; Update enemy x,y positions.

       ld ix,ashdir        ; point to enemy Ash data.
       call enemy          ; move Ash down and left/right.

; ..... code continues...

; --------------------------------------------------------------
; UPDATE ENEMY.
; Calculate new x,y positions for an enemy car.
; IX = start of enemy data block.

enemy  ld a,(ix+0)         ; test direction.
       cp 0                ; moving left (0=left, 1=right)?
       jp nz,enem0         ; no - then enemy is moving right.

; Direction: Left - test left border.

       ld a,(ix+2)         ; load enemy's x-coordinate.
       cp leftb            ; compare it to left border constant.
       jp nc,+             ; branch if accumulator (x) > leftb.
                           ; else - enemy is on the left border
       ld a,1              ; shift direction to 'right'.
       ld (ix+0),a         ; load it into direction byte.
       jp enem1            ; skip forward to vertical movement.

; Direction: Left - subtract from enemy x coordinate.

+      ld b,hspeed         ; load horizontal speed into B.
       ld a,(ix+2)         ; load enemy x into A.
       sub b               ; subtract hspeed from x (move left).
       ld (ix+2),a         ; update enemy x coordinate.
       jp enem1            ; skip forward to vertical movement.

; Direction: Right - test right border.

enem0  ld a,(ix+2)         ; load enemy x.
       cp rightb           ; compare it to right border.
       jp c,+              ; skip if rightb > accumulator (x).

       xor a               ; else - shift direction to 0 = left.
       ld (ix+0),a         ; load new value into direction var.
       jp enem1            ; forward to vertical movement.

; Direction: Right - add to enemy x coordinate.

+      ld b,hspeed         ; load hspeed constant into B.
       ld a,(ix+2)         ; load enemy x into A.
       add a,b             ; add hspeed to enemy x.
       ld (ix+2),a         ; update enemy x coordinate.

; Vertical movement for enemy (move enemy car down).

enem1  ld a,(ix+1)         ; load enemy y into A.
       add a,espeed        ; add constant enemy vertical speed.
       ld (ix+1),a         ; update enemy y.
       ret