DOC: Run-Time Routines Assume Direction Flag Is Clear in C (50712)
The information in this article applies to:
- The C Run-Time (CRT), when used with:
- Microsoft Visual C++ for Windows, 16-bit edition 1.0
- Microsoft Visual C++ for Windows, 16-bit edition 1.5
- Microsoft Visual C++, 32-bit Editions 1.0
- Microsoft Visual C++, 32-bit Editions 2.0
- Microsoft Visual C++, 32-bit Editions 2.1
- Microsoft Visual C++, 32-bit Editions 4.0
- Microsoft Visual C++, 32-bit Editions 5.0
- Microsoft Visual C++, 32-bit Editions 6.0
This article was previously published under Q50712 SUMMARY
The C run-time routines assume that the direction flag is cleared. If
you are using other functions with the C run-time functions, you must
ensure that the other functions leave the direction flag alone or
restore it to its original condition. Expecting the direction flag to
be clear upon entry makes the run-time code faster and more efficient.
The direction flag is a CPU flag specific to Intel 80x86 processors.
It applies to all assembly instructions that use the REP (repeat)
prefix, such as MOVS, MOVSD, MOVSW, etc. Addresses provided to
applicable instructions are increased if the direction flag is cleared.
The CRT function, such as the string manipulation and buffer
manipulation routines, expect the direction flag to be clear. This is
usually the case and is documented in the "Microsoft Macro Assembler
Programmer's Guide," for versions 5.0 and 5.1, as follows:
Under DOS, the direction flag will normally be cleared if your
program has not changed it.
| Modification Type: | Major | Last Reviewed: | 12/12/2003 |
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| Keywords: | kbdocerr KB50712 kbAudDeveloper |
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