Path: utzoo!dptcdc!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!osu-cis!sppy00!jmv
From: jmv@sppy00.UUCP (Jim Vickroy)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Question 2 of 3 for assembly language programmers
Keywords: How do you call existing interrupt handlers in ass'y?
Message-ID: <298@sppy00.UUCP>
Date: 18 Apr 89 15:07:04 GMT
References: <4497@tekigm2.MEN.TEK.COM>
Reply-To: jmv@sppy00.UUCP (Jim Vickroy)
Distribution: na
Organization: Online Computer Library Center, Dublin, Ohio.
Lines: 53

In article <4497@tekigm2.MEN.TEK.COM> alanr@tekigm2.MEN.TEK.COM (Alan Rovner) writes:
=>
=>If I wanted to hook onto the keyboard interrupt handler to write my own
=>TSR in assembly language how would I call the original handler(s) that
=>were already in use?  I understand that when grabbing onto an existing
=>interrupt vector you should call however many functions have already been
=>in use there.  This is easy in C, one could call the function via a
=>pointer, like (*orig_name)().  But I'm unclear how to do this in assembly
=>language.  I'm sure if there's a way to screw up the segment registers
=>I'll find it.  Again, any examples would be helpful.

Try this:  (example int16 - keyboard i/o)


OLD_KEYBOARD	    LABEL   DWORD
OLD_KEYBOARD_OFF    DW	    ?
OLD_KEYBOARD_SEG    DW	    ?
		    :
		    :
		    MOV	    AX,3516H		; 'Get Interrupt Vector' 16H
		    INT	    21H			; DOS Function Call Interrupt
		    MOV	    OLD_KEYBOARD_OFF,BX	; Save the offset
		    MOV	    OLD_KEYBOARD_SEG,ES ; Save the segment    
		    :
		    [stuff left out like grabbing and handling the interrupt]
		    :
		    JMP	    OLD_KEYBOARD	; Transfer control to the old
						;   handler

You may have to use segment overrides if your program is sufficiently large.

The equivalent to your C example [ (*orig_name)() ] would be:

		    CALL    ORIG_NAME

**** Please note: You *would not* want to do this in a interrupt handler
		  because the old handler is going to return via the IRET
		  which will *not* transfer back to your handler.

jim
--
==============================================================================
:::: ::: ::  ::   :    :   Jim Vickroy
|OC| ||| ||  ||   |    |   Technical Services Department
|LC| ||| ||  ||   |    |   Online Computer Library Center, Inc.
:::: ::: ::  ::   :    :   Dublin, Ohio
------------------------------------------------------------------------------
UUCP:    {att|pyramid|killer}!osu-cis!sppy00!jmv  
domain:	 jmv@sppy00.uucp
USSNAIL: 6565 Frantz Rd., Dublin, Ohio 43017-0702
------------------------------------------------------------------------------
"Hey, wow man ... pooby shooby"                                            -cm
==============================================================================