data is reread
and the sprite redrawn. Therefore, if you change the position data before the
sprite is redrawn, it will appear in a new position and will seem to be
moving.
You must take care that you are not moving the sprite (that is, changing
control word data) at the same time that the system is using that data to find
out where to display the object. If you do so, the system might find the start
position for one field and the stop position for the following field as it
retrieves data for display. This would cause a "glitch" and would mess up the
screen.
Therefore, you should change the content of the control words only during a
time when the system is not trying to read them. Usually, the vertical
blanking period is a safe time, so moving the sprites becomes part of the
vertical blanking tasks and is handled by the Copper as shown in the example
below.
As sprites move about on the screen, they can collide with each other or with
either of the two playfields. You can use the hardware to detect these
collisions and exploit this capability for special effects. In addition, you
can use collision detection to keep a moving object within specified on-screen
boundaries. Collision Detection is described in Chapter 7, "System Control
Hardware. "
In this example of moving a sprite, the spaceship is bounced around on the
screen, changing direction whenever it reaches an edge.
The sprite position data, containing VSTART and HSTART, lives in memory at
$25000. VSTOP is located at $25002. You write to these locations to move the
sprite. Once during each frame, VSTART is incrcmented (or decremented) by 1
and HSTART by 2. Then a new VSTOP is calculated, which will be the new VSTART
+6.
MOVE.B #151,d0 ; Initialize horizontal count
MOVE.B #194,d1 ; Initialize vertical count
MOVE.B #64,d2 ; Initialize horizontal position
MOVE.B #44,d3 ; Initialize vertical position
MOVE.B #1,d4 ; Initialize horizontal increment value
MOVE.B #1,d5 ; Initialize vertical increment value
;
; Here we wait for the start of the screen updating.
; This ensures a glitch-free display.
;
LEA CUSTOM,a0 ; Set custom chip base pointer
VLOOP:
MOVE.B VHPOSR(a0),d6 ; Read Vertical beam position.
; Only insert the folllowing line if you are using a PAL machine.
; CMP.B #$20,d6 ; Compare with end of PAL screen.
BNE.S VLOOP ; Loop if not end of screen.
; Alternatively you can use the following code:
; VLOOP:
- 110 Sprite Hardware -
; MOVE.W INTREQR(a0),d6 ; Read interrupt requeat word
; AND.W #$0020,d6 ; Mask off all but vertical blank bit
; BEQ VLOOP ; Loop until bit is a 1
; MOVE.W #$0020,INTREQ(a0) ; Vertical bit is on, so reset it
;Please note that this will only work if you have turned OFF the Vertical
;blanking interupt enable (not recommended for long periods).
ADD.B d4,d2 ; Increment horizontal value
SUBQ.B #1,d0 ; Decrement horizontal counter
BNE L1
MOVE.B #151,d0 ; Count exhausted, reset to 151
EOR.B #$FE,d4 ; Negate the increment value
L1:
MOVE.B d2,$25001 ; Write new HSTART value to sprite
ADD.B d5,d3 ; Increment vertical value
SBQ.B #1,d1 ; Decrement vertical counter
BNE L2
MOVE.B #194,d1 ; Count exhausted, reset to 194
EOR.B #$FE,d5 ; Negate the increment value
L2:
MOVE.B d3,$25000 ; Write new VSTART value to sprite
MOVE.B d3,d6 ; Must now calculate new VSTOP
ADD.B #6,d6 ; VSTOP always VSTART+6 for spaceship
MOVE.B d6,$25002 ; Write new VSTOP to sprite
BRA VLOOP ; Loop forever
CREATING ADDITIONAL SPRITES
To use additional sprites, you must create a data structure for each one and
arrange the display as shown in the prcvious section, naming the pointers
SPR1PTH and SPR1PTL for sprite DMA channel 1, SPR2PTH and SPR2PTL for sprite
DMA channel 2, and so on.
NOTE
When you enable sprite DMA for one sprite, you enable DMA for all the
sprites and place them all in automatic mode. Thus, you do not need to
repeat this step when using additional sprite DMA channels.
Once the sprite DMA channels are enabled, all eight sprite pointers must be
initialized to either a real sprite or a safe null sprite. An uninitialized
sprite could cause spurious sprite video to appear.
