Jameco Electronics 2000 Manuale Utente

Rabbit 3000 Microprocessor

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

with the

address pointed to by

65535-B

, but simply indicates the difference between 65535 and

the 16-bit unsigned integer

;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.