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# Bugs:Super Mario Bros.

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## 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, which is what this patch does: