FIX: MASM 6.0 Does Not Align on Page Boundary
ID: Q82287
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The information in this article applies to:
-
Microsoft Macro Assembler for MS-DOS, versions 6.0, 6.0a, 6.0b
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Microsoft Macro Assembler for OS/2, versions 6.0, 6.0a, 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:
- 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
. .
. .
- 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.
- 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**
.
.
- 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
ALIGN 256
xor al, al
mov ax,4C00h
int 21h
_TEST ENDS
END start
Additional query words:
6.00 6.00a 6.00b buglist6.00 buglist6.00a buglist6.00b fixlist6.10
Keywords :
Version : :6.0,6.0a,6.0b
Platform :
Issue type :
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