DOCUMENT:Q80381 06-MAY-2001 [masm] TITLE :Relative Addresses (Relocatables) and the Minus Operator PRODUCT :Microsoft Macro Assembler PROD/VER::5.0,5.1,5.1a,6.0,6.0a,6.0b OPER/SYS: KEYWORDS: ====================================================================== ------------------------------------------------------------------------------- The information in this article applies to: - Microsoft Macro Assembler (MASM), versions 5.0, 5.1, 5.1a, 6.0, 6.0a, 6.0b ------------------------------------------------------------------------------- SUMMARY ======= The Microsoft Macro Assembler does not support using the minus operator (-) to negate a relative address, as in the following statement: lea ax, [bx - offset Table] Under MASM 5.1 and 5.1a, the error generated for this statement is: A2040: operands must be in same segment, or one must be constant Under MASM 6.0, the error generated is: A2094: operand must be relocatable MORE INFORMATION ================ Relocatable means not having an absolute address. Relocatables are often referred to as "relative addresses." The assembler does not know where the label, data, or code will be located in memory. The linker provides the address. The assembler generates a fixup record (FIXUPP) for each address it cannot determine at assembly time, which contains the information the linker will need to determine the address. The FIXUPP record contains the LOCATION of the address to be fixed up, the TARGET address to which the fixup refers, and the FRAME relative to which the address computation is performed. These fields are explained in detail on pages 682-693 in "The MS-DOS Encyclopedia." The FIXUPP record follows the data record that link needs to update. Note that an address that needs to be fixed up is indicated in the source listing produced by the assembler by an R following the instruction. This is a handy way to observe some of the common fixups. A statement such as the following lea ax, [offset Table1 - offset Table] (where Table1 and Table are in the same segment) includes a difference of relocatables and will be an immediate value. The assembler will perform the computation. This is one of the recommendations in the A2040 error shown in the summary. A statement like such as the following lea ax, [bx + offset Table] is equivalent to lea ax, Table[bx]. The offset of Table is known by the assembler, stored in a data record, fixed up, and added to bx at run time. In the following statement introduced in the summary lea ax, [bx - offset Table] the offset of Table is known, but the negation of an address is not defined for any language. The assembler could merely toggle the bits, but how would the linker interpret the new, negated address and perform the fixup? What would happen at run time? As it stands, the value produced would have no meaning. Since this fixup is not supported, an error is generated. Additional query words: kbinf 5.00 5.10 5.10a 6.00 6.00a 6.00b ====================================================================== Keywords : Technology : kbMASMsearch kbAudDeveloper kbMASM510 kbMASM600 kbMASM500 kbMASM600a kbMASM510a kbMASM600b Version : :5.0,5.1,5.1a,6.0,6.0a,6.0b ============================================================================= 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.