Scrolling BG (AE2)

; ===========================================================================
; Scrolling BG from the Aero the Acro-Bat 2.
; Ripped by TheBlad768.
; The code in the Aero the Acro-Bat is the same.
; ===========================================================================

; RAM Address
long_464A:                  = RAM+$00       ; 4 bytes
long_464E:                  = RAM+$04       ; 4 bytes
word_4652:                  = RAM+$08       ; 2 bytes
word_4654:                  = RAM+$0A       ; 2 bytes
word_4656:                  = RAM+$0C       ; 2 bytes
word_4658:                  = RAM+$0E       ; 2 bytes

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

Scrolling_BG_Init:
        move.w  #1,(word_4656).w        ; set X move
        move.w  #1,(word_4658).w        ; set Y move
        clr.w   (word_4652).w
        clr.w   (word_4654).w
        rts
; End of function Scrolling_BG_Init

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

Scrolling_BG:
        tst.w   (word_4652).w
        bmi.s   loc_2FD6
        cmpi.w  #512,(word_4652).w
        bhi.s   loc_2FE6
        bra.s   loc_3014
; ---------------------------------------------------------------------------

loc_2FD6:
        cmpi.w  #-512,(word_4652).w
        blt.s       loc_2FE6
        bra.s   loc_3014
; ---------------------------------------------------------------------------

loc_2FE6:
        neg.w   (word_4656).w
        move.w  (word_4656).w,d0
        add.w   d0,(word_4652).w
        bsr.w   Random_Number
        andi.w  #7,d0
        addq.w  #1,d0
        tst.w (word_4656).w
        bpl.s   +
        neg.w   d0
+       move.w  d0,(word_4656).w

loc_3014:
        move.w  (word_4656).w,d0
        add.w   d0,(word_4652).w
        move.w  (word_4652).w,d0
        ext.l   d0
        lsl.l   #8,d0
        add.l   d0,(long_464A).w
        tst.w   (word_4654).w
        bmi.s   loc_304A
        cmpi.w  #512,(word_4654).w
        bhi.s   loc_305A
        bra.s   loc_3088
; ---------------------------------------------------------------------------

loc_304A:
        cmpi.w  #-512,(word_4654).w
        blt.s       loc_305A
        bra.s   loc_3088
; ---------------------------------------------------------------------------

loc_305A:
        neg.w   (word_4658).w
        move.w  (word_4658).w,d0
        add.w   d0,(word_4654).w
        bsr.s   Random_Number
        andi.w  #7,d0
        addq.w  #1,d0
        tst.w   (word_4658).w
        bpl.s   +
        neg.w   d0
+       move.w  d0,(word_4658).w

loc_3088:
        move.w  (word_4658).w,d0
        add.w   d0,(word_4654).w
        move.w  (word_4654).w,d0
        ext.l   d0
        lsl.l   #8,d0
        add.l   d0,(long_464E).w
        move.w  (long_464A).w,(H_scroll_buffer+2).w
        move.w  (long_464E).w,(V_scroll_value+2).w
        rts
; End of function Scrolling_BG
; ---------------------------------------------------------------------------
; Generates a pseudo-random number in d0
; ---------------------------------------------------------------------------

byte_CE54:                  = RAM+$10       ; 1 byte
byte_CE55:                  = RAM+$11       ; 1 byte

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

Random_Number:
        movem.l d1-d2/a0,-(sp)
        move.w  (V_int_run_count).w,d2
        move.w  d2,d1
        andi.w  #$FF,d2
        lsr.w   #8,d1
        move.b  (byte_CE55).w,d0
        eor.b   d0,d2
        move.b  (byte_CE54).w,d0
        eor.b   d0,d1
        moveq   #0,d0
        move.w  d1,d0
        lsl.w   #8,d0
        or.b    d2,d0
        add.l   d0,d0
        movea.l d0,a0
        moveq   #0,d0
        move.w  (a0),d0
        sub.w   d0,d2
        sub.w   d0,d1
        ror.b   #1,d2
        rol.b   #1,d1
        add.b   d1,(byte_CE55).w
        add.b   d2,(byte_CE54).w
        eor.b   d2,d0
        lsl.w   #8,d1
        eor.b   d1,d0
        movem.l (sp)+,d1-d2/a0
        rts
; End of function Random_Number