Rand.asm
Functions Provided
| Entry point | Function provided | Notes |
|---|---|---|
| RAND | Generate an N-bit random number | N 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, for this function to store its random numbers in.
Also, consider calling this function periodically from idle loops, and/or mixing other random data into RNDHI/RNDLO to further increase randomness.
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 ;;
;; ======================================================================== ;;
