Microsoft KB Archive/49840

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Use of the /Gt Switch in C

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Q49840

6.00 6.00a 6.00ax 7.00 | 6.00 6.00a | 1.00 1.50 MS-DOS | OS/2 | WINDOWS kbtool kbfasttip --- The information in this article applies to: - The Microsoft C/C++ Compiler (CL.EXE) component, included with: - Microsoft C for MS-DOS, versions 6.0, 6.0a, and 6.0ax - Microsoft C for OS/2, versions 6.0, and 6.0a - Microsoft C/C++ for MS-DOS, version 7.0 - Microsoft Visual C++ for Windows, versions 1.0 and 1.5 --- In Microsoft products mentioned above, the way /Gt is used determines which data items are moved out of the default data segment. /Gt alone is the same as /Gt256, which means that ONLY data items of size 256 and greater get moved out. If you have a lot of data items smaller than that, that data will NOT get moved out. With /Gt256, the data left in DGROUP plus the stack may still exceed 64K. As the value on the /Gt switch becomes smaller, more and more data gets pushed out. /Gt0 pushes all the data generated by your program out. However, it also generates some constants in the logical segment _CONST in DGROUP -- 2 bytes for each extern or uninitialized variable moved. The following classes of variables generate a 2-byte segment address in DGROUP for EACH variable of that class declared when /Gt0 is used: 1. All extern declarations, such as &quot;extern int i;&quot; 2. All declarations without storage class that are not initialized, such as &quot;int i;&quot; Initialized variables, such as &quot;int i = 0;&quot;, provided that the sum of their sizes is less than 64K, are treated as a group and generate only one segment address in DGROUP per module. Therefore, the optimal number for minimizing DGROUP space is /Gt3. /Gt3 is better than /Gt0 for uninitialized and extern variables because /Gt0 will move out 1- and 2-byte data elements to the far heap, replacing them with 2-byte segment addresses. Not only does this NOT save you space in DGROUP V.S. /Gt3, but it increases the overall size of the executable V.S. /Gt3, as you also have to pay 2 bytes in the far heap per data element moved. Another reason to use /Gt3 over /Gt0 is that although the size of DGROUP will remain the same, /Gt0 will cause character and integer data elements to be addressed with far pointers, slowing execution. With /Gt0, all that remains in DGROUP is the stack, several kilobytes of variables associated with the run-time library, the near heap (from which will be allocated some file buffers and the local copy of the environment), and the 2-byte segment addresses for data items moved out as needed. Additional reference words: kbinf 1.00 1.50 6.00 6.00a 6.00ax 7.00 8.00 8.00c L2041 KBCategory: kbtool kbfasttip KBSubcategory: CLIss

Keywords : kb16bitonly

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Technology : kbVCsearch kbAudDeveloper kbCVCComp