Well gang, I'm finished school for the year. Wrote my Japanese test last
week (I'm pretty sure I did well). Now I've got time to do some 2600
programming.
So tonight I sat down to do some sprite experimenting, moving the sprites
horizontally with only the RSP0/1 commands.
But something seems to have gone horribly wrong with my little scheme and
it's got me questioning the functionability of my 2600.
When I run the programme on an emulator, everything works fine.
But when I try to run it on a real 2600, I get something unlike anything
I've ever gotten before.
Here's what the programme is supposed to do...
Dark grey background.
Blue Stickman at top of the screen moving back and forth.
Red Ghost under him moving in the opposite directions.
That's it, no fancy multisprite stuff.
Here's what I'm getting on a real 2600.
Three copies of each sprite spread out vertically across the screen
and black lines dividing the screen into 3rds.
The sprites all do what I want them to, it's just that there's too damn
many of them. And I don't know where these black lines are coming from.
The image is very stable and the VBLANK and OVERSCAN are just knabbed from
some other programmes of mine that work like a charm.
I can't see anything in the screen kernal that could possibly cause this odd
behaviour so either my 2600 or supercharger have gone wonky on me, or
there's some really odd bug I just can't see. Here's the source code.
Now it's a little speghetti right now because I was just sort of doing
"stream of consciousness programming"...
--SEXP0001.ASM---------------------------------------------------------------
processor 6502
VSYNC = $00
VBLANK = $01
WSYNC = $02
;NUSIZ0 = $04
;NUSIZ1 = $05
COLUPF = $08
COLUBK = $09
PF0 = $0D
PF1 = $0E
PF2 = $0F
;SWCHA = $280
INTIM = $284
TIM64T = $296
;CTRLPF = $0A
COLUP0 = $06
COLUP1 = $07
GP0 = $1B
GP1 = $1C
;HMOVE = $2a
RESP0 = $10
RESP1 = $11
;RAM
HPOS1 = $80
HPOS2 = $81
SPRITE1 = $82
SPRITE2 = $8A
DELAY = $92
MTOGGLE = $93
org $F000
start SEI
CLD
LDX #$FF
TXS
LDA #$00
zero STA $00,X ;looks familiar, right?
DEX ;typical zeroing routine
BNE zero
lda #$96 ;Just setting crap up.
sta COLUP0
lda #$38
sta COLUP1
lda #$04
sta COLUBK
lda #$00 ;starting position for each sprite
sta HPOS1
lda #$0D
sta HPOS2
main JSR vertb ;main loop
ldy #$07
JSR sprite
JSR draw
JSR clear
JMP main
vertb LDX #$00 ;vertical blank, We all know what this is about
LDA #$02
STA WSYNC
STA WSYNC
STA WSYNC ;I don't see anything any different here from
STA VSYNC ;any other VBLANK routine I've done.
STA WSYNC
STA WSYNC
LDA #$2C
STA TIM64T
LDA #$00
STA WSYNC
STA VSYNC
RTS
sprite lda sprt1,y ;just loading the sprite data into RAM
sta SPRITE1,y ;it'll make it easier when I animate them
dey
bpl sprite
ldy #$07
spritb lda sprt2,y
sta SPRITE2,y
dey
bpl spritb
rts
draw LDA INTIM ;check to see if it's time to draw a frame
BNE draw
sta WSYNC
sta VBLANK ;I don't see anything unusual here.
sta WSYNC
;insert display kernal
LDY HPOS1 ;pretty darn simple kernal. According to my
pos1 dey ;math it should be 192 scanlines on the bean
bpl pos1
sta RESP0
sta WSYNC
ldy #$07
sl1 lda SPRITE1,y
sta GP0
sta WSYNC
dey
bpl sl1
lda #$00
sta GP0
sta WSYNC
ldy HPOS2
pos2 dey
bpl pos2
sta RESP1
sta WSYNC
ldy #$07
sl2 lda SPRITE2,y
sta GP1
sta WSYNC
dey
bpl sl2
lda #$00
sta GP1
sta WSYNC
ldx #$ab
blow sta WSYNC
dex
bpl blow
rts
clear LDA #$24 ;set timer for overscan
STA TIM64T
LDA #$02 ;clear the screen and turn off the video
STA WSYNC
STA VBLANK
LDA #$00
STA PF0
STA PF1
STA PF2
sta COLUPF
sta COLUBK ;just my standard clearing routine.
dec DELAY
bpl oscan
lda #$0F
sta DELAY
lda MTOGGLE
cmp #$01
beq rightb
inc HPOS1
lda HPOS1
cmp #$0D
bpl righta
jmp oscan
righta lda #$01
sta MTOGGLE
rightb dec HPOS1
lda HPOS1
cmp #$02
bmi lefta
jmp oscan
lefta lda #$00
sta MTOGGLE ;all that just takes care of the movement
oscan sec
lda #$0D
sbc HPOS1
sta HPOS2
oscana lda INTIM ;ordinary overscan stuff.
bne oscana
sta WSYNC
rts
sprt1 .byte %01100110 ;sprite data
.byte %00100100
.byte %00111100
.byte %00011000
.byte %00011000
.byte %11111111
.byte %00111100
.byte %00111100
sprt2 .byte %10101010
.byte %10101010
.byte %11111111
.byte %11111111
.byte %10111011
.byte %10011001
.byte %01111110
.byte %00111100
org $FFFC
.word start
.word start
-----------------------------------------------------------------------------
Tiy would think if it were some sort of missed bit sync problem the picture
wouldn't be stable at all. I'm totally boggled here. All my other homebrews
still work okay.
The .BIN file will follow so you wont have to compile this mess.
CRACKERS
(What the.... from hell!!!!!)
--
Accordionist - Wethifl Musician - Atari 2600 Collector | /\/\
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