FIX: A2006: Phase Error May Be Caused By Incorrect NOP

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

Using the "SEG" directive in conjunction with forward reference to a procedure or a segment name will produce the error

   A2006 : phase error between passes

CAUSE

The assembler incorrectly places a "NOP" instruction in the code generated during Pass 1. This NOP is not generated in Pass 2, so the assembler generates a phase error. This phase error will most often happen on the next instruction that is associated with a fixup.

STATUS

Microsoft has confirmed this to be a problem in MASM versions 5.1 and 5.10a. This problem was fixed in MASM version 6.0.

RESOLUTION

To work around this problem, introduce a NOP that is seen by the assembler only during the second pass.

MORE INFORMATION

The following code produces the errors. The errors do not occur when the ax register is used. They occur when the bx, cx, and dx registers are used. The workaround is shown in commented out lines.

Sample Code:

;Assemble options needed: none

;filler          MACRO                     ;macro that codes a nop
;                if2                       ;in the second pass of the
;                nop                       ;assembler
;                endif

_text           SEGMENT PARA PUBLIC 'CODE'
                ASSUME CS:_TEXT

                MOV     cx, SEG proc1     ;error using cx, bx or dx
;                filler
                MOV     ax, SEG proc1     ;no error using ax
                MOV     cx, SEG stackseg  ;error using cx, bx, or dx
;                filler
                MOV     ax, SEG stackseg  ;no error using ax

STACKSEG        SEGMENT PARA PUBLIC 'STACK'
                db 100 dup (?)
STACKSEG        ENDS

proc1           PROC    FAR
                ret
proc1           ENDP
_text           ENDS
END