More realistic background.

pewpew.asm

@@ -54,9 +54,9 @@ Start:
     jsr LoadPaletteAndTileData
     jsr InitializeSpriteTables
 
-    ; Set screen mode: 16x16 tiles for BG2, 8x8 tiles elsewhere, mode 0.
-    lda #%00100000
-    sta BGMODE
+    ; Set screen mode: 16x16 tiles for backgrounds, mode 0.
+    lda #%11110000
+    sta SCREENMODE
 
     ; Set sprite size to 16x16 (small) and 32x32 (large).
     lda #%01100000
@@ -87,10 +87,8 @@ Start:
     ; Write something recognizable into our scratch space.
     jsr FillScratch
 
-    ; Start the background color as a sky blue.
-    lda #16
-    sta $23
-    lda #31
+    ; Start the background color as a dark blue.
+    lda #4
     sta $24
 
     ; Player's initial starting location.
@@ -202,6 +200,13 @@ LoadPaletteAndTileData:
     lda #%00000001
     sta DMAENABLE
 
+    ; Tell the system that the BG2 tilemap starts at $4000.
+    lda #%00100000
+    sta BG2TILEMAP
+    ; ... and that the background tile data for BG1 & BG2 starts at $2000.
+    lda #%00010001
+    sta BG12NBA
+
     ; Set up the BG2 tilemap.
     ; VRAM write mode: increments the address every time we write a word.
     lda #%10000000
@@ -210,21 +215,38 @@ LoadPaletteAndTileData:
     ldx #$2000  ; BG 2 tilemap starts here. (Byte address $4000.)
     stx VMADDR
     ; Now write entries into the tile map.
+    ; We have only a couple tiles, but we set the invert horizontal/vertical
+    ; bits so that we get more variation.  We also do 7 tiles per loop -- since
+    ; the tile map is 32 entries wide, this means that successive rows end up
+    ; looking different.
     ldy #0
 -
-    ldx #$0002  ; Write one tile into the map.
+    ldx #$0002
+    stx VMDATA
+    iny
+    ldx #$0004
+    stx VMDATA
+    iny
+    ldx #$4002
+    stx VMDATA
+    iny
+    ldx #$8002
+    stx VMDATA
+    iny
+    ldx #$8004
     stx VMDATA
     iny
-    cpy #1024  ; The tile map is 32x32 (1024 entries).
+    ldx #$A002
+    stx VMDATA
+    iny
+    ldx #$A004
+    stx VMDATA
+    iny
+    ; The tile map is 32x32 (1024 entries).
+    ; This is the next multiple of 7 above that.
+    cpy #1029
     bne -
 
-    ; The BG2 tilemap starts at $4000.
-    lda #%00100000
-    sta BG2TILEMAP
-    ; Background tile data for BG1 & BG2 starts at $2000.
-    lda #%00010001
-    sta BG12NBA
-
     rts
 
 
@@ -475,11 +497,18 @@ UpdateGraphics:
     and #%11111110
     sta $0300
 
-    ; Make the background scroll.
+    ; Make the background scroll. Horizontal over time; vertical depending on
+    ; player's y-coordinate.
     lda $14
     sta BG2HOFS
     lda $15
     sta BG2HOFS
+    lda $21
+    .rept 3
+        lsr
+    .endr
+    sta BG2VOFS
+    stz BG2VOFS
 
     rts
 

registers.asm

@@ -48,7 +48,7 @@
 ;             d: BG1 tile size (0=8x8, 1=16x16).
 ;             e: Highest priority for BG3 in MODE 1.
 ;             f: MODE definition.
-.define BGMODE $2105
+.define SCREENMODE $2105
 
 ; $2107-210A: BG1-4 tilemap registers [BGxSC]
 ; xxxxxxab    x: Base address (in VRAM, shifted left 11 bits).

tiles.asm

@@ -3,12 +3,10 @@
 TileData:
     .db $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00
     .db $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00
-    .db $07, $00, $18, $07, $20, $1F, $4C, $33, $50, $2F, $86, $79, $86, $79, $80, $7F
-    .db $E0, $00, $18, $E0, $04, $F8, $32, $CC, $0A, $F4, $61, $9E, $61, $9E, $01, $FE
-    .db $10, $00, $28, $10, $A8, $90, $64, $58, $E3, $DC, $30, $0F, $AC, $93, $60, $5F
-    .db $00, $00, $00, $00, $00, $00, $F8, $00, $80, $00, $70, $80, $0E, $F0, $01, $FE
-    .db $80, $00, $80, $00, $80, $00, $80, $00, $80, $00, $80, $00, $80, $00, $80, $00
     .db $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00
+    .db $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $20, $00, $00, $00, $00
+    .db $00, $00, $00, $00, $00, $10, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00
+    .db $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $04
     .db $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00
     .db $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00
     .db $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00
@@ -19,11 +17,13 @@ TileData:
     .db $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00
     .db $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00
     .db $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00
-    .db $80, $7F, $9F, $60, $9F, $6F, $4F, $37, $47, $38, $20, $1F, $18, $07, $07, $00
-    .db $01, $FE, $F9, $06, $F9, $F6, $F2, $EC, $E2, $1C, $04, $F8, $18, $E0, $E0, $00
-    .db $60, $5F, $AC, $93, $30, $0F, $E3, $DC, $64, $58, $A8, $90, $28, $10, $10, $00
-    .db $01, $FE, $0E, $F0, $70, $80, $80, $00, $F8, $00, $00, $00, $00, $00, $00, $00
-    .db $80, $00, $80, $00, $80, $00, $80, $00, $80, $00, $80, $00, $80, $00, $80, $00
+    .db $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00
+    .db $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00
+    .db $00, $00, $00, $00, $00, $00, $00, $00, $00, $08, $00, $00, $00, $00, $00, $00
+    .db $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00
+    .db $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00
+    .db $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00
+    .db $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00
     .db $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00
 
 TilePalette: