The 8051 Instruction Set
1-59 Atmel 8051 Microcontrollers Hardware Manual
4316B–8051–02/04
1.14.47 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 subtracted 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 CLR C instruction.
SUBB A,R
n
Bytes: 1
Cycles: 1
Encoding: 10011r r r
Operation: SUBB
(A) ← (A) - (C) - (R
n
)
SUBB A,direct
Bytes: 2
Cycles: 1
Encoding: 10010101 direct address
Operation: SUBB
(A) ← (A) - (C) - (direct)
SUBB A,@R
i
Bytes: 1
Cycles: 1
Encoding: 1001011 i
Operation: SUBB
(A) ← (A) - (C) - ((R
i
))
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