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A C64 Game - Step 97

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And here's a little gameplay update, the bats. The diagonal movement was too predictable, so now there's more randomness to it.

step97.png

The bat will move in curves. On every end of a curve the new direction will be decided randomly. Two tables are enough, however due to the C64 using two bit complement negative values are annoying to handle. Therefor I went the naive route and simply added code for every case. Ugly, but it works :)

;------------------------------------------------------------
;simply move diagonal
;------------------------------------------------------------
!zone BehaviourBatDiagonal
BehaviourBatDiagonal 
          jsr HandleHitBack 
          beq .NoHitBack 
          
          rts
          
.RandomDir 
          jsr GenerateRandomNumber 
          and #$07 
          sta SPRITE_DIRECTION,x 
          inc SPRITE_DIRECTION,x 
          lda #0 
          sta SPRITE_MOVE_POS,x 
          rts
          
.NoHitBack 
          lda DELAYED_GENERIC_COUNTER 
          and #$03 
          bne .NoAnimUpdate 
          inc SPRITE_ANIM_POS,x 
          lda SPRITE_ANIM_POS,x 
          and #$03 
          sta SPRITE_ANIM_POS,x 
          tay 
          lda BAT_ANIMATION,y 
          sta SPRITE_POINTER_BASE,x 
          
.NoAnimUpdate 
          lda SPRITE_DIRECTION,x 
          beq .RandomDir 
          cmp #1 
          beq .MoveCCWWN 
          cmp #2 
          beq .MoveCCWSW 
          cmp #3 
          beq .MoveCCWES 
          cmp #4 
          beq .MoveCCWNE 
          cmp #5 
          bne + 
          
          jmp .MoveCWWS
          
+ 
          cmp #6 
          bne + 
          
          jmp .MoveCWNW
          
+ 
          cmp #7 
          bne + 
          
          jmp .MoveCWEN
          
+ 
          cmp #8
          bne + 
          
          jmp .MoveCWSE
          
+ 
.NextStep 
          inc SPRITE_MOVE_POS,x 
          lda SPRITE_MOVE_POS,x 
          cmp #16 
          bne + 
          
          lda #0 
          sta SPRITE_DIRECTION,x 
          sta SPRITE_MOVE_POS,x
          
+ 
          rts 
          
.MoveCCWWN 
          ldy SPRITE_MOVE_POS,x 
          lda PATH_CURVE,y 
          sta PARAM3 
          jsr .TryMoveUp 
          
          ldy SPRITE_MOVE_POS,x 
          lda PATH_CURVE_R,y 
          sta PARAM3 
          jsr .TryMoveRight 
          jmp .NextStep
          
.MoveCCWSW 
          ldy SPRITE_MOVE_POS,x 
          lda PATH_CURVE,y 
          sta PARAM3 
          jsr .TryMoveLeft 
          ldy SPRITE_MOVE_POS,x 
          lda PATH_CURVE_R,y 
          sta PARAM3 
          jsr .TryMoveUp 
          jmp .NextStep
          
.MoveCCWES 
          ldy SPRITE_MOVE_POS,x 
          lda PATH_CURVE,y 
          sta PARAM3 
          jsr .TryMoveDown 
          ldy SPRITE_MOVE_POS,x 
          lda PATH_CURVE_R,y 
          sta PARAM3 
          jsr .TryMoveLeft 
          jmp .NextStep
          
.MoveCCWNE 
          ldy SPRITE_MOVE_POS,x 
          lda PATH_CURVE,y 
          sta PARAM3 
          jsr .TryMoveRight 
          ldy SPRITE_MOVE_POS,x 
          lda PATH_CURVE_R,y 
          sta PARAM3 
          jsr .TryMoveDown 
          jmp .NextStep
          
.MoveCWWS 
          ldy SPRITE_MOVE_POS,x 
          lda PATH_CURVE,y 
          sta PARAM3 
          jsr .TryMoveDown 
          ldy SPRITE_MOVE_POS,x 
          lda PATH_CURVE_R,y 
          sta PARAM3 
          jsr .TryMoveRight 
          jmp .NextStep
          
