ZiLOG Z80 Handbook page 224

LD
A,(OPERA) OPERAND A
LD
B,(OPERB)
OPERAND B
CALL CMPARE A:B
JR
LTHAN
RETURN HERE IF A < B
JR
EQUAL
RETURN HERE IF A = B
JR GTHAN
RETURN HERE IF A > B
As in some high-level languages, the missing equalities of A<=B
and A>=B may be constructed by proper use of the return points,
such as this sequence that returns to location LTEQL if A<=B and
to GTTHAN if A>B.
LD
A,(OPERA) OPERAND A
LD
B,(OPERB) OPERAND B
CALL CMPARE A:B
JR LTEQL
A < B RTN
JR LTEQL
A = B RTN
JR GTHAN
A > B RTN
The actual code within the subroutine would be an expansion of
that in Chapter 11.
CMPARE
CP
B.
A:B
EX (SP),HL GET RETURN
JR Z,EQUAL
GO IF A = B
PUSH AF
SAVE A, FLAGS
XOR B
JP
P,SAME
GO IF SIGNS EQUAL
POP AF
RESTORE A, FLAGS
TEST JP
C,LESST
A < B
JP GREAT
A > B
SAME POP
AF
CCF
JP
TEST
GREAT INC
HL
BUMP TO CALL + 4
INC HL
EQUAL INC HL
BUMP TO CALL + 2
INC HL
LESST EX (SP),HL
RESTORE RTN
RET
RETURN
The comparison is made after first retrieving the return address
from the stack (stack pointer remains unaffected). The return ad-
dress is then incremented to a return point to +2 or +4 bytes past
the return location stored by the call to reflect the return on <,
or >.
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