Super Mario Bros. 3/Notes

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Disassembly/reverse engineering notes

All values given are hexadecimal unless noted otherwise. Various important bits of code follow below. Unless marked as "subroutine" you can NOT jump directly to these addresses, and even then there might be side effects.

Player physics/movement

Horizontal movement

• If the player holds L/R the horizontal velocity $BD will start to increase (todo: what's the exact increment?) at around 1 per frame. This continues until a set limit.
• If walking, the limit is 18 (24 dec).
• If player holds B,
  • The limit is raised to 28 (40 dec) and velocity is allowed to increase again (same acceleration).
  • The P-meter timer $515 starts ticking down from 7. Every time it reaches 0, an arrow is added to the P-meter.
  • After the final arrow is lit and the P starts blinking:
    • Player is now in "flight-mode" (blinking P).
    • The limit is raised to 38 (56 dec) and velocity is allowed to increase again (same acceleration).
    • The P-meter timer is LOCKED to the value 15 (why that value? could be anything but 0).
    • Jumping now launches into flight, (if player has a suitable powerup, otherwise just jumping high).
  • Player exits "flight-mode" by touching ground, (todo: add more cases).
    • If not in "flight-mode", the P-meter timer starts ticking down from 23. Every time it reaches 0, an arrow is removed from the P-meter.

$AC5A Jump power

$AC5A:A5 BD     LDA $00BD			; Load player horz velocity
$AC5C:10 03     BPL $AC61			; If negative,
$AC5E:20 0F DD  JSR $DD0F			; 	take absolute value
$AC61:4A        LSR				; Divide by 16
$AC62:4A        LSR
$AC63:4A        LSR
$AC64:4A        LSR
$AC65:AA        TAX				; X = A, going to use it as an index
$AC66:AD 47 A6  LDA $A647			; Load default jump velocity
$AC69:38        SEC				
$AC6A:FD 48 A6  SBC $A648,X			; Subtract from the jump velocity (remember lower means more power) using the table 00,02,04,08.
						; Thus a higher horizontal speed means a more powerful jump.
$AC6D:85 CF     STA $00CF			; Store as new vertical velocity.

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$ACA1 Application of gravity

$ACA1:A0 05     LDY #$05		; Y = default falling gravity
$ACA3:A5 CF     LDA $00CF		; Load current vertical velocity
$ACA5:C9 E0     CMP #$E0		; 
$ACA7:10 0D     BPL $ACB6		; If currently rising and v vel is still faster than E0,
$ACA9:AD 79 05  LDA $0579		; 	Don't know what 0579 is... unused? Debugger never sees a nonzero value.
$ACAC:D0 0D     BNE $ACBB
$ACAE:A5 17     LDA $0017		; 	Read gamepad. 80 is jump key, so value will appear negative!
$ACB0:10 04     BPL $ACB6		; 	If jump pressed,
$ACB2:A0 01     LDY #$01		; 		Y = jump gravity (lower than normal)
$ACB4:D0 05     BNE $ACBB		; 
$ACB6:A9 00     LDA #$00		; This is run if jump key is NOT pressed
$ACB8:8D 79 05  STA $0579		; 	So what does it do? 
$ACBB:98        TYA			; A=Y
$ACBC:18        CLC			;
$ACBD:65 CF     ADC $00CF		; Add gravity to current vertical velocity
$ACBF:85 CF     STA $00CF		; And store back

So, jumps are aborted either by the user releasing the button (bit 7 in 0017 is zero) or the 00CF is more positive than E0. Since SOME gravity is always applied, this ensures jumps are aborted.

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$BFCC (Subroutine) Clamp Y velocity to the maximum

$BFCC:A5 CF     LDA $00CF			; Load current vertical velocity
$BFCE:30 08     BMI $BFD8			; Negative? Then skip clamping.
$BFD0:C9 40     CMP #$40			; #$40 is the maximum fall vel (note: gravity is added afterwards so the effective value is 45) 
$BFD2:30 04     BMI $BFD8			; Less than this? Then skip clamping.
$BFD4:A9 40     LDA #$40			; Replace $00CF with the maximum fall vel.
$BFD6:85 CF     STA $00CF
$BFD8:A2 12     LDX #$12			; UNKNOWN
$BFDA:20 93 BF  JSR $BF93			; UNKNOWN	
$BFDD:60        RTS