32-Bit Flat Memory Model MASM Code for Windows NT

• Microsoft Macro Assembler for MS-DOS, versions 6.0a, 6.0b, 6.1, 6.1a, and 6.11

SUMMARY

The Microsoft Macro Assembler (MASM) versions 6.0a and later support generating 32-bit flat memory model code through the .MODEL flat directive. When combined with the CVTOMF and LINK32 utilities and the KERNEL32.LIB library distributed with the Windows NT software development kit (SDK) or the LINK utility and the KERNEL32.LIB library distributed with Microsoft Visual C++ for Windows NT, MASM can generate a 32-bit flat memory model application for the Windows NT environment.

Because MASM version 6.0 does not properly support the _stdcall convention, it cannot be used to generate applications for the Windows NT environment.

MORE INFORMATION

Given some MASM code in a file named FLAT32.ASM where its entry point is _start, to assemble this code into a 32-bit flat memory model application for the Windows NT environment, perform the following four steps:

1. Place the .386 or .486 directive in the code prior to the .MODEL flat, stdcall directive to ensure that the code and data segments are correctly initialized for a 32-bit flat memory model application.

2. Assemble the source code using the following command:

         ML /c flat32.asm.
   

   The /c assembler option prevents the MASM from invoking the 16-bit linker provided with MASM.

3. For MASM 6.0a or 6.0b, use the CVTOMF utility provided with the Windows NT SDK to convert the generated object files from OMF format to COFF format. The command is as follows:

         cvtomf flat32.obj
   

   This step is unnecessary with MASM version 6.1 and later if you use the /coff switch because it will produce a COFF format object file.

4. Use the LINK32 utility provided with the Windows NT SDK or LINK utility provided with Visual C++ for Windows NT to link the object modules. Specify a path to KERNEL32.LIB. The command line is as follows:

         link32 -subsystem:console -entry:_start -out:flat32.exe
            flat32.obj kernel32.lib
   
      To create an executable module that contains symbolic debugging
      information, add the -Zi switch to the assembler options (Step 2)
      and add the following switches to the linker command line:
      -debug:full -debugtype:cv.
   
   

Windows NT does not support the MLX driver provided with MASM versions 6.0a and 6.0b. Also, Windows NT does not support the MS-DOS-extender used by MASM versions 6.1 and 6.1a. Therefore, it may be necessary to perform the assembly step on a machine running MS-DOS while the other steps run on a machine running Windows NT. Another option is to upgrade to MASM version 6.11.

The code below is a "Hello, world" application developed in assembler to use the 32-bit flat memory model of Windows NT.

Sample Code

; Assemble options needed: /c

.386 .MODEL flat, stdcall

STD_OUTPUT_HANDLE EQU -11

GetStdHandle PROTO NEAR32 stdcall,

    nStdHandle:DWORD

    hFile:DWORD, lpBuffer:NEAR32, nNumberOfBytesToWrite:DWORD,
    lpNumberOfBytesWritten:NEAR32, lpOverlapped:NEAR32

    dwExitCode:DWORD

.DATA

msg DB "Hello, world.", 13, 10 written DW 0 hStdOut DD 0

.CODE _start:

    INVOKE  GetStdHandle,
        STD_OUTPUT_HANDLE      ; Standard output handle
    mov hStdOut, eax

    INVOKE  WriteFile,
        hStdOut,               ; File handle for screen
        NEAR32 PTR msg,        ; Address of string
        LENGTHOF msg,          ; Length of string
        NEAR32 PTR written,    ; Bytes written
        0                      ; Overlapped mode

    INVOKE  ExitProcess,
        0                      ; Result code for parent process