Jump to content

  • Log In with Google      Sign In   
  • Create Account





A C64 Game - Step 97

Posted by Endurion, in C64 26 April 2013 · 491 views

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

Attached Image

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 Posted Image
;------------------------------------------------------------

;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


Previous Step Next Step

Attached Files






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.

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.

October 2014 »

S M T W T F S
   1234
567891011
12131415161718
19202122232425
2627282930 31  
PARTNERS