Handshake Solutions
HT80C51 User Manual
80C51 Family Instruction Set – Instruction definitions
Page 126 of 132
© Philips Electronics N.V. 2005
SUBB A, <src-byte>
Function: Subtract with borrow
Description: SUBB subtracts the indicated variable and the carry flag together from the Accumulator, leaving
the result in the Accumulator. SUBB sets the carry (borrow) flag if a borrow is needed for bit 7,
and clears C otherwise. (If C was set before executing a SUBB instruction, this indicates that a
borrow was needed for the previous step in a multiple precision subtraction, so the carry is sub-
tracted from the Accumulator along with the source operand.) AC is set if a borrow is needed for
bit 3, and cleared otherwise. OV is set if a borrow is needed into bit 6, but not into bit 7, or into bit
7, but not bit 6. When subtracting signed integers OV indicates a negative number produced when
a negative value is subtracted from a positive value, or a positive result when a positive number is
subtracted from a negative number.
The source operand allows four addressing modes: register, direct, register-indirect, or immediate.
Example: The Accumulator holds 0C9H (11001001B), register 2 holds 54H (01010100B), and the carry flag
is set. The instruction,
SUBB A,R2
will leave the value 74H (01110100B) in the Accumulator, with the carry flag and AC cleared but
OV set. Notice that 0C9H minus 54H is 75H. The difference between this and the above result is
due to the carry (borrow) flag being set before the operation. If the state of the carry is not known
before starting a single or multiple-precision subtraction, it should be explicitly cleared by a CLR C
instruction.
SUBB A,Rn
Bytes: 1
Cycles: 1
Encoding:
Operation: SUBB
(A)
← (A) – (C) – (Rn)
SUBB A,direct
Bytes: 2
Cycles: 1
Encoding:
Operation: SUBB
(A)
← (A) – (C) – (direct)
SUBB A,@Ri
Bytes: 1
Cycles: 1
Encoding:
Operation: SUBB
(A)
← (A) – (C) – (Ri)
SUBB A,#data
Bytes: 2
Cycles: 1
Encoding:
Operation: SUBB
(A)
← (A) – (C) – (#data)
1
0
0
1
1
r
r
r
1
0
0
1
0
1
0
1
direct address
1
0
0
1
0
1
1
i
1
0
0
1
0
1
0
0
immediate data
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