commented code for level loading

Makefile

@@ -14,7 +14,7 @@ clean:
 env:
 	mkdir -p {build,bin,res.bin}
 
-debug: $(ASM) $(RES)
+debug: env $(ASM) $(RES)
 	g++ -o bin/explodefont util/explodefont.cpp
 	dasm src/main.asm -Isrc/ -DSYSTEM=64 -DDEBUG=1 -sbuild/symbols.txt -obin/snake.prg
 

src/gamereset.asm

@@ -4,18 +4,6 @@ gamereset:
     ; Turn MultiColor mode on
     jsr multicolorOn
 
-    ; Clear screen
-    ldx #$00
-    lda #$20
-fullresetCLS:
-    dex
-    sta $400,x
-    sta $500,x
-    sta $600,x
-    sta $700,x
-    cpx #$00
-    bne fullresetCLS
-
     ; Set overscan
     lda #11
     sta $d020

src/levels.asm

@@ -1,47 +1,63 @@
-statusLevelSelect:  ; select new level
-
+; load new level on the screen
+statusLevelSelect:
+    ; initialize video pointer with first video memory address
+    ; (skip first line, used for the status bar)
     ldy #39
-    sty tempPointer
+    sty levelVideoPointer
     ldy #$04
-    sty tempPointer + 1
+    sty levelVideoPointer + 1
+
+; Level data is compressed with RLE. Array example:
+; +---+---+---+---+---+-..-+---+---+
+; | T | C | T | C | . |    | 0 | 0 |
+; +---+---+---+---+---+-..-+---+---+
+; T tile char
+; C count (how many repeated tile chars)
+; 0 end of level
 
 writeLevelLoop:
+    ; read `T`, and save onto stack
     ldy #0
     lda (levelPointer),y
-
     pha
 
+    ; increment array pointer
     lda #levelPointer
     sta nextPointerPointer
     ldx #1
     jsr nextPointer
 
+    ; read `C`, and save onto stack
     ldy #0
     lda (levelPointer),y
-
     pha
 
+    ; increment array pointer
     lda #levelPointer
     sta nextPointerPointer
     ldx #1
     jsr nextPointer
 
+    ; retrieve `C` from stack and put in X and Y
     pla
-
     tay
     tax
+    ; retrieve `T` from stack
     pla
 
+    ; check array end
     cmp #0
     beq writeLevelEnd
+
+    ; unpack char from RLE to the screen
 writeLevelElement:
-    sta (tempPointer),y
+    sta (levelVideoPointer),y
     dey
     bne writeLevelElement
 
-    lda #tempPointer
+    lda #levelVideoPointer
     sta nextPointerPointer
-    ; x still holds the total number
+    ; reg X is still holding the original `C`
     jsr nextPointer
     jmp writeLevelLoop
 
@@ -50,23 +66,3 @@ writeLevelEnd:
     sta status
     rts
 
-
-; Input:
-; regX = count
-nextPointer:
-    lda #0
-    sta nextPointerPointer + 1
-
-    txa
-
-    clc
-    ldy #0
-    adc (nextPointerPointer),y
-    sta (nextPointerPointer),y
-    ldy #1
-    lda (nextPointerPointer),y
-    adc #0
-    sta (nextPointerPointer),y
-
-    rts
-

src/subroutines.asm

@@ -132,4 +132,24 @@ printIntroEndCheck:
 printIntroEnd:
     rts
 
+; Increment a pointer in the zeropage
+; Input parameters:
+;   nextPointerPointer  pointer to the pointer in zeropage
+;   regX                value to increment
+nextPointer:
+    lda #0
+    sta nextPointerPointer + 1
+
+    txa
+
+    clc
+    ldy #0
+    adc (nextPointerPointer),y
+    sta (nextPointerPointer),y
+    ldy #1
+    lda (nextPointerPointer),y
+    adc #0
+    sta (nextPointerPointer),y
+
+    rts
 

src/zeropage.asm

@@ -15,8 +15,8 @@ introScreenStart DS 2
 ; Pointer to level struct
 levelPointer DS 2
 
-; Temporary pointer -- Use when you don't call a subroutine
-tempPointer DS 2
+; Pointer to video memory used in the level loading routine
+levelVideoPointer DS 2
 
 ; Pointer for Pointer in the NextPointer routine
 nextPointerPointer DS 2