FIX: MASM Allows Alignments > Segment Alignment

MS-DOS     | OS/2

The information in this article applies to:

• Microsoft Macro Assembler for MS-DOS, versions 5.1 and 5.1a
• Microsoft Macro Assembler for OS/2, versions 5.1 and 5.1a

SYMPTOMS

The Microsoft Macro Assembler (MASM) versions 5.1 and 5.1a incorrectly allow ALIGN directives that are greater than the alignment of the segment they appear within. MASM 5.1 does not generate errors for these statements; however, it cannot guarantee that the item following these ALIGN directives will be aligned on the given boundary.

RESOLUTION

Microsoft has confirmed this to be a problem in MASM versions 5.1 and 5.1a. This problem was corrected in MASM version 6.0.

MORE INFORMATION

The maximum ALIGN value for data or code is the alignment used when defining the segment. When a segment is declared, it is guaranteed to start only at its specified alignment. Since the assembler can align items relative only to the start of a segment, any ALIGN statement greater than the segment's alignment should generate an error. If the following sample code is assembled with MASM 5.1 or MASM 5.1a, no error is generated. Padding is inserted after the ALIGN 4 directive, but it will not align the next statement on the indicated boundary. MASM 6.0 correctly generates the following error:

   A2189: invalid combination with segment alignment : 4

Sample Code

; Assemble options needed: none

STACK SEGMENT PARA STACK 'STACK'

       DB      100h DUP('x')

_DATA SEGMENT WORD PUBLIC 'DATA'

msg    DB      "Hello World!", 13, 10, '$'

_TEXT1 SEGMENT WORD PUBLIC 'CODE'

       ASSUME CS:_TEXT1

_TEXT2 SEGMENT WORD PUBLIC 'CODE'

       ASSUME CS:_TEXT2, DS:_DATA
start: mov     ax, SEG msg
       mov     ds, ax

       ALIGN   4

       mov     dx, OFFSET msg
       mov     ah, 09h
       int     21h

       mov     ax, 4C00h
       int     21h

END start