Routines
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890D: Set Random Speed
Used by the routines at FireShot, SetFallingSpeed and ReverseVerticalSpeed.
Gives an object a random speed. The object is removed if it ends up with no speed at all.
Input
IY Address of the object
SetRandomSpeed 890D LD A,($9BCC) Fetch the second byte of the random seed.
8910 LD B,A Copy it into B.
8911 AND %00000110 Keep two bits.
8913 NEG Negate them.
8915 LD C,A Copy the result into C.
8916 LD (IY+$09),A Set the vertical speed of the object.
8919 LD A,B Fetch the random value again.
891A RLCA Rotate bits 5 to 3 into the bottom three bits.
891B RLCA
891C RLCA
891D AND %00000111 Keep them.
891F CP $07 Jump to SetRandomSpeed_Remove if the value is 07.
8921 JP Z,SetRandomSpeed_Remove
8924 SUB $03 Subtract three...
8926 ADD A,A ...and double it to get a horizontal speed.
8927 LD B,A
8928 LD (IY+$07),A Set the horizontal speed of the object.
892B OR C Combine the speeds.
892C JR NZ,FireShot_Sound Jump to FireShot_Sound if the object has any speed.
This entry point is used by the routine at SetFallingSpeed.
SetRandomSpeed_Remove 892E LD (IY+$00),$FF Free the slot of the object.
8932 JR FireShot_Sound Jump to FireShot_Sound.
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