Cpd compare and decrement (byte) – Zilog Z80380 User Manual
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Z380
™
U
SER
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S
M
ANUAL
DC-8297-03
Z
ILOG
CPD
COMPARE AND DECREMENT (BYTE)
CPD
Operation:
A - (HL)
if (XM) then begin
HL(31-0)
←
HL(31-0) - 1
end
else begin
HL(15-0)
←
HL(15-0) - 1
end
BC(15-0)
←
BC(15-0) - 1
This instruction is used for searching strings of byte data. The byte of data at the location
addressed by the HL register is compared with the contents of the accumulator and the Sign
and Zero flags are set to reflect the result of the comparison. The contents of the accumulator
and the memory bytes are unaffected. Two’s complement subtraction is performed. Next
the HL register is decremented by one, thus moving the pointer to the previous element in
the string. The BC register, used as a counter, is then decremented by one.
Flags:
S:
Set if the result is negative; cleared otherwise
Z:
Set if the result is zero, indicating that the contents of the accumulator and the memory
byte are equal; cleared otherwise
H:
Set if there is a borrow from bit 4 of the result; cleared otherwise
V:
Set if the result of decrementing BC is not equal to zero; cleared otherwise
N:
Set
C:
Unaffected
Addressing
Execute
Mode
Syntax
Instruction Format
Time
Note
CPD
11101101 10101001
3+r
X