FIX: Bad Code Produced When Using Location Arithmetic

• Microsoft Macro Assembler for MS-DOS and OS/2, version 6.0

SYMPTOMS

Microsoft Macro Assembler (MASM) version 6.0 may generate incorrect code when more than one label is used to calculate a relative offset.

RESOLUTION

Use another method to calculate offsets. For example, the sample program in this article generates the correct code by using structures to calculate the offsets.

STATUS

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

MORE INFORMATION

The sample program below illustrates this problem. The code actually generated by the assembler is included as comments within the source. As you can see from generated code, the address to place the value is calculated correctly in all cases. However, if location arithmetic is used, the assembler changes the first byte from hex 30 to hex 00. The result is independent of the format used for the constant ('0' behaves the same as 30h).

Sample Code

; Assemble options needed: none
;
; This program should display "Six Zeros -> 000000" on the screen.
;        Instead, it displays "Six Zeros ->  0  0 ".
;
; To work around this problem, uncomment the code involving
; structures and remove the code using location arithmetic.

.MODEL small
.STACK 4096
.DATA

VOLCANOS STRUCT
RAINER   BYTE    "Six"
BAKER    BYTE    "Zeros ->"
ADAMS    BYTE    6 DUP(01H)
STHELENS BYTE    '$'
VOLCANOS ENDS

MOUNTAINS     LABEL BYTE
MACKENZIE     BYTE  "Six "
FUJI          BYTE  "Zeros -> "
GODWIN_AUSTEN BYTE  6 DUP(01H)
KILIMANJARO   BYTE  '$'

.CODE
main:
         mov    ax, @DATA
         mov    ds, ax
         mov    di, OFFSET MOUNTAINS

;Offsets calculated using structures

;         mov    BYTE PTR VOLCANOS.ADAMS+2 [DI], '0'
;         mov    WORD PTR VOLCANOS.ADAMS   [DI], '00'
;
;3750:0008 C6450F30       MOV       Byte Ptr [DI+0F],30
;3750:000C C7450D3030     MOV       Word Ptr [DI+0D],3030
;
;         mov    BYTE PTR VOLCANOS.ADAMS+5 [DI], 30h
;         mov    WORD PTR VOLCANOS.ADAMS+3 [DI], 3030h
;
;3750:0011 C6451230       MOV       Byte Ptr [DI+12],30
;3750:0015 C745103030     MOV       Word Ptr [DI+10],3030

; Offsets calculated using location arithmetic

         mov    BYTE PTR [GODWIN_AUSTEN+2-MOUNTAINS+DI], '0'
         mov    WORD PTR [GODWIN_AUSTEN-MOUNTAINS+DI], '00'

;3750:001A C6450F00       MOV       Byte Ptr [DI+0F],00
;3750:001E C7450D0030     MOV       Word Ptr [DI+0D],3000

         mov    BYTE PTR [GODWIN_AUSTEN+5-MOUNTAINS+DI], 30H
         mov    WORD PTR [GODWIN_AUSTEN+3-MOUNTAINS+DI], 3030H

;3750:0023 C6451200       MOV       Byte Ptr [DI+12],00
;3750:0027 C745100030     MOV       Word Ptr [DI+10],3000

; Output
         mov    dx, OFFSET MOUNTAINS  ; Load DX with offset of
                             ;  MOUNTAINS (segment already in DS)
         mov    ax, 09h
         int    21h          ; Display String to stdout

         mov    ax, 4C00h    ; Exit functions with 0 in AL
         int    21h          ; Exit Program with Return Code

         END    main 

Additional query words: 6.00 EQU buglist6.00 fixlist6.00a

Keywords :
Version : MS-DOS:6.0
Platform : MS-DOS
Issue type :