INFO: How Bitfields Are Stored in Memory

 Last reviewed: October 1, 1997
Article ID: Q32841

The information in this article applies to:
  • The Microsoft C/C++ Compiler (CL.EXE) included with: - Microsoft C for MS-DOS, versions 5.1, 5.1a, 6.0, 6.0a, 6.0ax - Microsoft C for OS/2, versions 5.1, 6.0, 6.0a - Microsoft C/C++ for MS-DOS, version 7.0 - Microsoft Visual C++ for Windows, versions 1.0, 1.5 - Microsoft Visual C++, 32-bit Edition, versions 1.0, 2.0, 4.0, 5.0

SUMMARY

An application developed with one of the Microsoft compilers above stores bitfields from low memory to high memory. For example, consider the following declaration: 

   struct x {
      unsigned field_one:3;
      unsigned field_two:9;
      unsigned field_three:5;
   };
Compiled with a 16-bit compiler, the above code stores field_one in bits 0- 2 of the first word, field_two in bits 3-11 of the same word, and field_three in bits 0-4 of the next word (because the field cannot fit in the remaining four bits of the first word and bit fields do not cross byte boundaries). Compiled with a 32-bit compiler, the same code stores the struct in the 17 least significant bits of a word.

Additional query words: 8.00 8.00c 9.00
Keywords : CLIss
Version : MS-DOS:5.1,6.0,6.00a,6.00ax,7.0; OS/2:5.1,6.0,6.00a; WINDOWS:1.0,1.5; WINDOWS NT:1.0,2.0,4.0,5.0
Platform : MS-DOS NT OS/2 WINDOWS
Issue type : kbinfo

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.

Last reviewed: October 1, 1997
© 1998 Microsoft Corporation. All rights reserved. Terms of Use.