copy over some changes from the project skeleton

4 files changed, 67 insertions, 32 deletions

diff --git a/Makefile b/Makefile
index b3bfaac..4353c6b 100644
--- a/Makefile
+++ b/Makefile
@@ -1,5 +1,5 @@
 NAME = snake
-OBJS = snake.o
+OBJS = main.o
 
 CC = wla-6502
 LD = wlalink
@@ -15,11 +15,11 @@ $(NAME).nes: $(NAME).rom header.bin
 $(NAME).rom: $(OBJS) linkfile
 	$(LD) $(LDFLAGS) linkfile $@
 
-snake.o: snake.chr
-
 %.o: %.s
 	$(CC) $(CFLAGS) -o $<
 
+main.o: sprites.chr
+
 run: $(NAME).nes
 	fceux $(NAME).nes
 
diff --git a/linkfile b/linkfile
index c41ff42..3dddda7 100644
--- a/linkfile
+++ b/linkfile
@@ -1,2 +1,2 @@
 [objects]
-snake.o
+main.o
diff --git a/snake.s b/main.s
index 58665ba..1a12ed7 100644
--- a/snake.s
+++ b/main.s
@@ -15,34 +15,47 @@ BANKS       1
 .ENDRO
 
 
-  .enum $0000
+.ENUM $0000
 buttons_pressed DB
 head_x          DB
 head_y          DB
 direction       DB ; 0: up, 1: down, 2: left, 3: right
 frame_skip      DB
 frame_count     DB
-  .ende
+.ENDE
 
 
   .bank 0
   .org $0000
+; the ppu takes two frames to initialize, so we have some time to do whatever
+; initialization of our own that we want to while we wait. we choose here to
+; set up cpu flags in the first frame and clear out system ram in the second
+; frame (clearing out ram isn't at all necessary, but we can't do anything
+; useful at this point anyway, so we may as well in order to make things more
+; predictable). clearing out system ram actually takes quite a bit longer than
+; a frame (a frame is ~2273 cycles, or ~324-1136 opcodes), but we may as well
+; start the process while we wait.
 RESET:
-  SEI          ; disable IRQs
-  CLD          ; disable decimal mode
+  SEI              ; disable IRQs
+  CLD              ; disable decimal mode
   LDX #$40
-  STX $4017.W    ; disable APU frame IRQ
+  STX $4017.w      ; disable APU frame IRQ
   LDX #$FF
-  TXS          ; Set up stack
-  INX          ; now X = 0
-  STX $2000.W    ; disable NMI
-  STX $2001.W    ; disable rendering
-  STX $4010.W    ; disable DMC IRQs
+  TXS              ; Set up stack (grows down from $FF to $00, at $0100-$01FF)
+  INX              ; now X = 0
+  STX $2000.w      ; disable NMI (we'll enable it later once the ppu is ready)
+  STX $2001.w      ; disable rendering (same)
+  STX $4010.w      ; disable DMC IRQs
 
 vblankwait1:       ; First wait for vblank to make sure PPU is ready
-  BIT $2002
-  BPL vblankwait1
-
+  BIT $2002        ; bit 7 of $2002 is reset once vblank ends
+  BPL vblankwait1  ; and bit 7 is what is checked by BPL
+
+  ; set everything in ram ($0000-$07FF) to $00, except for $0200-$02FF which
+  ; is conventionally used to hold sprite attribute data. we set that range
+  ; to $FE, since that value as a position moves the sprites offscreen, and
+  ; when the sprites are offscreen, it doesn't matter which sprites are
+  ; selected or what their attributes are
 clrmem:
   LDA #$00
   STA $0000, x
@@ -53,14 +66,46 @@ clrmem:
   STA $0600, x
   STA $0700, x
   LDA #$FE
-  STA $0200, x    ;move all sprites off screen
+  STA $0200, x
   INX
   BNE clrmem
 
+  ; initialize variables in ram
+  LDA #$00
+  STA buttons_pressed
+  STA direction
+  STA frame_count
+  LDA #$80
+  STA head_x
+  STA head_y
+  LDA #30
+  STA frame_skip
+
+  LDA #$32
+  STA $0201 ; set the sprite number to display
+  LDA #$00
+  STA $0202 ; set the sprite attributes (palette, flipping, etc)
+
+  LDA #$33
+  STA $0205 ; set the sprite number to display
+  LDA #$00
+  STA $0206 ; set the sprite attributes (palette, flipping, etc)
+
+  LDA #$34
+  STA $0209 ; set the sprite number to display
+  LDA #$00
+  STA $020A ; set the sprite attributes (palette, flipping, etc)
+
+  LDA #$35
+  STA $020D ; set the sprite number to display
+  LDA #$00
+  STA $020E ; set the sprite attributes (palette, flipping, etc)
+
 vblankwait2:      ; Second wait for vblank, PPU is ready after this
   BIT $2002
   BPL vblankwait2
 
+  ; now that the ppu is ready, we can start initializing it
 LoadPalettes:
   LDA $2002    ; read PPU status to reset the high/low latch
   LDA #$3F
@@ -69,23 +114,12 @@ LoadPalettes:
   STA $2006    ; write the low byte of $3F00 address
   LDX #$00
 LoadPalettesLoop:
-  LDA palette.w, x        ;load palette byte
+  LDA palette.w, x      ;load palette byte
   STA $2007             ;write to PPU
   INX                   ;set index to next byte
-  CPX #$20            
+  CPX #$20
   BNE LoadPalettesLoop  ;if x = $20, 32 bytes copied, all done
 
-  ; initialize variables in ram
-  LDA #$00
-  STA buttons_pressed
-  STA direction
-  STA frame_count
-  LDA #$80
-  STA head_x
-  STA head_y
-  LDA #30
-  STA frame_skip
-
   LDA #%00010000   ; enable sprites
   STA $2001
 
@@ -95,6 +129,7 @@ LoadPalettesLoop:
 loop:
   JMP loop
 
+
 read_controller1:
   ; latch
   LDA #$01
@@ -216,4 +251,4 @@ palette:
 
   .bank 1 slot 1
   .org $0000
-  .incbin "snake.chr"
+  .incbin "sprites.chr"
diff --git a/snake.chr b/sprites.chr
index fa71045..fa71045 100644
--- a/snake.chr
+++ b/sprites.chr