Rand.asm

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Contents

Functions Provided

Entry pointFunction providedNotes
RANDGenerate an N-bit random numberN can be in the range 0..16



See source code below for calling convention.

Examples

(todo... please contribute!)

Notes

This function requires 2 words of 16-bit memory. Create two labels, RNDHI and RNDLO, that are set to the addresses for this function to store its random numbers state.

Also, consider calling this function periodically from idle loops, and/or mixing other random data into RNDHI/RNDLO to further increase randomness.

If this function is too bulky or slow, consider using randfast.asm.

Source Code

;* ======================================================================== *;
;*  These routines are placed into the public domain by their author.  All  *;
;*  copyright rights are hereby relinquished on the routines and data in    *;
;*  this file.  -- Joseph Zbiciak, 2008                                     *;
;* ======================================================================== *;

;; ======================================================================== ;;
;;  GLOBAL VARIABLES USED BY THESE ROUTINES                                 ;;
;;                                                                          ;;
;;  This routine needs two 16-bit global variables as shown below.          ;;
;;                                                                          ;;
;;  Example declarations for these routines are shown, commented out.       ;;
;;  You should use declarations such as these, or perhaps use macros such   ;;
;;  as those in "dseg.mac" to define these variables.  Make sure to pick    ;;
;;  locations that aren't used for anything else.                           ;;
;; ======================================================================== ;;

                        ; Used by       Req'd Width     Description
                        ;-----------------------------------------------------
;RNDLO  EQU     $320    ; RAND          16-bit          Random number state
;RNDHI  EQU     $321    ; RAND          16-bit          Random number state

;; ======================================================================== ;;
;;  RAND                                                                    ;;
;;      Returns random bits in R0.                                          ;;
;;                                                                          ;;
;;  INPUTS:                                                                 ;;
;;      R0 -- Number of bits desired                                        ;;
;;      R5 -- Return address                                                ;;
;;      Random state in RNDLO, RNDHI                                        ;;
;;                                                                          ;;
;;  OUTPUTS:                                                                ;;
;;      R0 -- N random bits.                                                ;;
;;      R1, R2, R3, R4 -- Saved and restored                                ;;
;;      R5 -- trashed.                                                      ;;
;;                                                                          ;;
;;  NOTES:                                                                  ;;
;;      You are encouraged to add additional "randomness" by adding or      ;;
;;      XORing other values into RNDLO or RNDHI.  Also, to initialize       ;;
;;      the random number generator, ensure that RNDLO and RNDHI are        ;;
;;      non-zero.                                                           ;;
;; ======================================================================== ;;
RAND    PROC
        PSHR    R5              ; Save return address and R1..R4
        PSHR    R4
        PSHR    R3
        PSHR    R2
        PSHR    R1

        MVII    #1,     R1      ; Our initial mask word
        TSTR    R0              ; Is R0 > 0?
        BEQ     @@nobits

        MVII    #$04C1, R5      ; period==(2**32 - 1) polynomial 
        MVII    #$1DB7, R4      ; (this is the CRC-32 polynomial)
        MVI     RNDHI,  R3      ; Read in our 32-bit random number state
        MVI     RNDLO,  R2

        TSTR    R3              ; If our random number generator is zero
        BNEQ    @@loop          ; jumpstart the process by forcing an XOR
        TSTR    R2              ; of our generator polynomal into R2/R3
        SETC                    ; up front.  Otherwise, we won't generate
        BEQ     @@forceit       ; any random numbers!
        
@@loop:
        SLLC    R2,     1       ; Shift our 32-bit random number left by 1
        RLC     R3,     1       ; ... by using the carry and an RLC.
@@forceit:
        SLL     R1,     1       ; Shift our mask bit left by 1
        BNC     @@nocarry
        XORR    R4,     R2      ; If the carry was set, XOR in our generator
        XORR    R5,     R3      ; polynomial.  
@@nocarry:
        DECR    R0              ; Keep generating bits.
        BNEQ    @@loop

        MVO     R3,     RNDHI   ; Store our new random number state.
        MVO     R2,     RNDLO

@@nobits:
        DECR    R1              ; Turn our mask bit into a mask word
        ANDR    R1,     R2      ; Mask the bits we actually want.
        MOVR    R2,     R0      ; Return our result in R0

        PULR    R1              ; Retstore our registers and return.
        PULR    R2
        PULR    R3
        PULR    R4
        PULR    PC

        ENDP

;; ======================================================================== ;;
;;  End of File:  rand.asm                                                  ;;
;; ======================================================================== ;;
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