Remember that some sprites can become unusable when additional DMA cycles are
allocated to displaying the screen, for example when an extra wide display or
horizontal scrolling is enabled (see Figure 6-9: DMA Time Slot Allocation).
Also, recall that each pair of sprites takes its color from different color
registers, as shown in Table 4-3.
- Sprite hardware 111 -
�� Table 4-3: Color Registers for Sprite Pairs
SPRITE NUMBERS COLOUR REGISTERS
0 and 1 17-19
2 and 3 21-23
4 and 5 25-27
6 and 7 29-31
NOTE
Some sprites become unusable when additinal DMA cycles are allocated to
displaying the screen, e.g. when enabling an extra wide display or horizontal
scrolling. (See Figure 6 DMA Time Slot Allocation.)�
SPRITE PRIORITY
When you have more Than one sprite on the screen, you may need to take into
consideration their relative video priority, that is, which sprite appears in
front of or behind another. Each sprite has a fixed video priority with
respect to all the others. The lowest numbered sprite has the highest priority
and appears in front of all other sprites; the highes�t numbered sprite has the
lowest priority. This is illustrated in Figure 4-8.
NOTE
See Chapter 7, "System Control Hardware", for more information on sprite
priorities.
____
__|_ 7|
__|_ 6|__|
__|_ 5|__|
__|_ 4|__|
__|_ 3|__|
__|_ 2|__|
__|_ 1|__|
| 0|__|
|____|
FIGURE 4-8: (Sprite Priority)
- 112 Sprite Hardware -
REUSING SPRITE DMA CHANNELS�
Each of the eight sprite DMA channels can produce more than one independently
controllable image. There may be times when you want more than eight objects,
or you may be left with fewer than eight objects because you have attached
some of the sprites to produce more colors or larger objects or overlapped
some to produce more complex images. You can reuse each sprite �DMA �channel
several times within the same display field, as shown in Figur�e 4-9.
SEE FIGURE 4-9: (Typical Example of Sprite Reuse�)
In single-sprite usage, two all-zero words are placed at the end of the data
structure to stop the DMA channel from retrieving any more data for that
particular sprite during that display field. To reuse a DMA channel, you �
replace this pair of zero words with another complete sprite data structure,
which describes the reuse of the DMA �channel at a position lower �on the screen
than the firstuse. You place the two all-zero words at the end, of the data
structure that contains the information for all usagcs of thc DMA channel. For
example, Figure 4-10 shows the data structure that describes the pi�ct�ure
above.
- Sprite Hardware 113 -
SPRITE DISPLAY LIST
------\ _ Data describing
________________________________ | / the 1st vertical
Increasing |________________________________| | / usage of this
RAM |________________________________| |/ sprite.
memory ________________________________ |
addresses |________________________________| |
|________________________________| |
| _________ |
| _________ |
| _________ |
| ________________________________ |
| |________________________________| |
| |________________________________| |
| -----/
|
| -----\ _ Data describing
| ________________________________ | / the 2nd vertical
| |________________________________| | / usage of this
| |________________________________| |/ sprite. Contents
| ________________________________ | of vertical start
| |________________________________| | word must be at
| |________________________________| | least one video
| _________ | line below actual
| _________ | end of preceding
| _________ | usage.
\|/ ________________________________ |
V |________________________________| |
|________________________________| |\
| \
-----/ \_ End-of-data words
ending the usage
of this sprite.
FIGURE 4-10: (Typical Data Structure for Sprite Re-use)
The only restrictions on the reuse of sprites during a single display field
is that the bottom line of one usage of a sprite must be separated from the
top line of the next usage by at least one horizontal scan line. This
restriction is necessary because only two DMA cycles per horizontal scan
line are allotted to each of the eight channels. The sprite channel needs the
time during the blank line to fetch the control word describing the next
usage of the sprite.
- 114 Sprite Hardware -
The following example displays the spaceship sprite and then redisplays it as �
a different object.��
Only the sprite data list is affected, so only the data list is shown here.
However, the sprite looks best with the color registers set as shown in the
example.
