From bc70a2447830145fb67ec50450b649846d1b5362 Mon Sep 17 00:00:00 2001
From: Jesse Luehrs <doy@tozt.net>
Date: Mon, 6 Oct 2014 03:16:09 -0400
Subject: cleanups and comments

---
 test.s | 74 +++++++++++++++++++++++++++++++++++++++---------------------------
 1 file changed, 44 insertions(+), 30 deletions(-)

diff --git a/test.s b/test.s
index bee2fea..35ce3d4 100644
--- a/test.s
+++ b/test.s
@@ -25,22 +25,35 @@ sprite_y        DB
 
   .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
@@ -51,28 +64,10 @@ clrmem:
   STA $0600, x
   STA $0700, x
   LDA #$FE
-  STA $0200, x    ;move all sprites off screen
+  STA $0200, x
   INX
   BNE clrmem
 
-vblankwait2:      ; Second wait for vblank, PPU is ready after this
-  BIT $2002
-  BPL vblankwait2
-
-LoadPalettes:
-  LDA $2002    ; read PPU status to reset the high/low latch
-  LDA #$3F
-  STA $2006    ; write the high byte of $3F00 address
-  LDA #$00
-  STA $2006    ; write the low byte of $3F00 address
-  LDX #$00
-LoadPalettesLoop:
-  LDA palette.w, x        ;load palette byte
-  STA $2007             ;write to PPU
-  INX                   ;set index to next byte
-  CPX #$20            
-  BNE LoadPalettesLoop  ;if x = $20, 32 bytes copied, all done
-
   ; initialize variables in ram
   LDA #$00
   STA buttons_pressed
@@ -101,6 +96,25 @@ LoadPalettesLoop:
   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
+  STA $2006    ; write the high byte of $3F00 address
+  LDA #$00
+  STA $2006    ; write the low byte of $3F00 address
+  LDX #$00
+LoadPalettesLoop:
+  LDA palette.w, x      ;load palette byte
+  STA $2007             ;write to PPU
+  INX                   ;set index to next byte
+  CPX #$20
+  BNE LoadPalettesLoop  ;if x = $20, 32 bytes copied, all done
+
   LDA #%00010000   ; enable sprites
   STA $2001
 
-- 
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