Homing Attack in Sonic 1 - Step 1 - Creating the Homing Attack, and make it work

Code:

      bsr.w   Sonic_Homingattack

Code:

Obj01_MdJump2:            ; XREF: Obj01_Modes
      bsr.w   Sonic_Homingattack
      bsr.w   Sonic_JumpHeight
      bsr.w   Sonic_ChgJumpDir
      bsr.w   Sonic_LevelBound
      jsr   ObjectFall
      btst   #6,$22(a0)
      beq.s   loc_12EA6
      subi.w   #$28,$12(a0)


Code:

; ---------------------------------------------------------------------------
; Subroutine Sonic_Homingattack
; ---------------------------------------------------------------------------

; ||||||||||||||| S U B   R O U T   I N E |||||||||||||||||||||||||||||||||||||||

Sonic_Homingattack:
      move.b   ($FFFFF603).w,d1   ; read controller
      andi.b   #$40,d1            ; is A pressed?
      beq.w   Sonic_HA_rts      ; if not, branch

Sonic_HA_rts:
      rts                  ; return
; Command of Homingattack end here

Code:

A = #$40
B = #$10
C = #$20
Start = #$80
Up = #1
Down = #2
Left = #4
Right = #8

Code:

      beq.w   Sonic_HA_rts      ; if not, branch
----->>>>>>        HERE        <<<<<<-----
      Sonic_HA_rts:


Code:


      lea      ($FFFFD800).w,a1   ; start at the first level object RAM

; ---------------------------------------------------------------------------

HA_enemylist:
      tst.b   (a1)            ; is a Null object
      bne.s   HA_calcdistance      ; if not, branch

HA_nextobject:
      adda.w  #$40,a1         ; jump to next object RAM entry
      cmpa.w  #$F000,a1      ; already tested all object RAM entry?
      blt.s  HA_enemylist   ; if not, return to enemy list
      rts

; ---------------------------------------------------------------------------

HA_calcdistance:   ; distance calculator
      move.w  8(a1),d1   ; move the object x-position to d1
      move.w  $C(a1),d2   ; move the object y-position to d2
      sub.w  8(a0),d1   ; sub sonic x-position of object x-position
      sub.w  $C(a0),d2   ; sub sonic y-position of object y-position

; ---------------------------------------------------------------------------

HA_Move:
      jsr      (CalcAngle).l   ; calculates the angle
      jsr      (CalcSine).l   ; calculates the sine and the cosine
      muls.w  #$C,d1         ; multiply cosine by $C
      move.w  d1,$10(a0)      ; move d1 to X-velocity
      muls.w  #$C,d0         ; multiply sine by $C
      move.w  d0,$12(a0)      ; move d0 to Y-velocity

Code:

      lea      ($FFFFD800).w,a1   ; start at the first level object RAM

Code:

   HA_enemylist:
         tst.b   (a1)            ; is a Null object
         bne.s   HA_calcdistance      ; if not, branch

   HA_nextobject:
         adda.w  #$40,a1         ; jump to next object RAM entry
         cmpa.w  #$F000,a1      ; already tested all object RAM entry?
         blt.s  HA_enemylist   ; if not, return to enemy list
         rts

Code:


         tst.b   (a1)            ; is a Null object
         bne.s   HA_calcdistance      ; if not, branch

Code:

         adda.w   #$40,a1         ; jump to next object RAM entry
         cmpa.w   #$F000,a1      ; already tested all object RAM entry?
         blt.s   HA_enemylist   ; if not, return to enemy list
         rts

Code:

      move.w  8(a1),d1   ; move the object x-position to d1
      move.w  $C(a1),d2   ; move the object y-position to d2
      sub.w  8(a0),d1   ; sub sonic x-position of object x-position
      sub.w  $C(a0),d2   ; sub sonic y-position of object y-position

Code:

      jsr      (CalcAngle).l   ; calculates the angle
      jsr      (CalcSine).l   ; calculates the sine and the cosine

Code:

      muls.w  #$C,d1         ; multiply cosine by $C
      move.w  d1,$10(a0)      ; move d1 to X-velocity
      muls.w  #$C,d0         ; multiply sine by $C
      move.w  d0,$12(a0)      ; move d0 to Y-velocity

hacker___ wrote:
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OuricoDoido wrote:Creating the Homing Attack

To add this skill to Sonic, you need go to Obj01_MdJump2 in sonic1.asm, and add this line:

Code:

 bsr.w Sonic_Homingattack

Your code will be:

Code:

Obj01_MdJump2: ; XREF: Obj01_Modes
 bsr.w Sonic_Homingattack
 bsr.w Sonic_JumpHeight
 bsr.w Sonic_ChgJumpDir
 bsr.w Sonic_LevelBound
 jsr ObjectFall
 btst #6,$22(a0)
 beq.s loc_12EA6
 subi.w #$28,$12(a0)


Go below Sonic_Roll subroutine (below this text: ; End of function Sonic_Roll) , and Add this (a normal skill code):

Code:

; ---------------------------------------------------------------------------
; Subroutine Sonic_Homingattack
; ---------------------------------------------------------------------------

; ||||||||||||||| S U B R O U T I N E |||||||||||||||||||||||||||||||||||||||

Sonic_Homingattack:
 move.b ($FFFFF603).w,d1 ; read controller
 andi.b #$40,d1 ; is A pressed?
 beq.w Sonic_HA_rts ; if not, branch

Sonic_HA_rts:
 rts ; return
; Command of Homingattack end here

If you want to "hold A" instead of "press A", modify $FFFFF603 to $FFFFF604.