LEA CUSTOM,a0
MOVE.W #$0F00,COLOR17(a0) ; Color 17 red
MOVE.W #$0FF0,COLOR18(a0) ; Color 18 yellow
MOVE.W #$0FFF,COLORl9(a0) ; Color 19 white
�SPRITE:
DC.W $6D60,$7200
DC.W $0990,$07E0
DC.W $13C8,$0FF0
DC.W $23C4,$1FF8
DC.W $13C8,$0FF0
DC.W $0990,$07E0
DC.W $8080,$8D00 ; VSTART, HSTART, VSTOP for new sprite
DC.W $1818,$0000
DC.W $7E7E,$0000
DC.W $7FFE,$0000
DC.W $FFFF,$2000
DC.W $FFFF,$2000
DC.W $FFFF,$3000
DC.W $FFFF,$3000
DC.W $7FFE,$1800
DC.W $7FFE,$0C00
DC.W $3FFC,$0000
DC.W $0FF0,$0000
DC.W $03C0,$0000
DC.W $0180,$�0000
DC.W $0000,$0000 ; End of sprite data
OVERLAPPED SPRITES
For more complex or larger moving objects, you can overlap sprites.
Overlapping simply mean that the sprites have the same or relatively close
screen positions. A relatively close screen position can result in an object
that is wider than 16 pixels.�
The built-in sprite video priority ensures that one sprite appears to be
behind the other when sprites are overlappcd. The priority circuitry gives
the lowest-numbered sprite the highest priority and the highest numbered
sprite the lowest priority. Therefore, when designing displays with �
overlapped sprites, make sure the "foreground" sprite has a lower number
than the "background" sprite. In Figure 4-11, for example, the cage should
be generated by a lower-numbered sprite DMA channel than the monkey.
- Sprite Hardware 115 -
SEE FIGURE 4-11: overlapping Sprites (Not attached)
You can create a wider sprite display by placing two sprites next to each
other. For instance, Figure 4-12 shows the spaceship sprite and how it
can be made twice as large by using two sprites placed next to each
other.
- 116 Sprite Hardware -
(128,65)
o_____________________
| _| |_ |
| _| |_ |
| |_ _| |
| |_ _| |
|______|________|_____|
(128,65) (144,65)
o_____________________o_____________________
| | | | |
| __| | |__ |
| | | | |
| __| | |__ |
| | | | |
| |__ | __| |
| | | | |
| |__ | __| |
| | | | |
|_________|___________|__________|__________|
Sprite 0 Sprite 1
FIGURE 4-12: Placing Sprites Next to Each Other
ATTACHED SPRITES
You can create sprites that have fifteen possible color choices (plus
transparent) instead of three (plus transparent), by "attaching" two
sprites. To create attached sprites, you must:
o Use two channels per sprite, creating two sprites of the same size and
located at the same position.
o Set a bit called ATTACH in the second sprite control word.
The fifteen colors are selected from the full range of color registers
available to sprites - registers 17 through 31. The extra color choices
are possible because each pixel contains four bits instead of only two as
in the normal, unattached sprite. Each sprite in the attached pair
contributes two bits to the binary color selector number. For example, if
you are using sprite DMA channels 0 and 1, the high- and low-order color
descriptor words for line 1 in both data structures are combined into
line 1 of the attached object.
- Sprite Hardware 117 -
Sprites can be attached in the following combinations:
Sprite 1 to sprite 0
Sprite 3 to sprite 2
Sprite 5 to sprite 4
Sprite 7 to sprite 6
Any or all of these attachments can be active during the same display
field. As an example, assume that you wish to have more colors in the
spaceship sprite and you are using sprite DMA channels 0 and 1. There are
five colors prus transparent in this sprite.
0000154444510000
0001564444651000
0015676446765100
0001564444651000
0000154444510000
The first line in this sprite requires the four data words shown in Table
4-4 to form the correct binary color selector numbers.
