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81 lines
1.3 KiB
NASM
81 lines
1.3 KiB
NASM
Random::
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; A simple hardware-based random number generator (RNG).
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; Two random numbers are generated by adding and subtracting
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; the divider to the respective values every time it's called.
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; The divider is a register that increments at a rate of 16384Hz.
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; For comparison, the Game Boy operates at a clock speed of 4.2MHz.
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; Additionally, an equivalent function is executed in VBlank.
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; This leaves a with the value in hRandomSub.
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push bc
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ld a, [rDIV]
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ld b, a
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ld a, [hRandomAdd]
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adc b
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ld [hRandomAdd], a
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ld a, [rDIV]
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ld b, a
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ld a, [hRandomSub]
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sbc b
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ld [hRandomSub], a
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pop bc
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ret
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BattleRandom::
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; _BattleRandom lives in another bank.
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; It handles all RNG calls in the battle engine, allowing
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; link battles to remain in sync using a shared PRNG.
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ld a, [hROMBank]
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push af
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ld a, BANK(_BattleRandom)
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rst Bankswitch
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call _BattleRandom
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ld [wPredefTemp + 1], a
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pop af
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rst Bankswitch
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ld a, [wPredefTemp + 1]
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ret
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RandomRange::
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; Return a random number between 0 and a (non-inclusive).
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push bc
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ld c, a
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; b = $100 % c
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xor a
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sub c
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.mod
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sub c
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jr nc, .mod
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add c
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ld b, a
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; Get a random number
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; from 0 to $ff - b.
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push bc
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.loop
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call Random
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ld a, [hRandomAdd]
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ld c, a
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add b
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jr c, .loop
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ld a, c
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pop bc
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call SimpleDivide
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pop bc
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ret
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