# Rand.asm

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## Contents |

# 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, 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 ;; ;; ======================================================================== ;;