Table 4-4: Data Words for First Line of Spaceship Sprite
PIXEL NUMBER
15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
----------------------------------------------------------------------
Line 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
Line 2 0 0 0 0 0 1 1 1 1 1 1 0 0 0 0 0
Line 3 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
Line 4 0 0 0 0 1 1 0 0 0 0 1 1 0 0 0 0
The highest numbered sprite (number 1, in this example) contributes the
highest order bits (left-most) in the binary number. The high-order data
word in each sprite contributes the leftmost digit. Therefore, the lines
above are written to the sprite data structures as follows:
Line 1 Sprite 1 high-order word for sprite line 1
Line 2 Sprite 1 low-order word for sprite line 1
Line 3 Sprite 0 high-order word for sprite line 1
Line 4 Sprite 0 low-order word for sprite line 1
See Figure 4-7 for the order these words are stored in memory. Remember
that this data is contained in two sprite structures.
- 118 Sprite Hardware -
The binary numbers 0 through 15 select registers 17 through 31 as shown
in Table 4-5.
Table 4-5: Color Registers in Attached Sprites
Decimal Binary ColorRegister
Number Number Number
0 0000 16 *
1 0001 17
2 0010 18
3 0011 19
4 0100 20
5 0101 21
6 0110 22
7 0111 23
8 1000 24
9 1001 25
10 1010 26
11 1011 27
12 1100 28
13 1101 29
14 lll0 30
15 1111 31
* Unused; yields transparent pixel.
Attachment is in effect only when the ATTACH bit, bit 7 in sprite control
word 2, is set to 1 in the data structure for the odd-numbered sprite.
So, in this example, you set bit 7 in sprite control word 2 in the data
structure for sprite 1.
When the sprites are moved, the Copper list must keep them both at
exactly the same position relative to each other. If they are not kept
together on the screen, their pixels will change color. Each sprite will
revert to three colors plus transparent, but the colors may be different
than if they were ordinary, unattached sprites. The color selection for
the lower numbered sprite will be from color registers 17-19. The color
selection for the higher numbered sprite will be from color registers 20,
24, and 28.
- Sprite Hardware 119 -
The following data structure is for the six-color spaceship made with two
attached sprites.
SPRITE0:
DC.W $6D60,$7200 ;VSTART = 65, HSTART = 128
DC.W $0C30,$0000 ;First color deacriptor word
DC.W $1818,$0420
DC.W $342C,$0E70
DC.W $1818,$0420
DC.W $0C30,$0000
DC.W $0000,$0000 ;End of sprite 0
SPRITE1:
DC.W $6D60,$7280 ;Same as sprite 0 except attach bit on
DC.W $07E0,$0000 ;First descriptor word for sprite 1
DC.W $0FF0,$0000
DC.W $1FF8,$0000
DC.W $0FF0,$0000
DC.W $07E0,$0000
DC.W $0000,$0000 ;End of sprite 1
MANUAL MODE
It is almost always best to load sprites using the automatic DMA
channels. Sometimes, however, it is useful to load these registers
directly from one of the microprocessors. Sprites may be activated
"manually" whenever they are not being used by a DMA channel. The same
sprite that is showing a DMA-controlled icon near the top of the screen
can also be reloaded manually to show a vertical colored bar near the
bottom of the screen. Sprites can be activated manually even when the
sprite DMA is tumed off.
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 lhen be displayed on every line, at the
horizontal position given in the "H" portion of the position 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.
- 120 Sprite Hardware -
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
pattems 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 -
_________________
| Beam counter |
|(Horizontel pos.)| ____________________
|_________________| |SPRxDATA load decode|
\ / | (68000 or DMA) |
________\/_______ |____________________|
| | Equal |
| Compensator |______ ___________________ |
|_________________| | |SPRxPOS load decode| |
/\ ________|______| (68000 or DMA) | |
_______/__\____|_ | |___________________| |
| | | |
| SPRxPOS (Horiz.)| | |
|_________________| | <-"ARM SPRITE"-> |
/\ ________|____________________________o
____________/ \ | |___ _______ |
| | | |AND |-| | | |
| ___________ | | \__/ |--------|Q S|-----|
|| \ / | | | | _____________
|| ________\/_____|_ | ----|Q R|--|SPRxCTL load |
|| | | | |_______| | decode |
|| | SPRxDATA | | |(68000 or DMA|
|| |_________________| ____o |_____________|
|| \ / | |
|| ______\/_______|_ | _____ ______________
|| ____| Parrallel to | | | |Sprite serial |
|| __|__ |serial coinverter|-----------------> | | video data |
|| ___ |_________________| | | | |
|| _ _________________ | | | Output to |
|| | Parrallel to | | | |video priority|
|| |serial converter |-----------------> | | logic |
|| |_________________| | _____| |______________|
|| /\ | |
|| _______/__\______ |____|
|| | |
|| | SPRxDATB |
|| |_________________| ____________________
|| /\ ^ |SPRxDATB load decode|
|| / \ |___________|(68000 or DMA |
||_______| | |____________________|
| __________|
||
||______________________________________________________________________
|_______________________________________________________________________
DATA BUS
FIGURE 4-13: Sprite Control Circuitry
Figure 4-13 shows the following:
o Writing to the sprite control registers disables the horizontal
comparator circuitry. This prevents the system from sending any output
from the data registers to the serial converter or to the screen.
