Homing Attack in Sonic 1 - Step 6 - Bounce Sonic upwards

Bounce Sonic upwards when do the Homing Attack

Above the Touch_Enemy subroutine (Why here? because this is the nearest place of two subroutines will call this, Touch_Monitor and Touch_KillEnemy), add this:

Code:

; ---------------------------------------------------------------------------
; Subroutine to Bounce Sonic upwards in Homing Attack
; ---------------------------------------------------------------------------

HA_Bounceup:
      tst.b   ($FFFFFFD1).w   ; is Sonic chasing some object?
      beq.s   HA_Bounceup_rts   ; if not, branch
      clr.b   ($FFFFFFD1).w   ; if yes, cancel the Homing Attack, clearing the number of frames Sonic can chasing the object

      clr.w   $10(a0)            ; clear Sonic X-vel (stop sonic horizontally)
      move.w   #-$600,$12(a0)      ; set -$600 for Y-velocity (move sonic upwards)
      btst   #6,$22(a0)         ; is sonic underwater?
      beq.s   HA_Bounceup_Shoes   ; if not, branch
      move.w   #-$320,$12(a0)      ; set -$320 for underwater Y-velocity (move sonic upwards)
      
HA_Bounceup_Shoes:
      tst.b   ($FFFFFE2E).w      ; does sonic has speed shoes?
      beq.s   HA_Bounceup_rts      ; if not, branch
      move.w   #-$650,$12(a0)      ; set -$650 for Y-velocity (move sonic upwards)

HA_Bounceup_rts:
      rts

; ===========================================================================

This subroutine is used to bounce up the Sonic when Hit a enemy or monitor, the speed is set depending on whether Sonic is with the speed shoes or underwater;
and cancel the Homing Attack, because you hit the enemy/monitor.


How to: Fix the bug when "destroy a monitor" of Step 4, improve the code and Bounce Sonic upwards when destroy a monitor

Search for Touch_Monitor (the code that says what should happen if Sonic touches a monitor) and you'll find this:

Code:

Touch_Monitor:
      tst.w   $12(a0)      ; is Sonic moving upwards?
      bpl.s   loc_1AF1E   ; if not, branch
      move.w   $C(a0),d0
      subi.w   #$10,d0
      cmp.w   $C(a1),d0
      bcs.s   locret_1AF2E
      neg.w   $12(a0)      ; reverse Sonic's y-motion
      move.w   #-$180,$12(a1)
      tst.b   $25(a1)
      bne.s   locret_1AF2E
      addq.b   #4,$25(a1)   ; advance the monitor's routine counter
      rts   
; ===========================================================================

loc_1AF1E:
      cmpi.b   #2,$1C(a0)   ; is Sonic rolling/jumping?
      bne.s   locret_1AF2E
      neg.w   $12(a0)      ; reverse Sonic's y-motion
      addq.b   #2,$24(a1)   ; advance the monitor's routine counter

locret_1AF2E:
      rts   

Below the text "Touch_Monitor:", add these lines:

Code:

      tst.b   ($FFFFFFD1).w      ; Sonic Homing Attack is chasing the monitor?
      bne.s   loc_1AF1E_bounce   ; if yes, destroy the monitor

Now replace this:

Code:

      addq.b   #2,$24(a1)   ; advance the monitor's routine counter

locret_1AF2E:
      rts   

to this:

Code:

loc_1AF1E_bounce:
      addq.b   #2,$24(a1)   ; advance the monitor's routine counter

locret_1AF2E:
   ;   rts

Why the "rts" has been commented?
Because below the Touch_Monitor subroutine is the HA_Bounceup subroutine, not needing call the HA_Bounceup. This to bounce Sonic upwards in Homing Attack when destroy a monitor.

Now, if the Sonic hit a monitor with the Homing Attack, the monitor will be destroyed immediately, improving the code.
The bug of Step 4 is fixed in HA_Bounceup, when the Homing Attack is cancelled.


How to: Fix the bug when "destroy a enemy" of Step 4 and Bounce Sonic upwards when destroy a enemy

Search for Touch_KillEnemy (the code that says what should happen if Sonic destroy a enemy) and you'll find this:

Code:

Touch_KillEnemy:
      bset   #7,$22(a1)
      moveq   #0,d0
      move.w   ($FFFFF7D0).w,d0
      addq.w   #2,($FFFFF7D0).w ; add 2 to item bonus counter
      cmpi.w   #6,d0
      bcs.s   loc_1AF82
      moveq   #6,d0

loc_1AF82:
      move.w   d0,$3E(a1)
      move.w   Enemy_Points(pc,d0.w),d0
      cmpi.w   #$20,($FFFFF7D0).w ; have 16 enemies been destroyed?
      bcs.s   loc_1AF9C   ; if not, branch
      move.w   #1000,d0   ; fix bonus to 10000
      move.w   #$A,$3E(a1)

loc_1AF9C:
      bsr.w   AddPoints
      move.b   #$27,0(a1)   ; change object   to points
      move.b   #0,$24(a1)
      tst.w   $12(a0)
      bmi.s   loc_1AFC2
      move.w   $C(a0),d0
      cmp.w   $C(a1),d0
      bcc.s   loc_1AFCA
      neg.w   $12(a0)
      rts   

Below this code:

Code:

loc_1AF9C:
      bsr.w   AddPoints

Add this line:

Code:

      bsr.w   HA_Bounceup   ; Bounce Sonic upwards if is Homing Attack

Now bounce Sonic upwards in Homing Attack when destroy a enemy, and in the HA_Bounceup subroutine the Homing Attack will be cleared to correct the bug of Step 4.

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