DP80390 datasheet(77/84 Pages) DCD

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DP80390 Instructions set details

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3.40.

SUBB

Instruction:

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,

3.40.1.

SUBB A, RN

Operation:

(PC)

← (PC) + 1

(A)

← (A) - (C) - (Rn)

Bytes:

Cycles:

Encoding:

3.40.2.

SUBB A, DIRECT

Operation:

(PC)

← (PC) + 2

(A)

← (A) - (C) - (direct)

Bytes:

Cycles:

Encoding:

direct address

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