- 122 Sprite Hardware -
o Writing to the sprite A data register enables the horizontal comparator.
This enables output to the screcn when the horizontal position of the
video beam equals the horizontal value in the position register.
o If the comparator is enabled, the sprite data will be sent to the
display, with the leftmost pixel of the sprite data placed at the
position defined in the horizontal part of SPRxPOS.
o As long as the comparator remains enabled, the current contents of the
sprite data register will be output at the selected horizontal position
on a video line.
o The data in the sprite data registers does not change. It is either
rewritten by the user or modified under DMA control.
The components described above produce the automatic DMA display as
follows: When the sprites are in DMA mode, the 18-bit sprite pointer
register (composed of SPRxPTH and SPRxPTL) is used to read the first two
words from the sprite data structure. These words contain the starting
and stopping position of the sprite. Next, the pointers write ese words
into SPRxPOS and SPRxCTL. After this write, the value in the pointers
points to the address of the first data word (low word of data for line 1
of the sprite.)
Writing into the SPRxCTL register disabled the sprite. Now the sprite DMA
channel will wait until the vertical beam counter value is the same as
the data in the VSTART (Y value) part of SPRxPOS. When these values
match, the system enables the sprite data access.
The sprite DMA channel examines the contents of VSTOP (from SPRxCTL,
which is the location of the line after the last line of the sprite) and
VSTART (from SPRxPOS) to see how many lines of sprite data are to be
fetched. Two words are fetched per line of sprite height, and these words
are written into the sprite data registers. The first word is stored in
SPRxDATA and the second word in SPRxDATB.
The fetch and store for each horizontal scan line occurs during a
horizontal blanking interval, far to the left of the start of the screen
display. This arms the sprite horizontal comparators and allows them to
start the output of the sprite data to the screen when the horizontal
beam count value matches the value stored in the HSTART (X value) part of
SPRxPOS.
If the count of VSTOP - VSTART equals zero, no sprite output occurs. The
next data word pair will be fetched, but it will not be stored into the
sprite data registers. It will instead become the next pair of data words
for SPRxPOS and SPRxCTL.
When a sprite is used only once within a single display field, the final
pair of data words, which follow the sprite color descriptor words, is
loaded automatically as the next contents of the SPRxPOS and SPRxCTL
registers. To stop the sprite after that first data set, the pair of
words should contain all zeros.
Thus, if you have fommed a sprite pattem in memory, this same pattem will
be produced as pixels automatically under DMA control one line at a time.
- Sprite Hardware 123 -
SUMMARY OF SPRITE REGISTERS
There are eight complete sets of registers used to describe the sprites.
Each set consists of five registers. Only the registers for sprite O are
described here. All of the others are the same, except for the name of
the register, which includes the appropriate number.
POINTERS
Pointers are registers that are used by the system to point to the
current data being used. During screen display, the registers are
incremented to point to the data being used as the screen display
progresses. Therefore, pointer registers must be freshly written during
the start of the vertical blanking period.
SPR0PTH and SPR0PTL
This pair of registers contains the 32-bit word address of Sprite 0 DMA
data.
Pointer register names for the other sprites are:
SPR1PTH SPR1PTL
SPR2PTH SPR2PTL
SPR3PTH SPR3PTL
SPR4PTH SPR4PTL
SPRSPTH SPRSPTL
SPR6PTH SPR6PTL
SPR7PTH SPR7PTL
CONTROL REGISTERS
SPR0POS
This is the sprite 0 position register. The word written into this
register contrls the position on the screen at which the upper left-hand
comer of the sprite is to be placed. The most significant bit of the
first data word will be placed in this position on the screen.
