38 lines
661 B
NASM
38 lines
661 B
NASM
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; 6502 LFSR PRNG - 16-bit
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; Brad Smith, 2019
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; http://rainwarrior.ca
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;
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; A 16-bit Galois LFSR
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; Possible feedback values that generate a full 65535 step sequence:
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; $2D = %00101101
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; $39 = %00111001
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; $3F = %00111111
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; $53 = %01010011
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; $BD = %10111101
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; $D7 = %11010111
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; $39 is chosen for its compact bit pattern
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; simplest version iterates the LFSR 8 times to generate 8 random bits
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; 133-141 cycles per call
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; 19 bytes
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galois16: subroutine
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ldy #8
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lda Seed+0
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.one
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asl ; shift the register
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rol Seed+1
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bcc .two
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eor #$39 ; apply XOR feedback whenever a 1 bit is shifted out
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.two
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dey
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bne .one
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sta Seed+0
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cmp #0 ; reload flags
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rts
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