Bugs:Super Mario Bros.
This page details bugs of Super Mario Bros..
Spiny Egg Bug
Here's how the Spiny egg's speed is calculated and stored.
DifLoop: lda PRDiffAdjustData,y ;get three values and save them sta $01,x ;to $01-$03 iny iny ;increment Y four bytes for each value iny iny dex ;decrement X for each one bpl DifLoop ;loop until all three are written ldx ObjectOffset ;get enemy object buffer offset jsr PlayerLakituDiff ;move enemy, change direction, get value - difference ldy Player_X_Speed ;check player's horizontal speed cpy #$08 bcs SetSpSpd ;if moving faster than a certain amount, branch elsewhere tay ;otherwise save value in A to Y for now lda PseudoRandomBitReg+1,x and #%00000011 ;get one of the LSFR parts and save the 2 LSB beq UsePosv ;branch if neither bits are set tya eor #%11111111 ;otherwise get two's compliment of Y tay iny UsePosv: tya ;put value from A in Y back to A A=SpeedVar SetSpSpd: jsr SmallBBox ;set bounding box control, etc. A=SpeedVar ...
Doing good so far. The A register stores the appropriate speed value.
SmallBBox: lda #$09 ;set specific bounding box size control A=09 bne SetBBox ;unconditional branch A=09
Oops, jumping to the bounding box subroutine overwrote A! All that work for nothing.
SetBBox: sta Enemy_BoundBoxCtrl,x ;set bounding box control here A=09 lda #$02 ;set moving direction for left A=02 sta Enemy_MovingDir,x A=02 InitVStf: lda #$00 ;initialize vertical speed A=00 sta Enemy_Y_Speed,x ;and movement force A=00 sta Enemy_Y_MoveForce,x A=00 rts A=00
Now A ends up equaling 00. Now the game jumps back to the SetSpSpd subroutine...
... ldy #$02 ;Set Y to 02 (Leftwards) A=00 sta Enemy_X_Speed,x ;Set Spiny egg speed to A (00) A=00 cmp #$00 ;Now check if speed is negative... A=00 bmi SpinyRte ;...but A is always 00. No branch. A=00 dey ;Set Y to 01 (Rightwards) SpinyRte: sty Enemy_MovingDir,x ;Set moving direction.
...and speed is set to 0. Moving direction is set to Rightwards, but since the egg has no horizontal movement and the direction is reset when the Spiny enemy spawns, this doesn't mean much.
To fix this bug, simply move the "jsr SmallBBox" to after the dey opcode.