AppleIIAsm-Collection/disks/disk4_math/T.RANDB

*
*``````````````````````````````*
* RANDB ::         RANDOM BYTE *
*-                            -*
* RETURNS A RANDOM BYTE VALUE  *
* BETWEEN THE HIGH AND LOW     *
* BOUNDS PASSED TO ROUTINE.    *
* NOTE THAT THIS ONLY WORKS    *
* WITH UNSIGNED VALUES.        *
*-                            -*
* CLOBBERS:                    *
*                              *
* FLAGS:  ????----  REG: AXYM  *
*-                            -*
* CYCLES: ???                  *
* SIZE:                        *
*-                            -*
* USAGE:                       *
*                              *
*  LDA #1 ;  LOW BOUND         *
*  PHA                         *
*  LDA #100 ; HIGH             *
*  PHA                         *
*  JSR RANDB                   *
*-                            -*
* ENTRY                        *
*                              *
* TOP OF STACK                 *
*                              *
* LOW BYTE OF RETURN ADDRESS   *
* HI BYTE OF RETURN ADDRESS    *
*-                            -*
* EXIT                         *
*                              *
* TOP OF STACK                 *
*                              *
* LOW BYTE OF RETURN ADDRESS   *
* HI BYTE OF RETURN ADDRESS    *
*                              *
* .Y = COUNTER; TRASH          *
* .X = COUNTER; TRASH          *
* .A = LOW BYTE OF RET ADDR    *
*,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,*
*
RANDB
*
** GET RETURN ADDRESS
*
         PLA
         STA   RETADR
         PLA
         STA   RETADR+1
*
** GET PARAMETERS
*
         PLA
         STA   :NEWMAX
         PLA
         STA   :NEWMIN
*
** GET OLDMIN,OLDMAX,OLDVAL
*
         LDA   #1
         STA   :OLDMIN
         LDA   #255
         STA   :OLDMAX
         JSR   RAND8      ; RANDOM BYTE 0..255
         STA   :OLDVAL
*
** NEWVALUE = (((OLDVAL-NEWMIN) * (NEWMAX-NEWMIN) /
**            (OLDMAX-OLDMIN)) + NEWMIN
*
** OR
*
** OLDRANGE = (OLDMAX-OLDMIN)
** NEWRANGE = (NEWMAX - NEWMIN)
** NEWVAL = (((OLDVAL-OLDMIN) * NEWRANGE) / OLDRANGE) + NEWMIN
*
** GET OLDRANGE: OLDMAX - OLDMIN
*
         LDA   :OLDMAX
         SEC
         SBC   :OLDMIN
         STA   :OLDRANGE
*
** NOW GET NEWRANGE: NEWMAX - NEWMIN
*
         LDA   :NEWMAX
         SEC
         SBC   :NEWMIN
         STA   :NEWRANGE
*
** NOW GET VALRANGE: OLDVAL - OLDMIN
*
         LDA   :OLDVAL
         SEC
         SBC   :OLDMIN
         STA   :VALRANGE
*
** GET MULRANGE: VALRANGE * NEWRANGE
*
         LDA   #00
         TAY
         STY   :NUM1HI
         BEQ   :ENTLP
*
:DOADD
         CLC
         ADC   :VALRANGE
         TAX
*
         TYA
         ADC   :NUM1HI
         TAY
         TXA
*
:MLP
         ASL   :VALRANGE
         ROL   :NUM1HI
:ENTLP
         LSR   :NEWRANGE
         BCS   :DOADD
         BNE   :MLP
*
         STA   :MULRANGE
         STY   :MULRANGE+1
*
** NOW GET DIVRANGE: MULRANGE / OLDRANGE
*
:DIVIDE
         LDA   #0
         STA   :REMAIN
         STA   :REMAIN+1
         LDX   #16
*
:DIVLP
         ASL   :MULRANGE
         ROL   :MULRANGE+1
         ROL   :REMAIN
         ROL   :REMAIN+1
         LDA   :REMAIN
         SEC
         SBC   :OLDRANGE
         TAY
         LDA   :REMAIN+1
         SBC   :OLDRANGE+1
         BCC   :SKIP
*
         STA   :REMAIN+1
         STY   :REMAIN
         INC   :DIVRANGE
*
:SKIP    DEX
         BNE   :DIVLP
*
** NOW ADD NEWMIN TO DIVRANGE
*
         LDA   :DIVRANGE  ; LO BYTE ONLY
         CLC
         ADC   :NEWMIN
         STA   :NEWVAL
*
** RESTORE RETURN ADDRESS
*
         LDA   RETADR+1
         PHA
         LDA   RETADR
         PHA
*
         LDA   :NEWVAL    ; RETURN # IN A
         STA   RETURN
         LDX   #1
         STX   RETLEN
         RTS
*
** DATA
*
:OLDRANGE DS   2
:NEWRANGE DS   2
:MULRANGE DS   2
:DIVRANGE DS   2
:VALRANGE DS   2
:OLDVAL  DS    2
:NEWVAL  DS    2
:OLDMIN  DS    2
:NEWMIN  DS    2
:OLDMAX  DS    2
:NEWMAX  DS    2
:NUM1HI  DS    2
:REMAIN  DS    2
*