Well gang. I'm starting work on my Virtual Pet for the 2600 now. But
the colours of the 2600 just seem kind of abstract to me. I mean just
how does 11011010 look? So here's a simple, very simple, damn simple
programme that let's you cycle through the colour pallet, one at a time
and gives you a binary representation of what colour you are on with the
PF2 register.
Maybe someone can use this programme to actually assign some RGB values
to the 2600's pallet.
I've only tested it out on an emulator so far. Here's the programme.
----------------------------------------------------------------------------
processor 6502
VSYNC = $00
VBLANK = $01
WSYNC = $02
COLUPF = $08
COLUBK = $09
PF0 = $0D
PF1 = $0E
PF2 = $0F
CXCLR = $2C
SWCHA = $280
INTIM = $284
TIM64T = $296
COLOUR = $80 ;the value for the colour
DELAY = $81 ;the value for the joystick delay
org $F000
start SEI
CLD
LDX #$FF
TXS
LDA #$00
zero STA $00,X ;looks familiar, right?
DEX ;typical zeroing routine
BNE zero
sta COLUBK
sta PF0
sta PF1
sta PF2
sta COLUPF
main JSR vertb ;main loop
JSR draw
JSR oscan
JMP main
vertb LDX #$00 ;vertical blank, We all know what this is about
LDA #$02
STA WSYNC
STA WSYNC
STA WSYNC
STA VSYNC
STA WSYNC
STA WSYNC
LDA #$2C
STA TIM64T
LDA #$00
STA CXCLR
STA WSYNC
STA VSYNC
RTS
draw LDA INTIM
BNE draw
sta WSYNC
sta VBLANK
ldx #$C0 ;192 scanlines
lda COLOUR ;loads the colour
sta COLUBK ;into the playfield background
sta PF1 ;also displays a binary representation to PF1
lda #$0F
sta PF2 ;just to help the binary number look clear.
doit STA WSYNC ;not much going on here, just repeat the
dex ;same scanline 192 times
beq done
jmp doit
done rts
oscan jsr joy ;go check the joystick
LDX #$1E
waste STA WSYNC ;no game logic so let's blow the overscan
DEX
BNE waste
RTS
joy lda SWCHA ;check the joystick ports
cmp #$7F ;see if it's moved right (joystick 1)
beq right
cmp #$BF ;see if it's moved left (joystick 1)
beq left
rts
right inc DELAY ;our joystick delay timer is increased
lda DELAY
cmp #$1E ;check to see if we've had enough delay (30 frames)
bpl clrup ;yep. Let's increase the colour
rts
left inc DELAY ;joystick timer increased
lda DELAY
cmp #$1E ;30 frames yet?
bpl clrdwn ;yep, decrease colour
rts
clrup lda COLOUR
cmp #$FE ;check to see if we've hit the maximum colour
bne upit ;nope, then let's increase it
lda #$00
sta DELAY ;reset joystick delay timer
rts
upit inc COLOUR ;okay... this is sloppy, but it works
inc COLOUR ;increases the colour value to next setting
lda #$00
sta DELAY ;reset joystick delay
rts
clrdwn lda COLOUR
cmp #$00 ;see if we've hit minimum colour
bne dwnit ;nope, then let's decrease it
lda #$00
sta DELAY ;reset the joystick delay
rts
dwnit dec COLOUR ;yeah, I know, not exactly eligant, but
dec COLOUR ;it increases the colour value to next setting
lda #$00
sta DELAY ;reset joystick delay
rts
org $FFFC
.word start
.word start
----------------------------------------------------------------------------
I'm sorry I have to post my work as source code, but you see I'm a freenet
user. So while I get the advantage of very inexpensive internet access, some
of the frills like being able to attach a binary to a text message are
unavailable to me.
Incidently, colour appearance varies greatly between an emulator and a
real 2600. So you'll want to check your colours on both systems to
make sure they're okay.
Now that I know what the colours look like for each value it will be
easier to assign specific colours for different elements in my future
projects.
I know I could have made this programme more useful by drawing a hex
representation of the colour with sprites, but I just wanted to
whip this off quickly so I could get to work on my real programme.
If someone else wants to fix this up then be my guest.
CRACKERS
(A fine capitalist tool from hell!!!!!)
--
Accordionist - Wethifl Musician - Atari 2600 Collector | /\/\
*NEW CrAB URL* http://www.hwcn.org/~ad329/crab.html ***| \^^/
Bira Bira Devotee - FES Member - Samurai Pizza Cats Fan| =\/=
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