AppleIIAsm-Collection/disks/disk3_math8_math16/T.RAND8

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