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From: Ralf.Brown@B.GP.CS.CMU.EDU
Newsgroups: comp.sys.ibm.pc
Subject: Re: Dos File Access In TSR's
Message-ID: <24a2448a@ralf>
Date: 23 Jun 89 14:09:14 GMT
Sender: ralf@b.gp.cs.cmu.edu
Organization: Carnegie Mellon University School of Computer Science
Lines: 62
In-Reply-To: <8064@saturn.ucsc.edu>

In article <8064@saturn.ucsc.edu>, lance@helios (Lance Bresee) writes:
}Isn't there a dos routine which sets the date stamp on a file?

Yes.

}If so, you can get this vector, and have your tsr execute a far jump..
}and the end of the routine, there will be an iret instruction, so
}dos will jump back to the application rather than back to your
}tsr.....

Doesn't work that way.  The DOS function call INT 21h does the following:
        setup
        indirect call to actual function handler
           function executes
           RET
        cleanup
        IRET
Besides, DOS is not reentrant, so you couldn't just call or jump to the
appropriate routine without first making sure that it is safe to call DOS.  If
you do that, you might as well just call DOS normally once you are sure it is
safe.

}Also, I believe that the interrupt hierarchy makes it impossible to
}interrupt a disk read...you may want to check on this though.....

Nope, disk reads/writes can be interrupted.  For a floppy disk or XT hard
drive, the basic sequence of events is
        seek to proper track, if necessary
        set up floppy controller
        set up DMA controller
        clear completion flag
        start DMA transfer
        loop until completion flag set
        loop until completion flag set
        loop until completion flag set
             DISK INTERRUPT: handler sets completion flag
        loop until completion flag set: loop terminates
        IRET
During this entire time, interrupts are enabled (have to be to allow the disk
interrupt).  The disk interrupt is not even at a very high priority.  The
floppy interrupt is IRQ6, and the XT hard drive is IRQ5.  The only thing with
a lower priority is the printer interrupt on IRQ7.

For an AT hard drive, the basic sequence of events is
        seek to proper track, if necessary
        set up HD controller
        REP INSW  or REP OUTSW     <- transfer data using the CPU
        IRET
Again, interrupts are enabled during the process.

DESQview 2.2 has no problems task-switching during heavy floppy access, such
as a large file copy or format.  Previous versions did not task switch during
floppy access, but that appears to have been a design decision (there is a
poorly-documented flag for DV 2.2 to disable task-switching during disk
accesses).

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