mirror of
https://github.com/nathanriggs/AppleIIAsm-Collection.git
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56 lines
1.5 KiB
Plaintext
56 lines
1.5 KiB
Plaintext
*
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*``````````````````````````````*
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* RAND8 :: 8-BIT RANDOM NUMBER *
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*- -*
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* GENERATES A PSEUDO-RANDOM *
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* NUMBER BETWEEN 1..255 (BYTE) *
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* AND RETURNS THE VALUE IN .A *
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* *
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* THIS USES THE GALOIS LINEAR *
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* FEEDBACK SHIFT REGISTER *
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* ALGORITHM WITH POLYNOMIAL *
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* $002D. THE SEQUENCE WILL *
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* REPEAT AFTER 65535 CALLS. *
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*- -*
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* CLOBBERS: *
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* *
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* FLAGS: ????---- REG: AX-- *
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*- -*
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* CYCLES: ??? *
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* SIZE: *
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*- -*
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* USAGE: JSR RND8 *
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*- -*
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* EXIT *
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* *
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* .Y = NOT AFFECTED *
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* .X = CLOBBERED *
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* .A = RANDOM NUM 1..255 *
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*- -*
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* NOTE: BASED ON THE PRNG *
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* LISTING FOUND IN THE NESDEV *
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* WIKI. *
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*,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,*
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*
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RAND8
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*
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LDX #8
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LDA RNDL+0
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:A
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ASL ;SHIFT THE REG
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ROL RNDL+1
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BCC :B
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EOR #$2D ; APPLY XOR FEEDBACK
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; WHENEVER A 1 BIT IS
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; SHIFTED OUT
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:B
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DEX
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BNE :A
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STA RNDL+0
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STA RETURN
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LDY #1
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STY RETLEN
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CMP #0 ; RELOAD FLAGS
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RTS
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*
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