INFO: How Bitfields Are Stored in Memory

ID: Q32841

The information in this article applies to:

The Microsoft C/C++ Compiler (CL.EXE)
- 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, 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

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 : kbCompiler kbVC100 kbVC150 kbVC200 kbVC400 kbVC500 kbVC600
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