Article: Q103885
Product(s): Windows for Workgroups and Windows NT Networking Issues
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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.
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