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https://github.com/blondie7575/GSCats.git
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53 lines
1.1 KiB
ArmAsm
53 lines
1.1 KiB
ArmAsm
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; random.s
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; GSCats
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;
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; Created by Quinn Dunki on 12/29/19.
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;
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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; random
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; A => Random 8 bits
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; Y => Less random 8 bits
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;
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; Taken from https://codebase64.org/doku.php?id=base:16bit_xorshift_random_generator
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; This routine is not very fast, but it'll do for now. It's intended
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; for 6502 so we're making due with it.
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;
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rng_zp_low = RANDOML
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rng_zp_high = RANDOMH
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random:
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SAVE_AXY
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EMULATION
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LDA rng_zp_high
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LSR
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LDA rng_zp_low
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ROR
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EOR rng_zp_high
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STA rng_zp_high ; high part of x ^= x << 7 done
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ROR ; A has now x >> 9 and high bit comes from low byte
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EOR rng_zp_low
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STA rng_zp_low ; x ^= x >> 9 and the low part of x ^= x << 7 done
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EOR rng_zp_high
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STA rng_zp_high ; x ^= x << 8 done
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NATIVE
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RESTORE_AXY
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RTS
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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; seedRandom
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; A = Seed
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;
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; Taken from https://codebase64.org/doku.php?id=base:16bit_xorshift_random_generator
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;
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seedRandom:
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SAVE_AXY
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EMULATION
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STA rng_zp_low
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LDA #0
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STA rng_zp_high
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NATIVE
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RESTORE_AXY
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rts
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