Routines
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27705: Generate Random Number
Used by the routines at Handler_RedBird, UpdateRedBirdMovement, Handle_Butterfly, Randomise_Butterfly_Position, Randomise_Car_Position, InitialiseCar, CheckCarEggCollision, Handler_CarType2 and Handler_CarType3.
Generates a pseudo-random number using the seed stored at *RedBirdAnimationSeed. The algorithm scrambles the seed value through a series of arithmetic operations and stores the result back as the new seed. Returns the random value in A.
Output
A Pseudo-random value
GenerateRandomNumber 27705 LD A,(27831) H=*RedBirdAnimationSeed.
27708 LD H,A
27709 LD L,0 Load both L and D with 0.
27711 LD D,L
27712 LD E,H Copy *RedBirdAnimationSeed into E.
HL = seed × 256 DE = seed. Subtract DE twice.
27713 AND A Clear carry.
27714 SBC HL,DE HL-=DE.
27716 SBC HL,DE HL-=DE.
27718 LD DE,254 HL+=0254.
27721 ADD HL,DE
Combine the high and low bytes.
27722 LD A,L A=L.
27723 SUB H A-=H.
27724 JR C,GenerateRandomNumber_Store Jump to GenerateRandomNumber_Store if carry set.
27726 DEC A Decrement A.
GenerateRandomNumber_Store 27727 LD (27831),A Write the new seed to *RedBirdAnimationSeed.
27730 RET Return.
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