Programming:Random Number Generator

This is a very simple generator. It's function is x[i + 1] = (5 * x[i] + 1) mod 256. The only advantage is the small size and it's simplicity.

Input: none

Output: A = a pseudo random number, period 256

Rand8	ld	a,Seed		; Seed is usually 0
	ld	b,a
	add	a,a
	add	a,a
	add	a,b
	inc	a		; another possibility is ADD A,7
	ld	(Rand8+1),a
	ret

To use this routine you need to define 1 Byte of RAM for the RandomPtr.

Input: none

Output: A = a pseudo random number, period 256

RandomNumber:
        push    hl
        ld      a,(RandomPtr)
        inc     a
        ld      (RandomPtr),a
        ld      hl,RandTable
        add     a,l
        ld      l,a
        jr      nc,.skip
        inc     h
.skip:  ld      a,(hl)
        pop     hl
        ret

RandTable:
        db      &3B,&02,&B7,&6B,&08,&74,&1A,&5D,&21,&99,&95,&66,&D5,&59,&05,&42
        db      &F8,&03,&0F,&53,&7D,&8F,&57,&FB,&48,&26,&F2,&4A,&3D,&E4,&1D,&D9
        db      &9D,&DC,&2F,&F5,&92,&5C,&CC,&00,&73,&15,&BF,&B1,&BB,&EB,&9E,&2E
        db      &32,&FC,&4B,&CD,&A7,&E6,&C2,&10,&11,&80,&52,&B2,&DA,&77,&4F,&EC
        db      &13,&54,&64,&ED,&94,&8C,&C6,&9A,&19,&9F,&75,&FA,&AA,&8D,&FE,&91
        db      &01,&23,&07,&C1,&40,&18,&51,&76,&3C,&BD,&2A,&88,&2D,&F1,&8A,&72
        db      &F6,&98,&35,&97,&68,&93,&B3,&0C,&82,&4E,&CB,&39,&D8,&5F,&C7,&D4
        db      &CE,&AE,&6D,&A3,&7C,&6A,&B8,&A6,&6F,&5E,&E5,&1B,&F4,&B5,&3A,&14
        db      &78,&FD,&D0,&7A,&47,&2C,&A8,&1E,&EA,&2B,&9C,&86,&83,&E1,&7B,&71
        db      &F0,&FF,&D1,&C3,&DB,&0E,&46,&1C,&C9,&16,&61,&55,&AD,&36,&81,&F3
        db      &DF,&43,&C5,&B4,&AF,&79,&7F,&AC,&F9,&37,&E7,&0A,&22,&D3,&A0,&5A
        db      &06,&17,&EF,&67,&60,&87,&20,&56,&45,&D7,&6E,&58,&A9,&B0,&62,&BA
        db      &E3,&0D,&25,&09,&DE,&44,&49,&69,&9B,&65,&B9,&E0,&41,&A4,&6C,&CF
        db      &A1,&31,&D6,&29,&A2,&3F,&E2,&96,&34,&EE,&DD,&C0,&CA,&63,&33,&5B
        db      &70,&27,&F7,&1F,&BE,&12,&B6,&50,&BC,&4D,&28,&C8,&84,&30,&A5,&4C
        db      &AB,&E9,&8E,&E8,&7E,&C4,&89,&8B,&0B,&24,&85,&3E,&38,&04,&D2,&90

This algorithm uses a similar method, but returns much better results.

Input: none

Output: A = a pseudo random number, period 65536

Rand16	ld	de,Seed		; Seed is usually 0
	ld	a,d
	ld	h,e
	ld	l,253
	or	a
	sbc	hl,de
	sbc	a,0
	sbc	hl,de
	ld	d,0
	sbc	a,d
	ld	e,a
	sbc	hl,de
	jr	nc,Rand
	inc	hl
Rand	ld	(Rand16+1),hl
	ret

A pseudo random generator using the 'r' register for the first byte of the random number. It then uses this number as a seed to feed the 8-bit random number generator to generate the other byte of the random value.

Input: none

Output: HL = a pseudo random number, period 65536

Rand16	ld a,r
	ld h,a

Rand8	ld b,a
	add a,a
	add a,a
	add a,b
	inc a
	ld (Rand8 + 1),a

	ld l,a          ; HL now contains a random 16-bit number
	ret