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Assembler Directives

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• A free Intel Assembler
  • For Windows:

    MAsm32: click here

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  • How to assemble an Intel assembler program:

    \masm32\bin\ml /c /Zd /coff my-prog.asm \masm32\bin\link /SUBSYSTEM:CONSOLE my-prog.obj

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• MASM Assembler Program structure
  • Program structure:

    .486 ;; Use 32 bit addresses .model flat, stdcall ;; Use the Protected mode option casemap :none ;; Make program case sensitive ;; Include commonly used header files include \masm32\include\kernel32.inc include \masm32\include\windows.inc include \masm32\include\masm32.inc include \masm32\include\msvcrt.inc ;; Include commonly used libraries includelib \masm32\lib\kernel32.lib includelib \masm32\lib\msvcrt.lib .data ;; Start data segment [Variable definitions] .... .code ;; Start code segment Start: ;; Segment name [Program] .... invoke ExitProcess, 0 ;; ***** Exit to Dos... with exit code 0 end Start ;; End program segment

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• Assembler directives
  • Commonly used assembler directives:

    equ = define symbolic constants DB = define byte size variables DW = define word size (16 bits) variables DD = define double word size (32 bits) variables DQ = define quad word size (64 bits) variables

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• Defining constants
  • equ directive:
    Used to define symbolic constants

  • Example

    MAX equ 10000000

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• Reserving space in memory for variables
  • .data directive:

    Used to start a data segment (a memory area that can be read and written)

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  • DB directive: (reserves 1 byte memory)
    define byte

    Example: un-initialized

    Java: Intel: char c; c: DB ?

    Example: initialized

    Java: Intel: char c = 45; c: DB 45

    Example: un-initialized char array

    Java: Intel: char c[100]; c: DB 100 dup (?)

    Example: initialized char array

    Java: Intel: char c[100] = {0,0, ..., 0}; c: DB 100 dup (0)

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  • DW, WORD directive: (reserve 2 bytes memory)
    define word (16 bytes)

    Example:

    C: short ListOfShortNumbers[] = {1000, 2000, 3000, 4000, 5000}; ListOfShortNumbers: DW 1000, 2000, 3000, 4000, 5000

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  • DD, DWORD directive: (reserve 4 bytes memory)
    define double word (32 bytes)

    Example:

    C: int ListOfInttNumbers[] = {100000, 200000, 300000, 400000, 500000}; ListOfIntNumbers: DD 1000000, 2000000, 3000000, 4000000, 5000000

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  • DQ, QWORD directive: (reserve 8 bytes memory)
    define quad word (32 bytes)

    Example:

    ListOfLongNumbers: DQ 1000000, 2000000, 3000000, 4000000, 5000000

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• More Information
  • MASM Programmer's Guide - Chapter 4 Defining and Using Simple Data Types: click here