64 lines
975 B
ArmAsm
64 lines
975 B
ArmAsm
// #LAYOUT# STD * #TAKE
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// #LAYOUT# * BASIC_0 #TAKE
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// #LAYOUT# * * #IGNORE
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//
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// A fast random generator, idea from:
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// - https://codebase64.org/doku.php?id=base:32bit_galois_lfsr
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//
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// Output:
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// - FAC1
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//
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// See also:
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// - https://www.atarimagazines.com/compute/issue72/random_numbers.php
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//
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rnd_generate:
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// Generate new 4-byte random number
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asl RNDX+1
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rol RNDX+2
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rol RNDX+3
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rol RNDX+4
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bcs rnd_generate_copy
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lda RNDX+1
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eor #$B7
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sta RNDX+1
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lda RNDX+2
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eor #$1D
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sta RNDX+2
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lda RNDX+3
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eor #$C1
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sta RNDX+3
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lda RNDX+4
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eor #$04
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sta RNDX+4
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// FALLTROUGH
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rnd_generate_copy:
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// Copy the number to FAC1, order changed on purpose
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lda RNDX+3
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sta FAC1_mantissa+0
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lda RNDX+1
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sta FAC1_mantissa+1
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lda RNDX+2
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sta FAC1_mantissa+2
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lda RNDX+4
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sta FAC1_mantissa+3
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lda #$00
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sta FACOV
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sta FAC1_sign
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lda #$80
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sta FAC1_exponent
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sta RNDX+0 // in case someone tries to use the float directly
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jmp normal_FAC1
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