Intel 80287, 80286 manual Address Manipulation Instructions

BASIC INSTRUCTION SET

seAS (Scan String) subtracts the destination string element at ES:DI from AX or AL and updates the flags AF, SF, ZF, PF, CF and OF. If the values are equal, ZF= 1; otherwise, ZF=O. If DF=O, the processor increments the memory pointer (DI) for the string. The segment register used for the source address can be changed with a segment override prefix while the destination segment register

cannot be overridden. .

Example: SCASW. Compares the value in AX with the destination string element.

REPE/REPZ (Repeat While ex Equal/Zero) and REPNE/REPNZ (Repeat While ex Not Equal/ Not Zero) are the prefixes that are used exclusively with the SCAS (ScanString) and CMPS (Compare String) primitives.

The difference between these two types of prefix bytes is that REPE/REPZ terminates when ZF=O and REPNE/REPNZ terminates when ZF= 1. ZF does not require initialization before execution of a repeated string instruction.

When these prefixes modify either the SCAS or CMPS primitives, the processor compares the value of the current string element with the value in AX for word elements or with the value in AL for byte elements. The resulting state of ZF can then limit the operation of the repeated operation as well as a zero value in CX.

Example: REPE SCASB. Causes the processor to scan the string pointed to by ES:DI until it encoun- ters a match with the byte value in AL or until CX decrements to zero.

LODS (Load String) places the source string element at DS:SI into AX for word strings or into AL for byte strings.

Example: LODSW. Loads AX with the value pointed to by DS:SI.

3.8 ADDRESS MANIPULATION INSTRUCTIONS

The set of address manipulation instructions provide a way to perform address calculations or to move

to a nc;v data segment or extra segment.

LEA (Load Effective Address) transfers the offset of the source operand (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 register (AX, DX, BX, CX, BP, SP, SI, or DI).

LEA does not affect any flags. This instruction is useful for initializing the registers before the execu- tion of the string primitives or the XLAT instruction.

Example: LEA BX EBCDIC_TABLE. Causes the processor to place the address of the starting location of the table labeled EBCDIC_TABLE intoBX.

LDS (Load Pointer Using DS) transfers a 32-bit pointer variable from the source operand to DS and the destination register. The source operand must be a memory operand, and the destination operand must be a 16-bit general register (AX, DX, BX, CX, BP, SP, SI or DI). DS receives the high-order segment word of the pointer. The destination register receives the low-order word, which points to a specific location within the segment.

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