Intel 80286, 80287 manual Addition Instructions, Subtraction Instructions

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Bit Position

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BASIC INSTRUCTION SET

Table 3·2. Control Flags'Functions

Name	Function
TF	Trap (Single Step) Flag-Once set, a single step interrupt
	occurs after the next instruction executes. TF is cleared by
	the single step interrupt.
IF	Interrupt-enable Flag-When set, maskable interrupts will
	cause the CPU to transfer control to an interrupt vector-
	specified location.
OF	Direction Flag-Causes string instructions to auto deere·
	ment the appropriate index registers when set. Clearing OF
	causes auto increment.

An arithmetic operation may consist of two register operands, a general register source operand with a memory destination operand, a memory source operand with a register destination operand, or an immediate field with either a register or memory destination operand, but not two memory operands. Arithmetic instructions can operate on either byte or word operands.

3.3.1 Addition Instructions

ADD (Add Integers) replaces the destination operand with the sum of the source and destination operands. ADD affects OF, SF, AF, PF, CF, and ZF.

Example: ADD BL, BYTEOPRND. Adds the contents of the memory byte labeled BYTEOPRND to the contents of BL, and replaces BL with the resulting sum.

ADC (Add Integers with Carry) sums the operands, adds one if CF is set, and replaces the destination operand with the result. ADC can be used to add numbers longer than 16 bits. ADt affects OF, SF, AF, PF, CF, and ZF.

Example: ADC BX, Cx. Replaces the contents of the destination operand BX with the sum of BX,

.CS, and 1 (if CF is set). If CF is cleared, ADC performs the same operation as the ADD instruction.

INC (Increment) adds one to the destination operand. The processor treats the operand as an unsigned binary number. INC updates AF, OF, PF, SF, and ZF, but it does not affect CF. Use ADD with an immediate value of 1 if an increment that updates carry (CF) is needed.

Example: INC BL. Adds 1 to the contents of BL.

3.3.2 Subtraction Instructions

SUB (Subtract Integers) subtracts the source operand from the destination operand and replaces the destination operand with the result. If a borrow is required, carry flag is set. The operands may be signed or unsigned bytes or words. SUB affects OF, SF, ZF, AF, PF, and CF.

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