398 lines
9.5 KiB
Plaintext
398 lines
9.5 KiB
Plaintext
processor 6502
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include "vcs.h"
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include "macro.h"
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include "xmacro.h"
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org $f000
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CurRow equ $8f
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NumRows equ 5
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EnemyRows0 equ $80
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EnemyYpos0 equ $85
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EnemyXofs0 equ $8a
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Temp equ $90
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MissileY0 equ $91
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MissileY1 equ $92
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MissileX0 equ $93
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BaseX equ $94
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ZPRoutine equ $a0
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ZPWrites equ ZPRoutine+(KernelStores-KernelStart)
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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Start
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CLEAN_START
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; Copy Grid Kernel routine into RAM so we can modify it.
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ldx #KernelEnd-KernelStart-1
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CopyRoutineLoop
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lda KernelStart,x
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sta ZPRoutine,x
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dex
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bpl CopyRoutineLoop
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; Set up interlocuters
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lda #$ff
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sta EnemyRows0+4
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sta EnemyRows0+3
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sta EnemyRows0+2
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sta EnemyRows0+1
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sta EnemyRows0+0
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lda #190
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sta EnemyYpos0+4
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lda #170
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sta EnemyYpos0+3
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lda #150
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sta EnemyYpos0+2
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lda #130
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sta EnemyYpos0+1
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lda #110
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sta EnemyYpos0+0
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lda #40
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sta MissileY0
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lda #50
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sta MissileY1
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lda #65
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sta BaseX
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lda #$df
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sta COLUPF ; use the ball for the player's missile
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; Try out the Grid Kernel (not neccessary except for the IDE to get timing)
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ldy #0 ; just one line
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jsr KernelStart
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NextFrame
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VERTICAL_SYNC
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; VBLANK
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TIMER_SETUP 37
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jsr ReadControls
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jsr MoveMissiles
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TIMER_WAIT
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; Main frame
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TIMER_SETUP 192
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jsr DrawEnemyFormation
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jsr DrawBelowFormation
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jsr DrawPlayerBase
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lda #$10
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sta COLUBK ; set ground color, just in time!
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lda #0
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sta ENABL
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sta ENAM1 ; turn off missile and ball
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; Overscan
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TIMER_SETUP 30
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TIMER_WAIT
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lda #0
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sta COLUBK ; clear ground color
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; Jump to next frame
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jmp NextFrame
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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; Draw all lines of the enemy formation.
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DrawEnemyFormation
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lda #4
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sta CurRow
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; We're going to use the timer to wait for the
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; scanline to approach the next row of enemies.
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; We'll draw any missiles while we're waiting.
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WaitForRow
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jsr DrawMissiles
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ldx CurRow
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lda EnemyYpos0,x
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cmp INTIM
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bcc WaitForRow
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; We've reached the right scanline, so we'll
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; set up the NUSIZ registers and clear collisions.
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lda #1 ; two copies, close for NUSIZ
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sta NUSIZ0
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sta NUSIZ1
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sta CXCLR ; clear collisions
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; WSYNC and draw the row of sprites.
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sta WSYNC
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jsr DrawFormation
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; See if any of the player objects collided with ball.
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lda CXP0FB
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ora CXP1FB
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and #$c0
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beq NoCollision
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; We've got a collision, now we need to see where it is.
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; Our grid spacing is 12 pixels, so we need to divide by 12.
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; We'll have to account for the X offset of the sprites, too.
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lda MissileX0
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ldx #$ff
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sec
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sbc #13
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DivideBy12
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inx
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sbc #12
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bcs DivideBy12
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txa
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; Now we lookup the bitmask to use, and erase the
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; bit that corresponds to the enemy.
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tax
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lda PowersOf2,x
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eor #$ff
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ldx CurRow
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and EnemyRows0,x
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sta EnemyRows0,x
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; Now we destroy the missile too.
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lda #0
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sta MissileY0
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NoCollision
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dec CurRow
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bpl WaitForRow
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lda #0 ; turn off two-copy mode
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sta NUSIZ0
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sta NUSIZ1
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rts
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; ;;
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; Draw a line of the formation.
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; First we'll modify the routine in RAM,
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; then we'll transfer control to it.
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DrawFormation
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ldx CurRow
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lda EnemyRows0,x
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ldx #1 ; start at KernelStores+1
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ShiftLoop
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ldy #RESP0
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ror
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bcs NoClearEnemy0
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ldy #$30 ; no-op
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NoClearEnemy0
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sty ZPWrites,x
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inx
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inx
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ldy #RESP1
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ror
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bcs NoClearEnemy1
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ldy #$30 ; no-op
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NoClearEnemy1
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sty ZPWrites,x
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inx
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inx
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cpx #16 ; 8*2 bytes
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bcc ShiftLoop
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; Now we jump to our newly-modified kernel in RAM
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; to draw the row of sprites.
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ldy EnemyColorFrame0 ; get height -> Y
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jsr ZPRoutine ; draw sprites
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rts
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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; The Grid Kernel, which we'll copy into RAM at the
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; start of the program. We'll use the "STA RESPn,x"
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; instruction since it takes up 4 cycles, which gives
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; us a horizontal spacing of 12 pixels.
