Overview: Interrupt 21 Hook Optimization

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

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.