Intel 210200-002 manual Accumulator-Specific Transfers

ARCHITECTURE AND INSTRUCTIONS

Accumulator-Specific Transfers

Three accumulator-specific transfer opera- tions are provided:

IN transfers a byte (or word) from an input port to the AL register (or AX register for a word). The port is specified either with an inline data byte, allowing fixed access to ports 0 through 255, or with a port number in the DX register, allowing variable access to 64K input ports.

OUT is similar to IN except that the transfer is from the accumulator to the output port.

XLAT performs a table lookup byte transla- tion. The AL register is used as an index into a 256-byte table whose base is addressed by the BX register. The byte operand so selected is transferred to AL.

Address-Object Transfers

Three address-object transfer operations are provided:

LEA (load effective address) transfers the off- set address of (rather than its value) to the destination operand. The source operand must be a memory operand and the destination operand must be a 16-bit general, pointer, or index register.

LDS (load pointer into DS) transfers a "pointer-object" (i.e., a 32-bit object contain- ing an offset address and a segment address) from the source operand (which must be a memory operand) to a pair of destination registers. The segment address is transferred to the DS segment register. The offset address must be transferred to a l6-bit gen- eral, pointer, or index register.

LES (load pointer into ES) is similar to LDS except that the segment address is transferred to the ES segment register.

Flag Register Transfers

Four flag register transfer operations are provided:

LAHF (load AH with flags) transfer the flag registers SF, ZF, AF, PF, and CF (the 8080

flags) into specific bits of the AH register.

SAHF (store AH into flags) transfers specific bits of the AH register to the flag register, SF, ZF, AF, PF, and CF.

PUSHF (push flags) decrements the SP reg- ister by two and transfers all of the flag registers into specific bits of the stack element addressed by SP.

POPF (pop flags) transfers specific bits of the stack element addressed by the SP register to the flag registers and then increments SP by two.

Arithmetic Instructions

The 8088 provides the four basic mathemati- cal operations in a variety of instructions. Both 8- and l6-bit operations and both signed and unsigned arithmetic are provided. Standard twos complement representation of signed values is used. The addition and sub- traction operations serve as both signed and unsigned operations to be made (see Condi- tional Transfer). Correction operations allow arithmetic to be performed directly on packed or unpacked decimal numbers.

Flag Register Settings

Six flag registers are set or cleared by arith- metic operations to reflect results of the operation. They generally follow these rules:

CF is set if the operation results in a carry out 'of (from addition) or a borrow into (from subtraction) the high-order bit of the result;

otherwise CF is cleared. .

AF is set if the operation results in a carry out of (from addition) or a borrow into (from subtraction) the low-order four bits of the result; otherwise AF is cleared.

ZF is set if the result of the operation is zero; otherwise ZF is cleared.

SF is set if the high-order bit of the result of the operation is set; otherwise SF is cleared.

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