Atari7800 sprite decompression routine

Ecernosoft · June 9, 2022

Ok, So, I just finished version 1 of my Atari 7800 sprite decompression system. Code is in the post below. Here's how it works:

This code can be used in your game.

Basically, each byte holds the color (0= transparent, 1, 2, or 3) and the width (0-63, but 0 is interpreted as 256)

The format goes like this:

76543210

\ /\/

Length,Color

Drawing starts in bottom left corner and ends in the top right corner.

Unfortunately, this uses up a lot of RAM. Since it decompresses the graphics into RAM, this eats 2K of the 7800's 4K. You can change the DLL's to be smaller, but then you get less sprites on a line. =/

Code is below for those who wanna look at it. ?

; Atari 7800 sprite sample
; Written by Daniel Boris (danlb_2000@yahoo.com)
; Modified by Steven Hugg @8bitworkshop
; Assemble with DASM
;

processor 6502

; ************ Hardware Adresses ***************************

INPTCTRL equ $01 ;Input control
AUDC0 equ $15 ;Audio Control Channel 0
AUDC1 equ $16 ;Audio Control Channel 1
AUDF0 equ $17 ;Audio Frequency Channel 0
AUDF1 equ $18 ;Audio Frequency Channel 1
AUDV0 equ $19 ;Audio Volume Channel 0
AUDV1 equ $1A ;Audio Volume Channel 1
INPT0 equ $08 ;Paddle Control Input 0
INPT1 equ $09 ;Paddle Control Input 1
INPT2 equ $0A ;Paddle Control Input 2
INPT3 equ $0B ;Paddle Control Input 3
INPT4 equ $0C ;Player 0 Fire Button Input
INPT5 equ $0D ;Player 1 Fire Button Input

BACKGRND equ $20 ;Background Color
P0C1 equ $21 ;Palette 0 - Color 1
P0C2 equ $22 ;Palette 0 - Color 2
P0C3 equ $23 ;Palette 0 - Color 3
WSYNC equ $24 ;Wait For Sync
P1C1 equ $25 ;Palette 1 - Color 1
P1C2 equ $26 ;Palette 1 - Color 2
P1C3 equ $27 ;Palette 1 - Color 3
MSTAT equ $28 ;Maria Status
P2C1 equ $29 ;Palette 2 - Color 1
P2C2 equ $2A ;Palette 2 - Color 2
P2C3 equ $2B ;Palette 2 - Color 3
DPPH equ $2C ;Display List List Pointer High
P3C1 equ $2D ;Palette 3 - Color 1
P3C2 equ $2E ;Palette 3 - Color 2
P3C3 equ $2F ;Palette 3 - Color 3
DPPL equ $30 ;Display List List Pointer Low
P4C1 equ $31 ;Palette 4 - Color 1
P4C2 equ $32 ;Palette 4 - Color 2
P4C3 equ $33 ;Palette 4 - Color 3
CHARBASE equ $34 ;Character Base Address
P5C1 equ $35 ;Palette 5 - Color 1
P5C2 equ $36 ;Palette 5 - Color 2
P5C3 equ $37 ;Palette 5 - Color 3
OFFSET equ $38 ;Unused - Store zero here
P6C1 equ $39 ;Palette 6 - Color 1
P6C2 equ $3A ;Palette 6 - Color 2
P6C3 equ $3B ;Palette 6 - Color 3
CTRL equ $3C ;Maria Control Register
P7C1 equ $3D ;Palette 7 - Color 1
P7C2 equ $3E ;Palette 7 - Color 2
P7C3 equ $3F ;Palette 7 - Color 3

SWCHA equ $280 ;P0, P1 Joystick Directional Input
SWCHB equ $282 ;Console Switches
CTLSWA equ $281 ;I/O Control for SCHWA
CTLSWB equ $283 ;I/O Control for SCHWB
;|
;||
;|||
;||||
;|||||
;||||||
;|||||||
;||||||||
;76543210 SAVE
SEG.U data

;******* Vairables ********************************

   org $40
A   ds.b 1
B   ds.b 1
C   ds.b 1
D   ds.b 1
E   ds.b 1
F   ds.b 1
G   ds.w 1
H   ds.w 1
I   ds.w 1
J   ds.b 1
K   ds.b 1
X   ds.b 1
Y   ds.b 1
temp   ds.b 1
temp2   ds.b 1
   org $80
DLLdata
   org $1800
GFXMEM
   org $2000; RAM
SPR1data   ds.b 64; graphics
   org $2100
SPR2data   ds.b 64; Graphics
   org $25C0; End of DLs, $340 bytes free (384+64 bytes, or about 448 bytes)