.MoveCWNW 
          ldy SPRITE_MOVE_POS,x 
          lda PATH_CURVE,y 
          sta PARAM3 
          jsr .TryMoveLeft 
          ldy SPRITE_MOVE_POS,x 
          lda PATH_CURVE_R,y 
          sta PARAM3 
          jsr .TryMoveDown 
          jmp .NextStep
          
.MoveCWEN 
          ldy SPRITE_MOVE_POS,x 
          lda PATH_CURVE,y 
          sta PARAM3 
          jsr .TryMoveUp 
          ldy SPRITE_MOVE_POS,x 
          lda PATH_CURVE_R,y 
          sta PARAM3 
          jsr .TryMoveLeft 
          jmp .NextStep
          
.MoveCWSE 
          ldy SPRITE_MOVE_POS,x 
          lda PATH_CURVE,y 
          sta PARAM3 
          jsr .TryMoveRight 
          ldy SPRITE_MOVE_POS,x 
          lda PATH_CURVE_R,y 
          sta PARAM3 
          jsr .TryMoveUp 
          jmp .NextStep
          
.Blocked 
          lda #0 
          sta SPRITE_DIRECTION,x 
          rts
          
.TryMoveUp 
          beq + 
          jsr ObjectMoveUpBlocking 
          beq .Blocked 
          
          dec PARAM3 
          jmp .TryMoveUp 
          
+ 
          rts 
          
.TryMoveDown 
          beq + 
          jsr ObjectMoveDownBlocking 
          beq .Blocked 
          dec PARAM3 
          jmp .TryMoveDown 
          
+ 
          rts 
          
.TryMoveLeft 
          beq + 
          jsr ObjectMoveLeftBlocking 
          beq .Blocked 
          
          dec PARAM3 
          jmp .TryMoveLeft 
          
+ 
          rts 
          
.TryMoveRight 
          beq + 
          jsr ObjectMoveRightBlocking 
          beq .Blocked 
          
          dec PARAM3 
          jmp .TryMoveRight 
          
+ 
          rts



The table are simple delta updates per frame, one being the reverse of the other:

PATH_CURVE 
          !byte 0 
          !byte 0 
          !byte 1 
          !byte 0 
          !byte 0 
          !byte 1 
          !byte 0 
          !byte 1 
          !byte 0 
          !byte 1 
          !byte 1 
          !byte 1 
          !byte 1 
          !byte 1 
          !byte 1 
          !byte 1
          
PATH_CURVE_R 
          !byte 1 
          !byte 1 
          !byte 1 
          !byte 1 
          !byte 1 
          !byte 1 
          !byte 1 
          !byte 0 
          !byte 1 
          !byte 0 
          !byte 1 
          !byte 0 
          !byte 0 
          !byte 1 
          !byte 0 
          !byte 0


The other thing is a little fix for a bug I found with the vanishing bats for the last portal stages. The bats could appear outside the playing area. The fix are two border values for left and right which will be set to be farther inside the screen if a portal level is run:

          ;set default 
          lda #10 
          sta SPAWN_LEFT_BORDER 
          lda #30 
          sta SPAWN_RIGHT_BORDER 
          
          ;adjust spawn border on portal level 
          lda LEVEL_CONFIG 
          and #$04 
          beq + 
          
          lda #15 
          sta SPAWN_LEFT_BORDER 
          lda #25 
          sta SPAWN_RIGHT_BORDER
          
+


...and adjust the spawn code inside BehaviourBatVanishing:

          ;position diagonal above/below player 
          lda SPRITE_CHAR_POS_X 
          cmp #SPAWN_LEFT_BORDER 
          
          ;was #10 
          bcc .SpawnOnRight 
          cmp #SPAWN_RIGHT_BORDER 
          
          ;was #30 
          bcs .SpawnOnLeft


step97.zip

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Doesn't the 6502 just use the same two's complement used in pretty much all modern CPUs? (some specialized DSPs being the only modern exception) I don't see what's so weird about it, in fact the whole point is that they can be treated the same as unsigned numbers for a lot of operations.

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Yeah it does, but having only the MSB set as a negative flag would make working with it easier. At least for that particular use :)

 

You could use BPL or BMI to check the flag and if BMI would be branched simply AND #$7f the value to have the result.

 

Two's complement is however really nice to simply "add" negative values.

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