; ********** PSEUDO-RANDOM NUMBER ROUTINE **********
; GENERATES A PSEUDO-RANDOM NUMBER FROM 0 TO 65535.
; 
; 	ENTRY:	NO PARAMETERS.
;	EXIT:	(BC) = RANDOM NUMBER 0 - 65535.
;           ALL REGISTERS SAVED EXCEPT BC.
; **************************************************

.ORG	0000h

SEED:	.EQU	78h		; ADDED FOR TESTING PURPOSES.

		LD	SP,	0080h	; ADDED FOR TESTING PURPOSES.
		CALL RND000		; ADDED FOR TESTING PURPOSES.
		HALT			; ADDED FOR TESTING PURPOSES.
		
RND000:	PUSH	AF			; SAVE REGISTERS.
		PUSH	DE	
		PUSH	HL	
		LD	DE,	(SEED)		; GET SEEND VALUE.
		LD	HL,	(SEED +2)	
		LD	B,	07h			; COUNT FOR MULTIPLY BY 128.
RND001:	CALL	SHIFT		; SHIFT ONE BIT LEFT.
		DJNZ	RND001		; SEED * 128.
		LD	B,	3			; FOR SUBTRACT.
RND002:	CALL	SUB			; SUBTRACT ONE.
		DJNZ	RND002		; (SEED * 128) - (3 * SEED) = (SEED * 125).
		LD	(SEED),	DE		; STORE NEW SEED.
		LD	(SEED +2),	HL
		LD	B,	E			; NOW IN B.
		LD	C,	H			; NOW IN BC.
		POP	HL				; RESTORE REGISTERS.
		POP	DE
		POP AF
		RET					; RETURN.
; SHIFT SUBROUTINE.
SHIFT:	ADD	HL,	HL			; SHIFT HL.
		EX	DE,	HL			; GET MS BYTE.
		ADC	HL,	HL			; SHIFT MS 2 BYTES.
		EX	DE,	HL			; NOW ORIGINAL * 2.
		RET					; RETURN.
; SUBTRACT SEED SUBROUTINE.
SUB:	PUSH	BC			; SAVE REGISTERS.
		LD	BC,	(SEED + 2)	; GET LS BYTE.
		OR	A				; RESET CARRY BIT.
		SBC	HL,	BC			; SUBTRACT LS 2 BYTES.
		EX	DE,	HL			; GET MS 2 BYTES.
		LD	BC,	(SEED)		; GET MS 2 BYTES.
		SBC	HL,	BC			; SUBTRACT MS 2 BYTES AND CARRY.
		EX	DE,	HL			; NOW ORIGINAL - SEED.
		POP	BC				; RESTORE REGISTERS.
		RET					; RETURN

.END