Routines
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31024: Generate Random Number
Used by the routines at Handler_Helicopter, Randomise_Position_Loop, Animate_Stunned_Hazard, Handler_UFO and Handler_Balloon.
Generates a pseudo-random number using a linear decay formula. Reads the current seed from RandomSeed, computes (seed + 1) * 254 then derives the new seed from the difference of the low and high bytes of the result. Returns the new value in A.
Output
A Pseudo-random value
Generate_Random_Number 31024 LD A,(31431) Fetch the current random seed from *RandomSeed.
31027 LD H,A Set HL to seed * 256 and copy into DE.
31028 LD L,0
31030 LD D,L
31031 LD E,H
31032 AND A Subtract DE twice from HL (i.e. HL = seed * 254).
31033 SBC HL,DE
31035 SBC HL,DE
31037 LD DE,254 Add 254 to HL.
31040 ADD HL,DE
31041 LD A,L Subtract H from L; if no borrow occurred, decrement the result by one.
31042 SUB H
31043 JR C,Store_Random_Seed
31045 DEC A
Store_Random_Seed 31046 LD (31431),A Write the new random seed to *RandomSeed.
31049 RET Return.
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