<code=asm>
;LFSR random generators
;
;with cc65 compiler/assembler package
;compile: ca65 -t c64 random.s && ld65 -t c64 -o random random.o c64.lib
;
;© 2007 Hanno Behrens (pebbles@schattenlauf.de)
;LGPL Licence
;
;Call random and you get a lowbyte in A and an highbyte in Y
;you should only use the 8 bit lowbyte cause thats best random. 
;The full 16 A/Y may be used but suffer of some bad disadvantages
;that should be best avoided by not using Y.
;The lowbyte of the one algo shifts right, the other left,
;so the result is mixed up a bit better than just with plain shifts
;The periode of this two combined generators is nearly 2^31 (exact 2^16-1*2^15-1)
;Make shure you don't feed either seeds just with plain zero cause
;the zero remains zero. All other seeds will do.
;
;DO NOT USE THIS ALGORITHMS FOR CRYPTOGRAPHY, THEY ARE WEAK 
;USE FORTUNA, YARROW OR OTHER STRONG (AND MUCH SLOWER THAN THIS) INSTEAD
;
;LFSR do produce on bit feedback periods as followed:
;0+1 = 3
;1+2 = 7
;2+3 = 15
;2+4 = 31
;4+5 = 63
;5+6 = 127
;1+2+3+7 = 255
;4+8 = 511
;6+9 = 1023 
;8+10 = 2047
;1+9+10+11 = 4095
;0+10+11+12 = 8191
;1+11+12+13 = 16383
;13+14 = 32767
;10+12+13+15 = 65535

		.setcpu "6502X"
		.macpack generic
		.macpack cbm
		
		.segment "STARTUP"
		
		.org $9000
		.word *
		.org *-2

;combination of generator 65535 and 32767 has periode of 2147385345
;result in a (lo) and y (hi)

.proc rand
		jsr rand64k       ;Factors of 65535: 3 5 17 257
		jsr rand32k       ;Factors of 32767: 7 31 151 are independent and can be combined
		lda sr1+1         ;can be left out 
		eor sr2+1         ;if you dont use
		tay               ;y as suggested
		lda sr1           ;mix up lowbytes of SR1
		eor sr2           ;and SR2 to combine both 
		rts
.endproc

;periode with 65535
;10+12+13+15
.proc rand64k
		lda sr1+1
		asl
		asl
		eor sr1+1
		asl
		eor sr1+1
		asl
		asl
		eor sr1+1
		asl
		rol sr1         ;shift this left, "random" bit comes from low
		rol sr1+1
		rts
.endproc

;periode with 32767
;13+14
.proc rand32k
		lda sr2+1
		asl
		eor sr2+1
		asl
		asl
		ror sr2         ;shift this right, random bit comes from high - nicer when eor with sr1
		rol sr2+1
		rts
.endproc

;feel free to set seeds as wished, if put in zeropage some speed-boost is 
;the result. For example sr1=$5c sr2=5e would fit
sr1:	.word $a55a
sr2:	.word $7653
</code>