Jameco Electronics 2000, 3000 manual test for

Some simplifications are possible if one of the unsigned numbers being compared is a constant. Note that the carry has a reverse sense from SBC. In the following examples, the pseudo-code in the form LD DE,(65535-B)does not indicate a load of DE with the address pointed to by 65535-B, but simply indicates the difference between 65535 and the 16-bit unsigned integer B.

;test for	HL>B	B is constant
LD DE,(65535-B)
ADD HL,DE	; carry set if HL>B
SBC HL,HL	; HL-HL-C - result -1 if carry set, else zero
BOOL HL	; 14 total clocks - true if HL>B
; HL>=B	B is constant not zero
LD DE,(65536-B)
ADD HL,DE
SBC HL,HL
BOOL HL	; 14 clocks
; HL>=B	and B is zero
LD HL,1	; 6	clocks

;HL<B B is a constant, not zero (if B==0 always false) LD DE,(65536-B)

ADD HL,DE	; not carry			if	HL<B
SBC HL,HL	;	-1	if carry,		else 0
INC HL	;	14	clocks	--0 if carry, else 1 if no carry
;

;HL <= B B is constant not zero LD DE,(65535-B)

ADD HL,DE	; ~C if HL<=B
CCF	; C if true
SBC HL,HL	; if C -1 else	0
INC HL	; 16 clocks --	1	if true, else 0
;
; HL <= B B is zero - true		if HL==0
BOOL HL	; result in HL
;

;HL==B and B is a constant not zero LD DE,(65536-B)

ADD HL,DE		;	zero if equal
BOOL HL
INC	HL
RES	1,l	;	16 clocks

;HL==B and B==0

BOOL HL INC HL

RES 1,l ; 8 clocks

For signed integers the conventional method to look at the zero flag, the minus flag and the overflow flag. Signed 8-bit integers span the range –128 to +127 (0x80 to 0x7F). Signed 16-bit integers span the range –32768 to + 32767 (0x8000 to 0x7FFF). The sign and zero flag tell which is the larger number after the subtraction unless the overflow is set, in which case the sign flag needs to be inverted in the logic, that is, it is wrong.

42 Rabbit 3000 Microprocessor