FIX: MASM 6.0 Does Not Align on Page Boundary

MS-DOS           | OS/2

The information in this article applies to:

• Microsoft Macro Assembler for MS-DOS, versions 6.0, 6.0a, and 6.0b
• Microsoft Macro Assembler for OS/2, versions 6.0, 6.0a, and 6.0b

SYMPTOMS

When using the Microsoft Macro Assembler (MASM), the ALIGN directive does not work properly when attempting to place data or code on a page (256 byte) boundary. If aligning inside of a code segment, the "ALIGN 256" directive has no effect on the placement of subsequent instructions; it may insert 256 bytes somewhere else later in that segment. When aligning inside of a data segment, the data to be aligned is placed 1 byte past the page boundary.

RESOLUTION

To work around this problem, follow the steps below:

   1. Edit your source to use an "ALIGN 128" where you previously
      used "ALIGN 256". For example,
      .                    .
      .                    .
      XOR   AL,AL          XOR   AL,AL
      ALIGN 256     --->   ALIGN 128
      MOV   CX,5           MOV   CX,5
      .                    .
      .                    .

   2. Assemble your source with the options "/Fl /c". This will
      generate a listing file with the same base name of your source
      file and a .LST extension.

   3. Viewing the .LST file, locate the line of code you want to
      align. The value to the left of your source code represents the
      offset of that item in the relative segment. This value will be
      a multiple of 128 (80h). There is a possibility that this value
      will fall on a page frame (100h). If this is the case then the
      code is aligned on a 256-byte boundary and you are done.

      Offset Machine Code  Source Code
      ------ ------------  -----------
      .
      .
      0000  32 C0          XOR   AL,AL
                           ALIGN 128
      0080  B9 0005        MOV   CX,5    **Not Aligned on page**
      .
      .
      .
      .
      0000  32 C0          XOR   AL,AL
                           ALIGN 128
      0100  B9 0005        MOV   CX,5    **Aligned on page**
      .
      .

   4. If the offset value is not a multiple of 256, go back to your
      source file and add the line "DB 128 DUP(?)" beneath the
      "ALIGN 128" line. This will add the bytes necessary to align
      your code to a page frame. Reassemble with "/Fl /c" and
      view the .LST file to verify.
      .                    .
      .                    .
      XOR   AL,AL          XOR   AL,AL
      ALIGN 128     --->   ALIGN 128
                           DB    128 DUP(?)
      MOV   CX,5           MOV   CX,5
      .                    .
      .                    .

STATUS

Microsoft has confirmed this to be a problem in MASM versions 6.0, 6.0a, and 6.0b. This problem was corrected in MASM for MS-DOS version 6.1.

MORE INFORMATION

The following sample code can be used to demonstrate the problem.

Sample Code

; Assemble options needed: none

_TEST SEGMENT PAGE PUBLIC 'CODE' start:

    xor al, al

    xor al, al
    mov ax,4C00h
    int 21h