Jameco Electronics 3000, 2000 manual Comparisons of Integers, User’s Manual

3.4.4 Comparisons of Integers

Unsigned integers may be compared by testing the zero and carry flags after a subtract operation. The zero flag is set if the numbers are equal. With the SBC instruction the carry cleared is set if the number subtracted is less than or equal to the number it is subtracted from. 8-bit unsigned integers span the range 0–255. 16-bit unsigned integers span the range 0–65535.

OR a		;	clear carry
SBC HL,DE		;	HL=A and DE=B
A>=B	!C
A<B	C
A==B	Z
A>B	!C & !Z
A<=B	C v Z

If A is in HL and B is in DE, these operations can be performed as follows assuming that the object is to set HL to 1 or 0 depending on whether the compare is true or false.

;compute HL<DE

;unsigned integers

;EX DE,HL ; uncomment for DE<HL

OR a ; clear carry

SBC	HL,DE	; C set if HL<DE
SBC	HL,HL	;	HL-HL-C --	-1	if carry set
BOOL HL		;	set to 1 if	carry, else zero

;else result == 0

;unsigned integers

;compute HL>=DE or DE>=HL - check for !C

;EX DE,HL ; uncomment for DE<=HL

OR a		; clear			carry
SBC HL,DE		; !C		if	HL>=DE
SBC	HL,HL	;	HL-HL-C - zero			if no	carry, -1 if C
INC	HL	;	14	/ 16 clocks		total	-if C after first SBC result 1,

;else 0

;0 if C , 1 if !C

: compute HL==DE

OR a ; clear carry SBC HL,DE ; zero is equal

BOOL HL	; force to zero, 1
DEC HL	;	invert logic
BOOL HL	;	12 clocks total -logical not, 1 for inputs equal
;

User’s Manual 41