For other buttons, change the #$40 to:

Code:

A = #$40
B = #$10
C = #$20
Start = #$80
Up = #1
Down = #2
Left = #4
Right = #8

Doing the Homing Attack work

Now, we start with main homing attack command, to move the Sonic in enemy direction.

Between these lines:

Code:

 beq.w Sonic_HA_rts ; if not, branch
----->>>>>>   HERE   <<<<<<-----
 Sonic_HA_rts:


Add this:

Code:


 lea ($FFFFD800).w,a1 ; start at the first level object RAM

; ---------------------------------------------------------------------------

HA_enemylist:
 tst.b (a1) ; is a Null object
 bne.s HA_calcdistance ; if not, branch

HA_nextobject:
 adda.w  #$40,a1 ; jump to next object RAM entry
 cmpa.w  #$F000,a1 ; already tested all object RAM entry?
 blt.s   HA_enemylist ; if not, return to enemy list
 rts

; ---------------------------------------------------------------------------

HA_calcdistance: ; distance calculator
 move.w  8(a1),d1 ; move the object x-position to d1
 move.w  $C(a1),d2 ; move the object y-position to d2
 sub.w   8(a0),d1 ; sub sonic x-position of object x-position
 sub.w   $C(a0),d2 ; sub sonic y-position of object y-position

; ---------------------------------------------------------------------------

HA_Move:
 jsr (CalcAngle).l ; calculates the angle
 jsr (CalcSine).l ; calculates the sine and the cosine
 muls.w  #$C,d1 ; multiply cosine by $C
 move.w  d1,$10(a0) ; move d1 to X-velocity
 muls.w  #$C,d0 ; multiply sine by $C
 move.w  d0,$12(a0) ; move d0 to Y-velocity

Compile and test, jump and hold or press A to you see what happens. (Note: If you used the "Hold A" instruction, you will see better the effect.)
In this code, the Sonic is going vs any object (rocks, bridge, spikes, .....)


I will explain this code by parts (the part 3, 4 and 5 have already been explained in Step 0.)


Part 1:

Code:

 lea ($FFFFD800).w,a1 ; start at the first level object RAM

This code aims to set the address $FFFFD800 in a1;

Why this address?
Because in Sonic 1, $FFFFD800 address, where is located "the first level object".
This code is completed by the 2nd part, to understand this part, read the 2nd part.


Part 2:

Code:

 HA_enemylist:
 tst.b (a1) ; is a Null object
 bne.s HA_calcdistance ; if not, branch

 HA_nextobject:
 adda.w  #$40,a1 ; jump to next object RAM entry
 cmpa.w  #$F000,a1 ; already tested all object RAM entry?
 blt.s   HA_enemylist ; if not, return to enemy list
 rts

This code aims to locate any existing object.

Here is the difference between the basic Homing Attack and the Pinball Bumper, in Pinball Bumper the two objects already have determined (the Sonic and own Pinball Bumper as you saw in Step 0),we already know the Pinball Bumper's RAM address (since the calculation is done within the object's subroutines, a0 being the address) and the Sonic's address in RAM is always fixed ($FFFFD000), no needing find the Sonic in RAM.

But in the Homing Attack, Sonic can attack any object, which in fact is not fixed in RAM, according the objects are being erased in the RAM (when the Sonic destroy him for example), new objects that will be loaded, will earn a RAM position, which may be or not, of the deleted object.

Knowing that the positions of objects in RAM are not fixed, we will be forced to find them in RAM, this way the Homing Attack is not limited to only one object.

Code:


 tst.b (a1) ; is a Null object
 bne.s HA_calcdistance ; if not, branch

This code checks if the object exists, if it does not exist, the subroutine continues to read, searching for another object (in the code below),
but if it exists, go to HA_calcdistance subroutine to calculate the distance, angle, sine and cosine .... and execute the Homing Attack.

Code:

 adda.w #$40,a1 ; jump to next object RAM entry
 cmpa.w #$F000,a1 ; already tested all object RAM entry?
 blt.s HA_enemylist ; if not, return to enemy list
 rts

If the object does not exist or is not part of the list of enemies (as we shall see in the step 2), will run this code, which aims to jump to the next object in RAM.

Now an explanation line by line:
1st - each object has a size of $40 bytes, so "adda.w  #$40, a1", to skip $40 bytes in a1, going to the next object.
2nd - "level object variable space" ends at address $FFFFF000, so "cmpa.w  #$F000, a1", compares a1 with address where ends "level object variable space".
3rd - The "blt.s  HA_enemylist" is if the RAM position that is in a1 is less than the address where ends "level object variable space", keep searching for an object, returning to the enemy list.


Part 3:

Code:

 move.w  8(a1),d1 ; move the object x-position to d1
 move.w  $C(a1),d2 ; move the object y-position to d2
 sub.w   8(a0),d1 ; sub sonic x-position of object x-position
 sub.w   $C(a0),d2 ; sub sonic y-position of object y-position

This is the calculation to obtain the object's coordinates as if Sonic's coordinates is (x=0, y=0), so we can calculate the angle, as explained in Step 0.


Part 4:

Code:

 jsr (CalcAngle).l ; calculates the angle
 jsr (CalcSine).l ; calculates the sine and the cosine

Here is done the math to figure out the angle (based on information received from Part 3), and figure out the Sine and Cosine (based on the angle).


Part 5:

Code:

 muls.w  #$C,d1 ; multiply cosine by $C
 move.w  d1,$10(a0) ; move d1 to X-velocity
 muls.w  #$C,d0 ; multiply sine by $C
 move.w  d0,$12(a0) ; move d0 to Y-velocity

This calculation serves to increase the sine and cosine's value that are very low, as explained in Step 0.
And after increase, set the sine at Sonic's y-velocity and set the cosine at Sonic's x-velocity.