Path: utzoo!utgpu!news-server.csri.toronto.edu!clyde.concordia.ca!uunet!mcsun!isgate!krafla!frisk
From: frisk@rhi.hi.is (Fridrik Skulason)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Interrupting An Interrupt Chain
Message-ID: <1600@krafla.rhi.hi.is>
Date: 28 Mar 90 12:01:13 GMT
References: <26909@ut-emx.UUCP> <322@pallas.athenanet.com> <2129@darkstar.ucsc.edu>
Reply-To: frisk@rhi.hi.is (Fridrik Skulason)
Organization: University of Iceland (RHI)
Lines: 45

In article <2129@darkstar.ucsc.edu> mee@ucscb.UCSC.EDU (Kevin Kahl) writes:
>Why not adopt a particular convention for each interrupt that has
>applications where it is necessary to block the rest of the chain?
>For example, why not have a global flag that is set when a TSR
>detects its hotkey?  All subsequent programs which obey the convention
>would then do anything that they needed to (except use the same hotkey)
>and then pass control along.

Consider the following case when one most certainly does not want any
previous interrupt handler to interfere:

In many countries a TSR program is used to remap the keyboard in order to
provide access to the national characters needed there.  For example,
here in Iceland when the '/' key is pressed, I want the TSR routine to insert
a special character (thorn) into the keyboard buffer.  If the TSR routine then
passed control to the original INT 9 handler, the character '/' would also
be inserted.

Of course 'a global flag' would solve the problem, but as the original INT
9 rouine is in ROM, this would require a ROM change - hardly a practical
solution.

No, I just will have to use the following code, which makes the
computer forget that the keypress ever occurred....

		IN	AL,61H
		MOV	AH,AL
		OR	AL,80H
		OUT	61H,AL
		XCHG	AH,AL
		JMP	SHORT $+2
		OUT	61H,AL
		CLI
		MOV	AL,20H
		OUT	20H,AL
		MOV	AX,9102H
		INT	15H
		POP	AX
		POP	DS
		IRET

-- 
Fridrik Skulason      University of Iceland  |       
Technical Editor of the Virus Bulletin (UK)  |  Reserved for future expansion
E-Mail: frisk@rhi.hi.is    Fax: 354-1-28801  |