Macro Definitions

The general format of a macro definition appears as follows :

  Name  MACRO  parameters  ...     ENDM 

Having defined the block once, it is possible to use it multiple times in the program. Depending on the values of the parameters, the section being replaced can take different values. If the specified section is presumed to be reused multiple times for example, within a loop then the macro definition provides indisputable advantages in comparison to a procedure because it speeds up the code execution, for example:

 EXC MACRO par1, par2         PUSH  par1         POP   par2         ENDM 

This macro definition will exchange contents between parameters. For example, EXC EAX , EBX is equivalent to PUSH EAX\POP EAX , EXC MEM1 , ESI is equivalent to PUSH MEM1\POP ESI , and so on. Note that if the first parameter is a number, this will load this number into the second operand.

The problem of labels is important with respect to macro definitions. If you use normal labels in macro definitions, then a collision would occur if you use that macro definition more than once. This collision can be avoided by declaring local labels.

To achieve this, the LOCAL keyword is used, for example:

 EXC MACRO par1, par2         LOCAL EXI         CMP   par1, par2         JE    EXI         PUSH  par1         POP   par2     EXI:     ENDM 

This macro definition can be reused as many times as needed, because the assembler will generate a unique label in the course of each substitution.

To exit a macro definition (to stop its generation), the EXITM directive is used. This directive will be, useful if you are using conditional constructs, such as IF ENDIF , in your macro definition.

Now, consider another example of a useful macro.

 ustring MACRO quoted_text, ptr_buf        LOCAL asc_txt           . data           asc_txt db quoted_text, 0           .code           invoke MultiByteToWideChar, CP_ACP,  0,           OFFSET asc_txt,  -1, OFFSET ptr_buf,  LENGTHOF ptr_buf           ENDM 

This macro converts the specified ASCII string into the Unicode string and loads it into the buffer pointed to by ptR_buf . In a more general form, this macro will appear as follows:

 ustring MACRO quoted_text, ptr_buf     LOCAL asc_txt     .data        asc_txt db quoted_text,  0     .code        PUSH   LENGTHOF ptr_buf        PUSH   OFFSET BUF ;  Buffer address        PUSH   -1        PUSH   OFFSET asc_txt        PUSH   0        PUSH   0        CALL  MultiByteToWideChar@24     ENDM 

For example, consider the following code fragment:

 ustring "Hello!", buf ; Buf --- In data segment     PUSH 0     PUSH OFFSET buf     PUSH OFFSET buf     PUSH 0     CALL MessageBoxW@16 ; Output of the Unicode string 

It is convenient ; do you agree?