random: trying to find a reasonable random number generator

random/Makefile

@@ -0,0 +1,26 @@
+CC = gcc
+CFLAGS = -O2 -Wall
+LFLAGS =
+
+all:	random16 random8 #random15
+
+###
+
+random16:	random16.o
+	$(CC) $(LFLAGS) -o random16 random16.o
+
+random16.o:	random16.c
+	$(CC) $(CFLAGS) -c random16.c
+
+###
+
+random8:	random8.o
+	$(CC) $(LFLAGS) -o random8 random8.o
+
+random8.o:	random8.c
+	$(CC) $(CFLAGS) -c random8.c
+
+###
+
+clean:
+	rm -f *~ *.o random16 random8 random15

random/README

@@ -0,0 +1,9 @@
+Trying to find a good pseudo-random number generator for 6502
+
+I had been using the "random16" one here, but at least on Peasant's Quest
+when used to generate a 0-31 random value it seemed to have "0" and "17"
+come up way more than exected
+
+the "random8" one here is better, but it's really only 7 bits as you can't
+actually generate a 0 and code might fail if you never get a 0 result?
+

random/random16.c

@@ -0,0 +1,109 @@
+#include <stdio.h>
+
+// 16-bit 6502 Random Number Generator
+
+// Linear feedback shift register PRNG by White Flame
+// http://codebase64.org/doku.php?id=base:small_fast_16-bit_prng
+
+// The Apple II KEYIN routine increments this field
+//while waiting for keypress
+
+//SEEDL = $4E
+//SEEDH = $4F
+
+//XOR_MAGIC = $7657	; "vW"
+
+static unsigned short xor_magic=0x7657;
+//static unsigned short xor_magic=0x002d;
+static unsigned short seed=0x0000;
+
+//	;=============================
+//	; random16
+//	;=============================
+
+unsigned short random16(void) {
+
+	if ((seed&0xff)!=0) {
+		if ((seed&0x8000)==0) {
+			// noEor
+			seed<<=1;
+			return seed;
+		}
+		else {
+			// doEor
+			seed<<=1;
+			seed=seed^xor_magic;
+			return seed;
+		}
+	}
+	else {
+		// if bottom 0
+		if (((seed>>8)&0xff)==0) {
+			seed<<=1;
+			seed=seed^xor_magic;
+			return seed;
+		}
+
+		else {
+			if ((seed & 0x8000)==0) {
+				seed<<=1;
+				return seed;
+			}
+			else {
+				seed<<=1;
+				seed=seed^xor_magic;
+				return seed;
+			}
+		}
+
+	}
+}
+
+int main(int argc, char **argv) {
+
+	int i,r;
+
+	for(i=0;i<65537;i++) {
+		r=random16();
+		printf("%04hx %04hx\n",r,r&0x1f);
+	}
+
+	return 0;
+}
+//	lda	SEEDL							; 3
+//	beq	lowZero		; $0000 and $8000 are special values	; 2
+
+//	asl	SEEDL		; Do a normal shift			; 5
+//	lda	SEEDH							; 3
+//	rol								; 2
+//	bcc	noEor							; 2
+
+//doEor:
+//				; high byte is in A
+
+
+//	eor	#>XOR_MAGIC						; 2
+//	sta	SEEDH							; 3
+//	lda	SEEDL							; 3
+//	eor	#<XOR_MAGIC						; 2
+//	sta	SEEDL							; 3
+//	rts								; 6
+
+//lowZero:
+//									; 1
+//	lda	SEEDH							; 3
+//	beq	doEor		; High byte is also zero		; 3
+//				; so apply the EOR
+//									; -1
+//				; wasn't zero, check for $8000
+//	asl								; 2
+//	beq	noEor		; if $00 is left after the shift	; 2
+//				; then it was $80
+//	bcs	doEor		; else, do the EOR based on the carry	; 3
+
+//noEor:
+//									; 1
+//	sta	SEEDH							; 3
+
+//	lda	SEEDL	; always return SEEDL
+//	rts								; 6

random/random8.c

@@ -0,0 +1,47 @@
+// http://www.6502.org/users/mycorner/6502/code/prng.html
+
+//; returns pseudo random 8 bit number in A. Affects A. (r_seed) is the
+//; byte from which the number is generated and MUST be initialised to a
+//; non zero value or this function will always return zero. Also r_seed
+//; must be in RAM, you can see why......
+
+//rand_8
+//	LDA	r_seed		; get seed
+//	ASL			; shift byte
+//	BCC	no_eor		; branch if no carry
+//
+//	EOR	#$CF		; else EOR with $CF
+//no_eor
+//	STA	r_seed		; save number as next seed
+//	RTS			; done
+
+//r_seed
+//	.byte	1		; prng seed byte, must not be zero
+
+
+#include <stdio.h>
+
+unsigned char r_seed=1;
+
+int rand8(void) {
+	if (r_seed&0x80) {
+		r_seed<<=1;
+		r_seed^=0xcf;
+	}
+	else {
+		r_seed<<=1;
+	}
+	return r_seed;
+}
+
+int main(int argc, char **argv) {
+
+	int i,r;
+
+	for(i=0;i<1024;i++) {
+		r=rand8();
+		printf("%02X m%02X\n",r,r&0x1f);
+	}
+
+	return 0;
+}