Q103885: Overview: Interrupt 21 Hook Optimization

Article: Q103885
Product(s): Windows for Workgroups and Windows NT Networking Issues
Version(s): 
Operating System(s): 
Keyword(s): 
Last Modified: 30-JUL-2001

OVERVIEW: INTERRUPT 21 HOOK OPTIMIZATION

In Microsoft LAN Manager version 2.1, an important performance
optimization was implemented in the DOS redirector--the Interrupt 21
Hook. The redirector initialization software registers a handler at
interrupt x21. Since the redirector loads after MS-DOS, the redirector
handler gets control, before MS-DOS, when interrupt x21 is called.

When an application wants to read from or write to a file, it loads
selected CPU registers with information such as:

- the file handle

- the number of bytes to read/write

- a pointer to a buffer that will accept the data to be read

- a pointer to a buffer that contains the data to be written

The application then passes the read/write request to MS-DOS by
calling interrupt x21 with the appropriate function number.

Before the Int 21 Hook
----------------------

The request is processed by MS-DOS. If the handle for the specified
file has been designated as "remote," MS-DOS passes the read/write
request to the redirector.

With the Int 21 Hook
--------------------

The request goes directly to the redirector, which quickly determines
whether the request is a repeated sequential read or write. If so, the
redirector handles the request immediately. If not, the redirector
passes control to the next registered interrupt x21 handler, and
MS-DOS gets control. MS-DOS performs its standard check to determine
if the request is local or remote. If remote, MS-DOS passes the
request back to the redirector.

Additional query words:

======================================================================
Keywords          :  

=============================================================================

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.

Copyright Microsoft Corporation 1986-2002.