Q32841: INFO: How Bitfields Are Stored in Memory

Article: Q32841
Product(s): Microsoft C Compiler
Version(s): 1.0,1.5,2.0,4.0,5.0,6.0
Operating System(s): 
Keyword(s): kbCompiler kbVC100 kbVC150 kbVC200 kbVC400 kbVC500 kbVC600
Last Modified: 28-SEP-2001

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The information in this article applies to:

- Microsoft Visual C++ for Windows, 16-bit edition, versions 1.0, 1.5 
- Microsoft Visual C++, 32-bit Editions, versions 1.0, 2.0, 4.0, 5.0, 6.0 
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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

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Keywords          : kbCompiler kbVC100 kbVC150 kbVC200 kbVC400 kbVC500 kbVC600 
Technology        : kbVCsearch kbVC400 kbAudDeveloper kbvc150 kbvc100 kbVC500 kbVC600 kbVC200 kbVC32bitSearch kbVC16bitSearch kbVC500Search
Version           : :1.0,1.5,2.0,4.0,5.0,6.0
Issue type        : kbinfo

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