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From: djb@wjh12.harvard.edu (David J. Birnbaum)
Newsgroups: comp.sys.ibm.pc.programmer,alt.msdos.programmer
Subject: which timer interrupt?
Message-ID: <484@wjh12.harvard.edu>
Date: 3 May 90 14:46:44 GMT
Reply-To: djb@wjh12.UUCP (David J. Birnbaum)
Distribution: na
Organization: Harvard University, Cambridge MA
Lines: 30

Tsrs that have to do something regularly usually do so by
hooking one of the two available timer interrupts: 8 and 1ch.
I understand that 8 gets called first, updates the system
clock, checks the diskette motor and turns it off when neces-
sary, and then issues int 1ch.  After regaining control, int 8
issues the eoi command.

According to the Phoenix Technical Reference for the System
BIOS for IBM PC/XT/AT Computers and Compatibles, p. 430,
"The user can revector INT 1Ch to a user-supplied routine to
be executed on every system clock tick.  No action is taken by
the BIOS when INT 1Ch is invoked unless this vector is modified
by the caller...."

Why do some programs hook int 8, while others hook int 1ch?  Is
there any reason to favor one approach over the other for certain
applications?  I understand that int 8 operates first, but unless
you plan to do something that depends on the reading of the system
clock before int 8 updates it, why would you need to get in that
early?  I am particularly curious about how chosing one or the
approach would affect a screen blanking utility?

Thanks for any comments or suggestions,

David

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David J. Birnbaum         djb@wjh12.harvard.edu [Internet]
                          djb@harvunxw.bitnet   [Bitnet]
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