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Defining Constants

Data Exchange and Translate Instructions

The XCHG Instruction

The xchg instruction exchanges 8-, 16, or 32-bit source and destination operands. The syntax is similar to that of the mov instruction.

xchg EAX,EDX

xchg [response],CL

xchg [total],DX

As in the mov instruction, both operands cannot be located in memory. Note that this restriction is applicable to most instructions. Thus,

xchg [response] , [namel ] ; illegal

is invalid. The xchg instruction is convenient because we do not need a third register to hold a temporary value in order to swap two values.

For example, we need three mov instructions

mov ECX,EAX

mov EAX,EDX

mov EDX,ECX

Another instruction , bswap, can be used to perform such conversions on 32-bit data.

The format is

bswap register

This instruction works only on the data located in a 32-bit register.

The XLAT Instruction

The xlat (translate ) instruction can be used to perform character translation. The format of this instruction is shown below: xlatb

To use this instruction , the EBX register must to be loaded with the starting address of the translation table and AL must contain an index value into the table.

The xlat instruction adds contents of AL to EBX and reads the byte at the resulting address.

This byte replaces the index value in the AL register

Shift and Rotate Instructions

Shift Instructions

The instruction set provides several shift instructions . We discuss the following two instructions here: shl (SHif t Left ) and shr (SHif t Right) .

The shl instruction can be used to left shif a destination operand. Each shift to the left by one bit position causes the leftmost bit to move to the carry flag (CF).

The shr instruction works similarly but shifts bits to the right as shown below:

The general formats of these instructions are:

shl destination,count

shl destination,CL

shr destination,count

shr destination,CL

The destination can be an 8-, 16 - or 32-bit operand stored either in a register or in memory.

Rotate Instructions

A drawback with the shift instructions is that the bits shifted out are lost. There may be situations where we want to keep these bits.

The rotate family of instructions provides this facility. These instructions can be divided into two types: rotate without involving the carry flag, or through the carry flag.

Rotate Without Carry

There are two instructions in this group: rol (Rotat e Left) ror (Rotate Right)

The format of instructions

rol destination,count

rol destination,CL

ror destination,count

ror destination,CL

MACROS

Macros provide a means by which a block of text (code, data, etc.) can be represented by a name (called the macro name)

Whe n the assembler encounters that name later in the program , the block of text associated with the macro name is substituted.

In NASM, macros can be defined with %macro and %endmacro directives . The macro text begins with the %macro directive and ends with the %endmacro directive. The macro definition syntax is

%macro macro_name para_count <macro body>

%endmacro

The para_count specifie s the number parameters used in the macro. The macro_name i s the name of the macro that, when used later in the program, causes macro expansion. To invoke or call a macro, use the macro_name and supply the necessary parameter values.

Macros with Parameters

The previous macro always multiplies EAX by 16 . B y using parameters, we can generalize this macro to operate on a byte, word, or doubleword located either in a general-purpose register or memory. The modified macro is

%macro mult_by_16 1

sal %1, 4

%endmacro

The EQU Directive

The %assign Directive

The %define Directive

The syntax of the EQU directive is: name EQU expression

This directive can be used to define numeric constants like the EQU directive. However, %as s ign allows redefinition. For example, we define

This directive is similar to the #define in C. It can be used to define numeric as well as string constants. For example