# Randfast.asm

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## Contents |

# Functions Provided

Entry point | Function provided | Notes |
---|---|---|

RANDFAST | Return a 16-bit random number | Always returns 16 bits, but only advances state by 4 bits. |

See source code below for calling convention.

# Examples

(todo... please contribute!)

# Notes

This function requires 1 word of 16-bit memory. Create a label, RSEED, that is set to the address for this function to store its random numbers state.

RANDFAST always returns a 16 bit random value, but only advances its internal state 4 bits. If you need greater separation between consecutive random numbers, either modify the count in the REPEAT loop, or call RANDFAST multiple times. If you need a more flexible and robust random number generator, consider rand.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 *; ;* ======================================================================== *; ;; ======================================================================== ;; ;; RANDFAST ;; ;; Returns random bits in R0. ;; ;; ;; ;; INPUTS: ;; ;; R5 -- Return address ;; ;; Random state in RSEED ;; ;; ;; ;; OUTPUTS: ;; ;; R0 -- 16 random bits. ;; ;; R1 -- Set to $AB19 ;; ;; R2..R4 -- Unmodified ;; ;; ;; ;; NOTES: ;; ;; You are encouraged to add additional "randomness" by adding or ;; ;; XORing other values into RSEED. ;; ;; ;; ;; Implementation is a Galois realization of a 16-bit LFSR with the ;; ;; following polynomial: ;; ;; ;; ;; x^16 = x^15 + x^13 + x^11 + x^9 + x^8 + x^4 + x^3 + x^0 ;; ;; ;; ;; The random number generator is only advanced by 4 bits per call. ;; ;; Call twice if you need 8 truly random bits. The generator uses ;; ;; a dense polynomial, so the top 12 bits will be different than the ;; ;; bottom 12 bits returned on the previous call 15 out of 16 times. ;; ;; ======================================================================== ;; RANDFAST PROC MVI RSEED, R0 ; 10 Get rand seed XORR R5, R0 ; 8 XOR in caller ret address MVII #$AB19, R1 ; 8 Field polynomial ;---- ; 26 REPEAT 4 SLLC R0, 1 ; 6 Multiply by x^1 ADCR PC ; 7 \_ If x^16 generated, XOR in XORR R1, R0 ; 6 / polynomial (-ve logic) ;---- ; 19 ENDR ; 76 (4 unrolled iterations) ; 26 (carried forward) ;---- ; 102 MVO R0, RSEED ; 11 JR R5 ; 7 ;---- ; 18 ; 102 ;---- ; 130 ENDP