- 124 Sprite Hardware -
NOTE
The sprites have a placement resolution on a full screen of 320 by 200
NTSC (320 by 256 PAL). The sprite resolution is independent of the bit-
plane resolution.
BIT POSITIONS:
o Bits 15-8 specify the vertical start position, bits V7 - V0.
o Bits 7-0 specify the horizontal start position, bits H8 - H1.
NOTE
This register is normally only written by the lsprite DMA channel itself.
See the details above regarding the organization of the sprite data. This
register is usually updated directly by DMA.
SPR0CTL
This register is normally used only by the sprite DMA channel. It
contains control information that is used to control the sprite data-
fetch process. Bit positions:
o Bits 15-8 specify vertical stop position for a sprite image, bits
V7 - V0.
o Bit 7 is the attach bit. This bit is valid only for odd-numbered
sprites. It indicates that sprites 0, 1 (or 2,3 or 4,5 or 6,7) will, for
color interpretation, be considered as paired, and as such will be called
four bits deep. The odd-numbered (higher number) sprite contains bits
with the higher binary significance.
During attach mode, the attached sprites are normally moved horizontally
and vertically together under processor control. This allows a greater
selection of colors within the boundaries of the sprite itself. The
sprites, although attached, remain capable of independent motion,
however, and they will assume this larger color set only when their edges
overlay one another.
o Bits 6-3 are reserved for future use (make zero).
o Bit 2 is bit V8 of vertical start.
o Bit 1 is bit V8 of vertical stop.
o Bit 0 is bit H0 of horizontal start.
- Sprite Hardware 125 -
Position and control registers for the other sprites are:
SPR1POS SPR1CTL
SPR2POS SPR2CTL
SPR3POS SPR3CTL
SPR4POS SPR4CTL
SPR5POS SPRSCTL
SPR6POS SPR6CTL
SPR7POS SPR7CTL
DATA REGISTERS
The following registers, although defined in the address space of the
main processor, are normally used only by the display processor. They are
the holding registers for the data obtained by DMA cycles.
SPR0DATA, SPR0DATB data registers for Sprite 0
SPR1DATA, SPR1DATB data registers for Sprite 1
SPR2DATA, SPR2DATB data registers for Sprite 2
SPR3DATA, SPR3DATB data registers for Sprite 3
SPR4DATA, SPR4DATB data registers for Sprite 4
SPR5DATA, SPR5DATB data registers for Sprite 5
SPR6DATA, SPR6DATB data registers for Sprite 6
SPR7DATA, SPR7DATB data registers for Sprite 7
SUMMARY OF SPRITE COLOR REGISTERS
Sprite data words are used to select the color of the sprite pixels from
the system color register set as indicated in the following tables.
If the bit combinations from single sprites are as shown in Table 4-6,
then the colors will be taken from the registers shown.
- 126 Sprite Hardware -
Table 4-6: Color Registers for Single Sprites
SINGLE SPRITES COLOR
Sprite Value Register
0 or 1 00 Not used *
01 17
10 18
11 19
2 or 3 00 Not used *
01 21
10 22
11 23
4 or 5 00 Not used *
01 25
10 26
11 27
6 or 7 00 Not used *
01 29
10 30
11 31
* Selects transparent mode.
If the bit combinations from attached sprites are as shown in Table 4-7,
then the colors will be taken from the registers shown.
- Sprite Hardware 127 -
Table 4-7: Color Registers for Attached Sprites
ATTACHED SPRITES
Color
Value Register
0000 Selects transparent mode
0001 17
0010 18
0011 19
0100 20
0101 21
0110 22
0111 23
1000 24
1001 25
1010 26
1011 27
1100 28
1101 29
1110 30
1111 31
INTERACTIONS AMONG SPRITES AND OTHER OBJECTS
Playfields share the display with sprites. Chapter 7, "System Control
Hardware," shows how playfields can be given different video display
priorities relative to the sprites and how playfields can collide with
(overlap) the sprites or each other.
- 128 Sprite Hardware -
End.
|