A C64 Game - Step 28

The levels are currently rather ermm. sparse. Remember that a screen is built by several primitve types. To get things rolling a few new building types are added:

Namely a character/color area, an alternating line horizontally and vertically (alternating means toggling the characters +1/-1) and a quad (a 2x2 tile).



We'll start with the new constants:

LD_AREA = 4          ;data contains x,y,width,height,char,color
LD_LINE_H_ALT = 5    ;data contains x,y,width,char,color
LD_LINE_V_ALT = 6    ;data contains x,y,height,char,color
LD_QUAD = 7          ;data contains x,y,quad_id

...the new level building code table entries...

LEVEL_ELEMENT_TABLE_LO
          !byte <‍.LevelComplete
          !byte <‍LevelLineH
          !byte <‍LevelLineV
          !byte <‍LevelObject
          !byte <‍LevelArea
          !byte <‍LevelLineHAlternating
          !byte <‍LevelLineVAlternating
          !byte <‍LevelQuad

LEVEL_ELEMENT_TABLE_HI
          !byte >.LevelComplete
          !byte >LevelLineH
          !byte >LevelLineV
          !byte >LevelObject
          !byte >LevelArea
          !byte >LevelLineHAlternating
          !byte >LevelLineVAlternating
          !byte >LevelQuad

Here's the area code. Straight forward, fetch all needed parameters and build the area row by row:

!zone LevelAreaLevelArea
          ;X pos
          iny
          lda (ZEROPAGE_POINTER_1),y
          sta PARAM1

          ;Y pos
          iny
          lda (ZEROPAGE_POINTER_1),y
          tax
          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

          ;width
          iny
          lda (ZEROPAGE_POINTER_1),y
          sta PARAM2
          sta PARAM6

          ;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
          tya
          pha

.NextLineArea
          ldy PARAM1

.NextCharArea
          lda PARAM4
          sta (ZEROPAGE_POINTER_2),y
          lda PARAM5
          sta (ZEROPAGE_POINTER_3),y
          iny
          dec PARAM2
          bne .NextCharArea

          dec PARAM3
          beq .AreaDone

          ;move pointers down a line
          tya
          sec
          sbc #40
          tay
          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

          lda PARAM6
          sta PARAM2
          jmp .NextLineArea

.AreaDone
          jmp NextLevelData

I won't go into details about the alternating lines, they are almost the same as the common lines, with one small addition: After every set character the lowest bit of the character number is toggled.


Way more interesting is the quad where we fall back to a hint from one of the first steps: Tables, tables, tables!

The quad primitive uses indices into quad tables. To easy the workload every quad is split into 8 tables, characters upper left, upper right, lower left, lower right and the same for the colors. This way the quad index can also be used for the content.

For the record: This makes building the quad tables really annoying but the speedup is worth it.

!zone LevelQuad
LevelQuad

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

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

          ;item
          iny
          lda (ZEROPAGE_POINTER_1),y
          sta PARAM3

          ;store y for next data
          tya
          pha

          ldy PARAM2
          lda SCREEN_LINE_OFFSET_TABLE_LO,y
          sta ZEROPAGE_POINTER_2
          sta ZEROPAGE_POINTER_3
          lda SCREEN_LINE_OFFSET_TABLE_HI,y
          sta ZEROPAGE_POINTER_2 + 1
          clc
          adc #( ( SCREEN_COLOR - SCREEN_CHAR ) & 0xff00 ) >> 8
          sta ZEROPAGE_POINTER_3 + 1

          ldy PARAM1
          ldx PARAM3

          ;put image
          lda BLOCK_TABLE_UL_LOCATION0,x
          sta (ZEROPAGE_POINTER_2),y
          lda BLOCK_TABLE_UL_COLOR_LOCATION0,x
          sta (ZEROPAGE_POINTER_3),y
          iny
          lda BLOCK_TABLE_UR_LOCATION0,x
          sta (ZEROPAGE_POINTER_2),y
          lda BLOCK_TABLE_UR_COLOR_LOCATION0,x
          sta (ZEROPAGE_POINTER_3),y
          tya
          clc
          adc #39
          tay
          lda BLOCK_TABLE_LL_LOCATION0,x
          sta (ZEROPAGE_POINTER_2),y
          lda BLOCK_TABLE_LL_COLOR_LOCATION0,x
          sta (ZEROPAGE_POINTER_3),y
          iny
          lda BLOCK_TABLE_LR_LOCATION0,x
          sta (ZEROPAGE_POINTER_2),y
          lda BLOCK_TABLE_LR_COLOR_LOCATION0,x
          sta (ZEROPAGE_POINTER_3),y
          jmp NextLevelData

step28.zip