Hello stella programmers:
Just recently got started with atari 2600 programming,
and I am learning a lot. Searching through the archives answers
most of my questions.
I am learning how to draw a playfield. I have read Nick's famous
playfield example and tried different variations that have
been posted on the list. I have written a small program
which only purpose is to draw a rectangle repeatedly
from the top to the bottom of the screen (without worrying about the
reflected portion of the screen). I initialized the playflied
registers to draw something like this:
* * * * * * * * * * * * * * * *
* *
* *
* *
* *
* *
* *
* * * * * * * * * * * * * * * *
When I run the code, the atari draws the rectangle
omitting one of the long lines (leaving the rectangle 'open').
How can I fix this? Let me know what I am doing wrong.
My guess I am not correctly counting which playfield line to draw.
Thank you for you time.
--Rico Dones
Here is the code:
; Learning atari playfields
;
; Objective: Draw a rectangle many times
; using the playfield registers.
;
processor 6502
include eckvcs.h
org $F000
START
SEI
CLD
LDX #$FF
TXS
LDA #0
CLEARMEM
STA 0,X
DEX
BNE CLEARMEM
LDA #$00 ; SET BACKGROUND BLACK
STA COLUBK
LDA #$1E ; SET PLAYFIELD YELLOW
STA COLUPF
MAINLOOP
LDA #2 ; TURN ON VSYNC
STA VSYNC
STA WSYNC
STA WSYNC
STA WSYNC
LDA #43
STA TIM64T
LDA #0 ; TURN OFF VSYNC
STA VSYNC
VBLANKTIME
LDA INTIM
BNE VBLANKTIME
LDX #7 ; Use X register to count
; eight playfield lines
; to draw.
LDY #191
SCANLOOP
STA WSYNC ; Accumulator is still 0
STA VBLANK
LDA PFData0,X
STA PF0
LDA PFData1,X
STA PF1
LDA PFData2,X
STA PF2
DEX ; Decrement X.
BNE SKIPINITX ; If X != 0 then skip next line
LDX #7 ; Repeat playfield drawing
; by setting X to decimal 7.
SKIPINITX
DEY
BNE SCANLOOP
LDX #30
OVERSCAN
STA WSYNC
DEX
BNE OVERSCAN
JMP MAINLOOP
org $FF00 ; GRAPHICS DATA STARTS HERE
; Playfield graphic is supposed to look like this:
;
; * * * * * * * * * * * * * * * *
; * *
; * *
; * *
; * *
; * *
; * *
; * * * * * * * * * * * * * * * *
;
PFData0
.byte $F0,$10,$10,$10,$10,$10,$10,$F0
PFData1
.byte $FF,$00,$00,$00,$00,$00,$00,$FF
PFData2
.byte $FF,$A0,$A0,$A0,$A0,$A0,$A0,$FF
ORG $FFFC
.WORD START
.WORD START
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