A C64 Game - Step 5

Our next big step:

Obviously we want some play screen, and more obviously, we are not going to store full screens (we've got only 64Kb Ram after all). Therefore there's a level build routine that allows to build a screen with various building elements. For now we'll start out with vertical and horizontal lines. Since we always have a level border at the screen edges we'll have the border as a second screen data block (LEVEL_BORDER_DATA).
To allow faster screen building we also have a table with the precalculated char offsets of a vertical line (SCREEN_LINE_OFFSET_TABLE_LO and SCREEN_LINE_OFFSET_TABLE_HI). Note that with the C64s mnemonics the best way to get stuff done is tables, tables, tables.



The BuildScreen sub routine clears the play screen area, builds the level data and then the border. The level data is a collection of primitives which are then worked through until LD_END is hit.

The first addition is the call to the buildroutine:

          ;setup level
          lda #0
          sta LEVEL_NR
          jsr BuildScreen

The BuildScreen routine sets up a level completely. It starts out with clearing the current play area (not the full screen). Then LEVEL_NR is used to look up the location of the level data in table SCREEN_DATA_TABLE.
.BuildLevel is jumped to to actually work through the level primitives and put them on screen. Then we use LEVEL_BORDER_DATA as second level to display a border on the screens edges.

.BuildLevel uses Y as index through the data. Depending on the first byte different routines are called (.LineH, .LineV, .LevelComplete). Since I think levels may be complex and use more than 256 bytes we add Y to the level data pointers so we can start out with Y=0 for the next primitive.

;------------------------------------------------------------
;BuildScreen
;creates a screen from level data
;------------------------------------------------------------
!zone BuildScreen
BuildScreen
          lda #0
          ldy #6
          jsr ClearPlayScreen

          ;get pointer to real level data from table
          ldx LEVEL_NR
          lda SCREEN_DATA_TABLE,x
          sta ZEROPAGE_POINTER_1
          lda SCREEN_DATA_TABLE + 1,x
          sta ZEROPAGE_POINTER_1 + 1
          jsr .BuildLevel

          ;get pointer to real level data from table
          lda #<‍LEVEL_BORDER_DATA
          sta ZEROPAGE_POINTER_1
          lda #>LEVEL_BORDER_DATA
          sta ZEROPAGE_POINTER_1 + 1
          jsr .BuildLevel
          rts

.BuildLevel
          ;work through data
          ldy #255
.LevelDataLoop
          iny
          lda (ZEROPAGE_POINTER_1),y
          cmp #LD_END
          beq .LevelComplete
          cmp #LD_LINE_H
          beq .LineH
          cmp #LD_LINE_V
          beq .LineV
.LevelComplete
          rts

.NextLevelData
          pla

          ;adjust pointers so we are able to access more
          ;than 256 bytes of level data
          clc
          adc #1
          adc ZEROPAGE_POINTER_1
          sta ZEROPAGE_POINTER_1
          lda ZEROPAGE_POINTER_1 + 1
          adc #0
          sta ZEROPAGE_POINTER_1 + 1
          ldy #255
          jmp .LevelDataLoop

The primitive display routines .LineH and .LineV are rather straight forward. Read the parameters, and put the character and color values in place.

.LineH
          ;X pos
          iny
          lda (ZEROPAGE_POINTER_1),y
          sta PARAM1

          ;Y pos
          iny
          lda (ZEROPAGE_POINTER_1),y
          sta PARAM2

          ;width
          iny
          lda (ZEROPAGE_POINTER_1),y
          sta PARAM3

          ;char
          iny
          lda (ZEROPAGE_POINTER_1),y
          sta PARAM4

          ;color
          iny
          lda (ZEROPAGE_POINTER_1),y
          sta PARAM5

          ;store target pointers to screen and color ram
          ldx PARAM2
          lda SCREEN_LINE_OFFSET_TABLE_LO,x
          sta ZEROPAGE_POINTER_2
          sta ZEROPAGE_POINTER_3
          lda SCREEN_LINE_OFFSET_TABLE_HI,x
          sta ZEROPAGE_POINTER_2 + 1
          clc
          adc #( ( SCREEN_COLOR - SCREEN_CHAR ) & 0xff00 ) >> 8
          sta ZEROPAGE_POINTER_3 + 1
          tya
          pha
          ldy PARAM1
.NextChar
          lda PARAM4
          sta (ZEROPAGE_POINTER_2),y
          lda PARAM5
          sta (ZEROPAGE_POINTER_3),y
          iny
          dec PARAM3
          bne .NextChar
          jmp .NextLevelData

.LineV
          ;X pos
          iny
          lda (ZEROPAGE_POINTER_1),y
          sta PARAM1

          ;Y pos
          iny
          lda (ZEROPAGE_POINTER_1),y
          sta PARAM2

          ;height
          iny
          lda (ZEROPAGE_POINTER_1),y
          sta PARAM3

          ;char
          iny
          lda (ZEROPAGE_POINTER_1),y
          sta PARAM4

          ;color
          iny
          lda (ZEROPAGE_POINTER_1),y
          sta PARAM5

          ;store target pointers to screen and color ram
          ldx PARAM2
          lda SCREEN_LINE_OFFSET_TABLE_LO,x
          sta ZEROPAGE_POINTER_2
          sta ZEROPAGE_POINTER_3
          lda SCREEN_LINE_OFFSET_TABLE_HI,x
          sta ZEROPAGE_POINTER_2 + 1
          clc
          adc #( ( SCREEN_COLOR - SCREEN_CHAR ) & 0xff00 ) >> 8
          sta ZEROPAGE_POINTER_3 + 1
          tya
          pha

          ldy PARAM1
.NextCharV
          lda PARAM4
          sta (ZEROPAGE_POINTER_2),y
          lda PARAM5
          sta (ZEROPAGE_POINTER_3),y

          ;adjust pointer
          lda ZEROPAGE_POINTER_2
          clc
          adc #40
          sta ZEROPAGE_POINTER_2
          sta ZEROPAGE_POINTER_3
          lda ZEROPAGE_POINTER_2 + 1
          adc #0
          sta ZEROPAGE_POINTER_2 + 1
          clc
          adc #( ( SCREEN_COLOR - SCREEN_CHAR ) & 0xff00 ) >> 8
          sta ZEROPAGE_POINTER_3 + 1
          dec PARAM3
          bne .NextCharV
          jmp .NextLevelData

step5.zip