;**********************************************************

    SEG code

org $4000 ;Start of code

START
   sei ;Disable interrupts
   cld ;Clear decimal mode

;******** Atari recommended startup procedure

   lda #$00
   sta INPTCTRL ;Lock into 7800 mode
   lda #%11111111
   sta CTRL ;Disable DMA
   lda #$00
   sta OFFSET
   sta INPTCTRL
   ldx #$FF ;Reset stack pointer
   txs

;************** Clear zero page and hardware ******

   ldx #$40
   lda #$00
crloop1
   sta $00,x ;Clear zero page
   sta   $100,x       ;Clear page 1
   inx
   bne crloop1

ldy #$00 ;Clear Ram
crloop2
sta   $1800,y
sta   $1900,y
sta   $1a00,y
sta   $1b00,y
sta   $1c00,y
sta   $1d00,y
sta   $1e00,y
sta   $1f00,y
sta   $2200,y
sta   $2300,y
sta   $2400,y
sta   $2500,y
sta   $2600,y
sta   $2700,y
iny
bne   crloop2

ldx #$00
crloop5 ;Clear 2100-213F
sta $2100,x
inx
cpx #$40
bne crloop5

;************* Build DLL *******************

; 20 blank lines

mainloop
   jsr Makesprite
jsr MakeDLL
lda #%01010000
sta CTRL
   jmp TEST
   jmp mainloop   ;Loop\
TEST
   lda #>Graphics
sta G+1
lda #<Graphics
sta G
lda #$19
sta K
lda #$FF
sta J
lda #$
sta F
jsr DRAW
jsr Doit
jmp Forever
DataPacket

   inc G
lda G
bne Noinc2
inc G+1
Noinc2
   lda (G),y
sta D

   inc G
lda G
bne step1
inc G+1
step1
   ldy #0
   lda (G),y
sta A

step1.1
   lda A
and #%11000000
sta (H),y
inc H+1
lda H+1
cmp #$20
bne Continue3
lda #$18
sta H+1
lda H
adc #$07
sta H
and #%11111000
cmp #$30
bne Continue3
lda H
sbc #$2F
sta H
Continue3
   lsr A
lsr A
lda A
bne nofetch2
inc G
lda (G),y
sta A
lda G
bne nofetch2
inc G+1
nofetch2
   dec D
lda D
beq Step1

DRAW
   lda #$18
sta H+1
lda #$80
sta E
lda #%11000000
sta F

Step1
   ldy #0
   lda (G),y
sta temp
and #%00000011
tax
lda WRBITS,x
sta E
lda temp

lsr
lsr

tax
Step2
   lda E
and F
ora (H),y
sta (H),y
inc H+1
lda H+1
cmp #$20
bne Continue
lda #$18
sta H+1
lda H
adc #$07
sta H
and #%11111000
cmp #$30
bne Continue
lda H
sbc #$30
sta H
lsr F
lsr F
lda F
bne Continue
lda #%11000000
sta F
inc H
lda H
and #%00001000
cmp #$08
beq DONE

Continue
   dex
bne Step2
inc G
lda G
bne Continue2
   inc G+1
Continue2
jmp Step1
DONE   rts
NMI
   RTI

IRQ
   RTI
Forever


   jmp Forever
WRBITS
   byte #%00000000
byte #%01010101
byte #%10101010
byte #%11111111

;Pointers to the DLs

DLPOINTH
.byte $22,$22,$22,$22,$23,$23,$23,$23,$24,$24,$24,$24,#$25,#$25,#$25,#$25,#$25,#$25,#$25,#$25,#$25,#$25,#$25,#$25
DLPOINTL
.byte $00,$40,$80,$C0,$00,$40,$80,$C0,$00,$40,$80,$C0,#$00,#$10,#$20,#$30,#$40,#$50,#$60,#$70,#$80,#$90,#$A0,#$B0
Fronts
hex 28589305843945FABCDAFCB2958923F475BA
Backs
MakeDLL
Doit ldy #$18
ldx #0

