DOCUMENT:Q80384 06-MAY-2001 [masm] TITLE :PRB: Span-Dependent Value Behavior Changes in N-Pass Assembly PRODUCT :Microsoft Macro Assembler PROD/VER::6.0,6.0a,6.0b,6.1,6.11,6.1a OPER/SYS: KEYWORDS: ====================================================================== ------------------------------------------------------------------------------- The information in this article applies to: - Microsoft Macro Assembler (MASM), versions 6.0, 6.0a, 6.0b, 6.1, 6.1a, 6.11 ------------------------------------------------------------------------------- SYMPTOMS ======== When the Microsoft Macro Assembler (MASM) assembles code that depends on the difference between the values of two labels (a span-dependent value), the results of an assembly by MASM versions 6.0 or later differs from the results produces by an earlier version of MASM. Specifically, the difference occurs when the application uses a span- dependent value in one of the following cases: - In the expression for a preprocessor directive, like IF - In the expression for a predefined macro, like REPEAT or WHILE - In the arguments to a MACRO or TEXTEQU statement preceded by % (percent sign) CAUSE ===== MASM versions 5.1 and earlier are two-pass assemblers; MASM versions 6.0 and later are N-pass assemblers. RESOLUTION ========== Avoid using a span-dependent value in a preprocessor directive or in a predefined macro. If the code requires a span-dependent value, create and review a listing file to determine that each span-dependent value is evaluated as desired. Avoid using the % operator with span-dependent values. The assembler evaluates an expression containing the % operator on the first pass. In many cases, the assembler produces the desired results when you remove the % operator and delay expression evaluation to a later pass. STATUS ====== This behavior is under review and may or may not change in a future release. MORE INFORMATION ================ MASM versions 5.1 and earlier are two-pass assemblers. The source file is read twice to determine the size of the instructions and the location of labels. MASM versions 6.0 and later are N-pass assemblers. The assembler scans the source until it can determine the size of all instructions and the location of all labels. However, an N-pass assembler must determine certain values on its first pass through the application code. These values include expressions in preprocessor directives, expressions in predefined macros, and arguments preceded by the % operator. If one of these values is based on the difference between the values of two labels, the results may differ from those generated by MASM versions 5.1 and earlier. The difference occurs because the assembler evaluates expressions and arguments based on the undetermined label locations available during the first assembly pass that may change in a subsequent assembly pass. Typically, the value changes when a forward reference occurs between the two labels. Because the assembler adds padding bytes for the forward reference and removes the padding in subsequent assembly passes, the second label has a larger value during the first assembly pass than in subsequent passes. The code example below demonstrates this behavior. Specify the /Fl assembler option switch to create a listing file. Review the listing file to see how the assembler processes a span-dependent value. The following information is part of the README.TXT file distributed with MASM version 6.1: Span-Dependent Expressions used in Macros ----------------------------------------- MASM 6.1 evaluates macro expressions only on the first pass of assembly, but code and data are reevaluated on subsequent passes. Because of this, macro expressions which depend on the span between two addresses may not evaluate correctly. For example, the following code will not evaluate correctly: Label1: JMP Label2 Label2: REPEAT Label2 - Label1 ; Evaluates incorrectly INC AX ENDM View the listing file to determine if a questionable macro expression was evaluated as desired. Span-Dependent Text Equates --------------------------- The TEXTEQU operator is evaluated on the first assembly pass. If TEXTEQU is used with an expression that depends on the difference between two addresses, the resulting constant may be incorrect. For example, the following code will not evaluate correctly: Label1: JMP Label2 Label2: WrongNum TEXTEQU %Label2-Label1 ; WrongNum will be incorrect Sample Code ----------- ; Assemble options needed in MASM 6.x: /c /Fl _text SEGMENT para public 'CODE' ASSUME cs:_text start: jmp SHORT forward forward: mov ax, 4C00h int 21h _text ENDS count = offset forward - offset start IF count LT 6 %OUT two-pass assembler ; MASM 5.1 and earlier shows this message ELSE %OUT n-pass assembler ; MASM 6.0 and later shows this message ENDIF tst1 SEGMENT para public index = 0 WHILE index LT count DB 1 ; This will be repeated 10 times in MASM 6.x. index = index + 1 ENDM tst1 ENDS tst2 SEGMENT para public REPT count ; This will be repeated 10 times in MASM 6.x, DB 1 ; twice in MASM 5.1. ENDM tst2 ENDS mac3 MACRO arg1 macvar = arg1 ENDM tst3 SEGMENT para public mac3 %count ; This will set macvar to 10 in MASM 6.x tst3 ENDS ; and to 2 in MASM 5.1. txt4 TEXTEQU %count tst4 SEGMENT para public txtvar DB txt4 ; This will set txtvar to 10 in MASM 6.x. tst4 ENDS END start Additional query words: 6.00 6.00a 6.00b 6.10 ====================================================================== Keywords : Technology : kbMASMsearch kbAudDeveloper kbMASM600 kbMASM610 kbMASM611 kbMASM610a kbMASM600a kbMASM600b Version : :6.0,6.0a,6.0b,6.1,6.11,6.1a ============================================================================= THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. MICROSOFT DISCLAIMS ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL, CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. SOME STATES DO NOT ALLOW THE EXCLUSION OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES SO THE FOREGOING LIMITATION MAY NOT APPLY. Copyright Microsoft Corporation 2001.