reset: Do not zero out RAM

This is usually done so the programmer can forget about initializing in
the future, but it not only hides programming mistakes, but we are also
resetting memory addresses which are never to be used by this simple
game. Hence, just skip resetting RAM altogether.

Last but not least, sprite initialization in `reset` code has also been
optimized so instead of around 2000 cycles it takes half of that. This
is done by only touching the first byte of the four bytes for a sprite
in OAM, which is enough for hiding random sprites and the cost of extra
`inx` instructions is far cheaper than all the extra `sta` to absolute
address with X-index from the old code.

Signed-off-by: Miquel Sabaté Solà <mikisabate@gmail.com>

2 files changed, 23 insertions, 25 deletions

diff --git a/src/jetpac.s b/src/jetpac.s
index 65b5d11..10f6df3 100644
--- a/src/jetpac.s
+++ b/src/jetpac.s
@@ -45,9 +45,17 @@
 .include "vectors.s"
 
 .proc main
-    ;; Disable the PPU.
+    ;; Disable the PPU and zero out variables which shadow PPU registers.
     lda #0
     sta PPU::MASK
+    sta PPU::zp_mask
+    sta PPU::zp_control
+
+    ;; Initialize other global variables which the rest of the game assume to
+    ;; have zero as their initial values.
+    sta Globals::zp_flags
+    sta Joypad::zp_buttons1
+    sta Joypad::zp_buttons2
 
     ;; Initialize the assets for the game.
     jsr Assets::init
diff --git a/src/vectors.s b/src/vectors.s
index 559a59d..8928aaf 100644
--- a/src/vectors.s
+++ b/src/vectors.s
@@ -5,64 +5,54 @@
 
 ;; Pretty standard reset function, nothing crazy.
 .proc reset
+    ;; Disable interrupts and decimal mode.
     sei
     cld
 
+    ;; Disable APU frame counter.
     ldx #$40
     stx APU::FRAME_COUNTER
 
+    ;; Setup the stack.
     ldx #$FF
     txs
 
+    ;; Disable NMIs and the APU's DMC.
     inx
     stx PPU::CONTROL
     stx PPU::MASK
     stx APU::DMC
 
+    ;; First PPU wait.
     bit PPU::STATUS
 @vblankwait1:
     bit PPU::STATUS
     bpl @vblankwait1
 
-    ldx #0
-    lda #0
-@ram_reset_loop:
-    sta $000, x
-    sta $100, x
-    sta $300, x
-    sta $400, x
-    sta $500, x
-    sta $600, x
-    sta $700, x
-    inx
-    bne @ram_reset_loop
-
+    ;; Reset all sprites by simply moving the Y coordinate out of screen.
     lda #$EF
+    ldx #0
 @sprite_reset_loop:
     sta $200, x
     inx
+    inx
+    inx
+    inx
     bne @sprite_reset_loop
 
+    ;; DMA setup for sprite reset.
     lda #$00
     sta OAM::ADDRESS
     lda #$02
     sta OAM::DMA
 
+    ;; Second PPU wait. After that the PPU is stable.
 @vblankwait2:
     bit PPU::STATUS
     bpl @vblankwait2
 
-    lda #$3F
-    sta PPU::ADDRESS
-    lda #$00
-    sta PPU::ADDRESS
-
-    lda #$0F
-    ldx #$20
-@palettes_reset_loop:
-    sta PPU::DATA
-    dex
-    bne @palettes_reset_loop
+    ;; NOTE: palettes are not initialized here as it's going to be one of the
+    ;; first things done on `main` code.
 
     jmp main
 .endproc