Assembly Language

Assembly Language Data Transfer Instructions

The MOV instruction • The MOV instruction copies data from one operand to another using either 8-bit or 16-bit operand. • Syntax: • MOV destination, source • Data is merely copied from the source to the destination, so the source operand is not changed. • The source operand may be immediate data, a register, or a memory operand. • The destination operand may be a register or a memory operand. Data Transfer Instructions

The MOV instruction • The following types of data transfers are possible: Immediate data to register or memory register to register register to memory memory to register • Register operands • A move involving only registers is the fastest type. • Registers should be used when an instruction must execute quickly. • Any registers may be used as a source operand, and any register except CS and IP may be destination operands, • Mov ax,bx • Mov dl, al • Mov bx,cs Data Transfer Instructions

The MOV instruction • Immediate operands • An immediate value (integer constant) may be moved to any register (except a segment register or IP) & may be moved to any memory operand. • A common error is to make the immediate value larger than the destination operand. • Mov bl, 01 ; move 8-bit value to BL • Mov bx, 01 ; mov 16-bit value to BX • Mov bx, 1000h ; mov 16-bit value to BX • Mov total, 1000 ; mov16-bit value to a variable Data Transfer Instructions

The MOV instruction • Direct addressing • A variable may be one (but not both) of the operands in a MOV instruction • The content of the variable are either the source or destination of the move. • Count is an 8-bit variable which contains the number 1. • The following statement copies the contents of count into AL: mov al, count Count: (AL register) 01 01 Data Transfer Instructions

The MOV instruction • Mov al, count ; 8-bit memory to register • Mov total, ax ; 16-bit register to memory • Mov total, 1000h ; 16-bit immediate to memory Data Transfer Instructions

The MOV instruction • There are a few limitations on the type of operands (which are based on both the physical design of the processor and its instruction set): • CS and IP may never be destination operands • Immediate data and segment registers may not be moved to segment registers • The source and destination operands must be the same size. • If the source is immediate data, it must not exceed 255(FFh) for an 8-bit destination or 65,535 (FFFFh) for a 16-bit destination. • Memory to memory moves is not allowed Data Transfer Instructions

The MOV instruction • Samples of illegal moves: • mov word_1, word_2 ; memory to memory • mov ds, 1000h ; immediate to segment • mov ip, ax ; IP as destination • mov al, bx ; mismatching register types • mov 1000h, ax ; immediate data as destination Data Transfer Instructions

The XCHG instruction • The XCHG instruction exchanges the contents of two registers or of a register and a variable. • Syntax: • XCHG operand 1, operand 2 • The two operands may be registers or memory operands, as long as one operand is a register. Data Transfer Instructions

The XCHG instruction • The XCHG instruction is the most efficient way to exchange two 8-bit or 16-bit operands because you don’t need a third register or variable to hold a temporary value. • This instruction provides a speed advantage (especially in sorting applications). • One or both operands may be registers or a register may be combined with a memory operand. • Two memory operands may not be used together. Data Transfer Instructions

The XCHG instruction • Xchg ax, bx ; exchange BX with AX • Xchg ah, al ; exchange upper, lower register halves. • Xchg var1, bx ; exchange memory operand with BX Data Transfer Instructions

Assembly Language