STC89C52RC datasheet(74/165 Pages) STC

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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 a

CLR C instruction.

SUBB A, Rn

Bytes:

Cycles:

Encoding:

1 0 0 1

1 r r r

Operation:

SUBB

(A) ← (A) - (C) - (Rn)

← (A) - (C) - (Rn)

(A) - (C) - (Rn)

SUBB A, direct

Bytes:

Cycles:

Encoding:

1 0 0

0 1 0 1

direct address

Operation:

SUBB

(A) ← (A) - (C) - (direct)

← (A) - (C) - (direct)

(A) - (C) - (direct)

SUBB A, @Ri

Bytes:

Cycles:

Encoding:

1 0 0

0 1 1 i

Operation:

SUBB

(A) ← (A) - (C) - ((Ri))

← (A) - (C) - ((Ri))

(A) - (C) - ((Ri))

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