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KernelStart
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KernelLoop
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lda EnemyFrame0,y ; load bitmap
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sta WSYNC
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ldx EnemyColorFrame0,y ; load color
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sta GRP0
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sta GRP1
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stx COLUP0
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stx COLUP1
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ldx #0 ; so we can do the STA RESPn,x variant
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KernelStores
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; These STAs are meant to be modified
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sta RESP0,x
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sta RESP1,x
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sta RESP0,x
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sta RESP1,x
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sta RESP0,x
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sta RESP1,x
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sta RESP0,x
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sta RESP1,x
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; End of modifiable section
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dey ; also acts as 2-cycle delay
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stx.w GRP0 ; clear player 0 bitmap (4-cycle version)
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sta RESP0 ; reset player 0 position
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stx GRP1 ; clear player 1 bitmap
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sta RESP1 ; reset player 1 position
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bpl KernelLoop ; repeat until Y < 0
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rts
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KernelEnd
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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; Draw the empty space in between interlocuters
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; and the base, which consists of missiles.
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DrawBelowFormation
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lda #$FF ; set missile colors
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sta COLUP0
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sta COLUP1
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jsr DrawMissiles
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lda INTIM
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cmp #17 ; are we close to the bottom?
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bcs DrawBelowFormation
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sta WSYNC ; exit in a known state
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rts
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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; Read the timer to see if we should draw either
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; or both missiles.
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; We do this in constant time by using the carry flag of
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; the CMP operation.
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DrawMissiles
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lda INTIM ; load timer value
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pha
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sec
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sbc MissileY0
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cmp #8 ; within 8 lines of missile?
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lda #3 ; bit 1 now set
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adc #0 ; if carry set, bit 1 cleared
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sta ENABL ; enable/disable ball
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pla
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sec
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sbc MissileY1
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cmp #8 ; within 8 lines of missile?
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lda #3 ; bit 1 now set
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adc #0 ; if carry set, bit 1 cleared
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sta ENAM1 ; enable/disable missile
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rts
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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; Draw the player's base
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DrawPlayerBase
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lda BaseX
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ldx #0
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jsr SetHorizPos ; first set the horizontal position
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ldy ColorFrame0 ; get sprite height
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DrawBaseLoop
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lda Frame0,y
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ldx ColorFrame0,y
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sta WSYNC
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sta GRP0
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stx COLUP0
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dey
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bpl DrawBaseLoop ; repeat until Y < 0
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rts
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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; Move missiles
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; Missile 0 moves up, missile 1 moves down.
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MoveMissiles
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lda MissileY0
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beq NoMoveMiss0
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inc MissileY0
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NoMoveMiss0
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lda MissileY1
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beq NoMoveMiss1
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dec MissileY1
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NoMoveMiss1
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rts
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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; Move player and shoot missiles with joystick/button
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ReadControls
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ldx BaseX
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bit SWCHA ; read joystick
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bvs NoMoveLeft ; bit 7 set?
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lda #%0
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sta REFP0 ; reset sprite flip
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dex
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bne NoMoveRight
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NoMoveLeft
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bmi NoMoveRight ; bit 6 set?
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lda #%1000
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sta REFP0 ; set sprite flip
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inx
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bmi NoStoreX
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NoMoveRight
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stx BaseX
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NoStoreX
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; Shoot a missile when fire button pressed
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bit INPT4 ; read button
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bmi NoFireButton ; bit 7 set?
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lda #10
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sta MissileY0 ; reset missile
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lda BaseX
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clc
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adc #6
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sta MissileX0
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ldx #4
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jsr SetHorizPos ; set ball X pos to player's X
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NoFireButton
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rts
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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; Set horizontal position of object
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SetHorizPos
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sta WSYNC ; start a new line
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sec ; set carry flag
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DivideLoop
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sbc #15 ; subtract 15
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bcs DivideLoop ; branch until negative
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eor #7 ; calculate fine offset
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asl
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asl
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asl
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asl
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sta HMP0,x ; set fine offset
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sta RESP0,x ; fix coarse position
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sta WSYNC
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sta HMOVE
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sta HMCLR
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rts ; return to caller
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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; Make sure the bitmap tables used by the Grid Kernel
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; do not cross page boundaries, because if so the CPU will
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; add an extra cycle and mess up our timing!
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org $ff00
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; Enemy cat-head graphics data
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EnemyFrame0
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.byte #0
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.byte #%00111100;$AE
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.byte #%01000010;$AE
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.byte #%11100111;$AE
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.byte #%11111111;$AC
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.byte #%10011001;$8E
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.byte #%01111110;$8E
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.byte #%11000011;$98
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.byte #%10000001;$98
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EnemyColorFrame0
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.byte #8 ; height
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.byte #$AE;
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.byte #$AC;
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.byte #$A8;
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.byte #$AC;
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.byte #$8E;
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.byte #$8E;
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.byte #$98;
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.byte #$94;
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; Player graphics data, such bitmap
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Frame0
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.byte #0
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.byte #%11111111;$0E
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.byte #%11111111;$0E
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.byte #%11101111;$0E
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.byte #%10010111;$0E
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.byte #%10011111;$FE
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.byte #%11111111;$FE
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.byte #%11010111;$FE
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.byte #%01111110;$FA
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.byte #%00111110;$FA
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.byte #%01000001;$FA
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ColorFrame0
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.byte #10 ; height
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.byte #$0E;
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.byte #$0E;
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.byte #$0E;
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.byte #$0E;
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.byte #$FE;
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.byte #$FE;
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.byte #$FE;
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.byte #$FA;
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.byte #$FA;
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.byte #$FA;
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PowersOf2
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.byte #$1,#$2,#$4,#$8,#$10,#$20,#$40,#$80
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; Epilogue
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org $fffc
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.word Start
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.word Start
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