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Random

PRNG

This is a simple Pseudo-Random Number Generator I wrote. It is very simple, and use the same algorithm as most other PRNG's in high level languages - Linear congruential generator.

The following code is written to be used in a library. To learn how to create and use a library, take a look here.

rand.asm

;---------------------------------------------------------------
	%TITLE "rand.asm"
;---------------------------------------------------------------
 
	IDEAL
	MODEL	small

;---------------------------------------------------------------
	DATASEG
;---------------------------------------------------------------

magic_no_1	DD	1103515245
magic_no_2	DW	12345

;---------------------------------------------------------------
	UDATASEG
;---------------------------------------------------------------

dd_result	DD	?
rand_seed	DW	?

;---------------------------------------------------------------
	CODESEG
;--------------------------------------------------------------- 

	PUBLIC	seed_random, get_random
	PUBLIC	magic_no_1, magic_no_2

;---------------------------------------------------------------
;
;	SEED_RANDOM
;
;	return: seed in dx and rand_seed
; 
;---------------------------------------------------------------

PROC	seed_random

	push	bx			; save registers
	push	cx
	push	dx
@@L10:	
	mov	ah, 2Ch			; get ticks
	int	21h			;

	or	dx, dx			; make sure it's not zero
	jz	@@L10

	mov	[rand_seed], dx		; copy seed to global var

	pop	dx			; restore registers
	pop	cx
	pop	bx

@@L99:
	ret

ENDP	seed_random

;---------------------------------------------------------------
;
;	GET_RANDOM
;
;	x = (xa + c) mod 32767
;
;	expect: bx = max
;		cx = min
;
;	return:	ax = result
;		rand_seed = new value between 0 and 32767
;
;---------------------------------------------------------------

PROC	get_random

	push	cx				; save registers
	push	bx

@@L10:
	mov	ax, [word magic_no_1 + 2]	; copy high word
	mov	bx, [rand_seed]
	mul	bx				; result in dx:ax

	mov	[word dd_result + 2], dx	; high word of result
	mov	[word dd_result], ax		; copy low word

	mov	ax, [word magic_no_1]		; copy low word
	mov	bx, [rand_seed]
	mul	bx				; result in dx:ax
						; - don't use dx
	add	ax, [magic_no_2]		; + c
	and	ax, 7FFFh			; mod 32767

	or	ax, ax				;
	jnz	@@L20				;

	call	seed_random			; if zero, get new seed
	jmp	@@L10				; and start again

@@L20:
	mov	[rand_seed], ax			; make new seed

@@L99:
	ret
 
ENDP	get_random

	END

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