Ok here's my code. It's pieced together from several other code snippets
I've found in the archives. It use to display some multi-colored stripes on
the screen but I disabled those for the time being by clearing the Playfield
registers.
What it should do is display my stupid ship graphic on scanline 92 and
recognize the joystick input (that part works), but I'm getting nothing but
a blank screen. Maybe I shouldn't write zero's to the Playfield registers?
If someone can fill me in on what stupid mistake I'm making I'd appreciate
it.
Tempest
;***************************************
;* Galactic Tactic *
;* Ver. 0.01 *
;***************************************
processor 6502
include vcs.h
org $F000
;*****************************************************
;* Equates *
;*****************************************************
Scanctrl equ $91
;*****************************************************
;* Clear Memory *
;*****************************************************
Start
SEI ; Disable interrupts, if there are any.
CLD ; Clear BCD math bit.
LDX #$FF
TXS ; Set stack to top of RAM.
LDA #0 ;Zero everything except VSYNC.
B1 STA 0,X
DEX
BNE B1
JSR GameInit
;******************************************************
; Main Loop *
;******************************************************
MainLoop
JSR VerticalBlank ;Execute the vertical blank.
JSR CheckSwitches ;Check console switches.
JSR GameCalc ;Do calculations during Vblank
JSR DrawScreen ;Draw the screen
JSR OverScan ;Do more calculations during overscan
JMP MainLoop ;Continue forever.
;******************************************************
; Verticle Blanking *
;******************************************************
VerticalBlank
LDA #2 ;VBLANK was set at the beginning of overscan.
STA WSYNC
STA WSYNC
STA WSYNC
STA VSYNC ;Begin vertical sync.
STA WSYNC ;First line of VSYNC
STA WSYNC ;Second line of VSYNC.
LDA #44 ;Set timer to activate during the last line of VBLANK.
STA TIM64T
LDA #0 ;Now we can end the VSYNC period.
STA WSYNC ;Third line of VSYNC.
STA VSYNC ;Writing zero to VSYNC ends vertical sync period.
RTS
;******************************************************
; Check Switches *
;******************************************************
CheckSwitches
LDA #0 ;Clear collision latches
STA CXCLR ;In a real game, we'd probably check the collision
;registers before clearing them.
RTS
;******************************************************
; Game Calculations *
;******************************************************
GameCalc
LDA #0
STA COLUBK ;Background will be black.
LDA #$80
BIT SWCHA
BEQ Right
LSR
BIT SWCHA
BEQ Left
LSR
BIT SWCHA
BEQ Down
LSR
BIT SWCHA
BEQ Up
JMP NoStick
Right
JMP NoStick
Left
JMP NoStick
Down
JMP NoStick
Up
JMP NoStick
NoStick
LDA #$00 ;Pixels: 00000000
STA PF0
STA PF2 ;Store alternating bit pattern to the playfield registers
ASL ;Because PF1 displays in the opposite bit order from PF0
STA PF1
LDA #1
STA CTRLPF ;Let's reflect the playfield just cause we feel like it =)
STA RESP0
RTS
;******************************************************
; Draw Screen *
;******************************************************
DrawScreen
LDA INTIM
BNE DrawScreen ;Loops until the timer is done - that means we're
;somewhere in the last line of vertical blank.
STA WSYNC ;End the current scanline - the last line of VBLANK.
STA VBLANK ;End the VBLANK period. The TIA will display stuff
;starting with the next scanline. We do have 23 cycles
;of horizontal blank before it displays anything.
LDY #192 ;We're going to use Y to count scanlines.
ScanLoop
STY COLUPF ;Keep changing the playfield color every line
STA WSYNC ;Wait for end of scanline
DEY
TYA
CMP #$91
BEQ DrawSprite
BNE ScanLoop ;Count scanlines.
RTS
DrawSprite
LDX #07
cont STA WSYNC
LDA Sprite,X
STA GRP0
DEX
BNE cont
RTS
;******************************************************
; Overscan *
;******************************************************
OverScan ;We've got 30 scanlines to kill.
LDX #30 ;In a real game, we'd probably be doing calculations here,
KillLines ;possibly using the timer again to tell us when overscan
STA WSYNC ;is done instead of counting the scanlines.
DEX
BNE KillLines
RTS
GameInit ;Usually called to start a new game.. we're not using it yet.
RTS
Sprite
.byte %00000000
.byte %00011000
.byte %00011000
.byte %10011001
.byte %10100101
.byte %11000011
.byte %00000000
org $FFFC
.word Start
.word Start
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