A C64 Game - Step 68 | GameDev.net

Added a first version of Sams dark force, making it visible. It's not looking quite what I wanted it, but it is getting there.

To make things faster for removal of the beam we calc the required length and direction of the beam:

          ;redraw black beam
          ldx PARAM6

          ldy SPRITE_CHAR_POS_Y,x
          dey
          sty SAM_FORCE_START_Y
          lda SCREEN_LINE_OFFSET_TABLE_LO,y
          sta ZEROPAGE_POINTER_1
          sta ZEROPAGE_POINTER_2
          lda SCREEN_LINE_OFFSET_TABLE_HI,y
          sta ZEROPAGE_POINTER_1 + 1clcadc #( ( SCREEN_COLOR - SCREEN_CHAR ) >> 8 )
          sta ZEROPAGE_POINTER_2 + 1

          ldy SPRITE_CHAR_POS_X,x
          lda SPRITE_DIRECTION,x
          sta PARAM1

          lda #0
          sta SAM_FORCE_LENGTH

          ldx SPRITE_HELD
          dex

-
          lda #253
          sta (ZEROPAGE_POINTER_1),y
          lda #6
          sta (ZEROPAGE_POINTER_2),y

          inc SAM_FORCE_LENGTH

          lda PARAM1
          bne ++

          iny
          jmp +

++
          dey

+
          tya
          cmp SPRITE_CHAR_POS_X,x
          bne -

          ;store for later removal
          ldx PARAM6
          lda PARAM1
          bne +

          ;going right
          lda SPRITE_CHAR_POS_X,x
          sta SAM_FORCE_START_X
          jmp ++


+
          ;going left
          sty SAM_FORCE_START_X
          inc SAM_FORCE_START_X

++
          ldy SPRITE_HELD
          dey

The beam needs to be removed once Sam stops pressing the fire button or he or his enemy dies. Aren't we glad that we already have a back buffer we can use to restore the background properly?

;restore play field from force beam
!zone RemoveForceBeam
RemoveForceBeam
          ldy SAM_FORCE_START_Y

          lda SCREEN_LINE_OFFSET_TABLE_LO,y
          sta ZEROPAGE_POINTER_1
          sta ZEROPAGE_POINTER_2
          sta ZEROPAGE_POINTER_3
          sta ZEROPAGE_POINTER_4
          lda SCREEN_LINE_OFFSET_TABLE_HI,y
          sta ZEROPAGE_POINTER_1 + 1
          clc
          adc #( ( SCREEN_COLOR - SCREEN_CHAR ) & 0xff00 ) >> 8
          sta ZEROPAGE_POINTER_2 + 1
          sec
          sbc #( ( SCREEN_COLOR - SCREEN_BACK_CHAR ) & 0xff00 ) >> 8
          sta ZEROPAGE_POINTER_3 + 1
          sec
          sbc #( ( SCREEN_BACK_CHAR - SCREEN_BACK_COLOR ) & 0xff00 ) >> 8
          sta ZEROPAGE_POINTER_4 + 1

          ldy SAM_FORCE_START_X

-
          lda (ZEROPAGE_POINTER_3),y
          sta (ZEROPAGE_POINTER_1),y
          lda (ZEROPAGE_POINTER_4),y
          sta (ZEROPAGE_POINTER_2),y

          iny
          dec SAM_FORCE_LENGTH
          bne -

          rts

So when Sam dies we modify existing code to:

          lda SPRITE_HELD
          beq +

          jsr RemoveForceBeam
          lda #0
          sta SPRITE_HELD
+

Voila! Carnage made more visible ;)

step68.zip

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