Dort   lda #$47
sta $80,x
inx

   lda DLPOINTH,y
sta $80,x
inx
lda DLPOINTL,y
sta $80,x
inx
dey
bne Dort
rts
Makesprite
   lda #$80
sta DPPL
lda #$00
sta DPPH
   lda #$0F
sta P0C1
lda #$0A
sta P0C2
lda #$04
sta P0C3
   lda #$00
sta $2500
lda #$08
sta $2510
lda #$10
sta $2520
lda #$18
sta $2530
lda #$20
sta $2540
lda #$28
sta $2550
lda #%01000000
sta $2501
sta $2511
sta $2521
sta $2531
sta $2541
sta $2551
lda #$18
sta $2502
lda #$18
sta $2512
sta $2522
sta $2532
sta $2542
sta $2552
lda #%00011000
sta $2503
sta $2513
sta $2523
sta $2533
sta $2543
sta $2553
lda #%00000000
sta $2504
sta $2514
sta $2524
sta $2534
sta $2544
sta $2554
lda #%01010000
sta CTRL
lda #$18
sta CHARBASE

   ldy #$40
TEST2
   tya
sta $2000,y
dey
bne TEST2

rts
TEST1; Format:
   ;76543210
;||||||||
;||||||color of pixels
;# of times to repeat pixel
   byte #%00000111

;************** Graphic Data *****************************
;set org and fill character
org $8000,0
Graphics; 100000-000111=011000
   byte #%00001000
byte #%00110101
byte #%10001000
byte #%00000101
byte #%00101111
byte #%00001101
byte #%10000000
byte #%00011001
byte #%00001011
byte #%00100101
byte #%01111100
byte #%00000101
byte #%00110011
byte #%00010001
byte #%01111100
byte #%01000101
byte #%01111100
byte #%00000101
byte #%00011011
byte #%00101001
byte #%01111100
byte #%00000101
byte #%00001011
byte #%00111001
byte #%01111100
byte #%00000101
byte #%00011011
byte #%00101001
byte #%01111100
byte #%01000101
byte #%01111100
byte #%00000101
byte #%00110011
byte #%00010001
byte #%01111100
byte #%00011001
byte #%00001011
byte #%00100101
byte #%01111100
byte #%00000101
byte #%00011011
byte #%00101001
byte #%01111100
byte #%00000111
byte #%01000001
byte #%01111100
byte #%00000101
byte #%00001011
byte #%00111001
byte #%01111100
byte #%01000101
byte #%01111100
byte #%00000101
byte #%00110111
byte #%00001101
byte #%01111100
byte #%00100101
byte #%00001111
byte #%00010101
byte #%01111100
byte #%00011101
byte #%00010011
byte #%00011001
byte #%01111100
byte #%00000101
byte #%00110111
byte #%00001101
byte #%01111100
byte #%01000101
byte #%01111100
byte #%00000101
byte #%00101011
byte #%00011001
byte #%01111100
byte #%00000101
byte #%00001011
byte #%00001001
byte #%00001011
byte #%00001001
byte #%00001011
byte #%00011001
byte #%01111100
byte #%01000101
byte #%01111100
byte #%00000101
byte #%00100011
byte #%00100001
byte #%01111100
byte #%00010101
byte #%00001011
byte #%00001001
byte #%00001011
byte #%00011001
byte #%01111100
byte #%00000101
byte #%00100011
byte #%00100001
byte #%01111100
byte #%01000101
byte #%01111100
byte #%00100001
byte #%00001011
byte #%00011101
byte #%01111100
byte #%00000101
byte #%00100111
byte #%00011101
byte #%01111100
byte #%00100001
byte #%00001011
byte #%00011101
byte #%01111100
byte #%00000101
byte #%00001011
byte #%00111001
byte #%10000000
byte #%00111101

;************** Cart reset vector **************************

   org $fff8
   .byte $FF ;Region verification
   .byte $87 ;ROM start $4000
   .word #NMI
   .word #START
   .word #IRQ

Ecernosoft · June 9, 2022

This is only version 1. Version 2 will have more modes and support for data packets.

Ecernosoft · June 9, 2022

Well, a data packet being traditional graphics data, with a terminator or set length, just it will be laid out vertically like this:

76543210

\ /\ /\ / L[]

| | L __[]

| L ____[]

L ______[]

(Of course there will be no offset)

Ecernosoft · June 9, 2022

Sorry, here's an example of the example of the above code:

image.png.7c6bfe9902f6a6472b2bd4a8d3d97ab4.png

The maximum size is 128 8*8 characters, or a canvas of 128*64 at 2bpp or 256*64 at 1bpp. The way I've set it up is 64*64 at 2bpp.

Ecernosoft · June 9, 2022

Actually, my bad. This example is 32*40.

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Atari7800 sprite decompression routine

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