Xref: utzoo comp.sys.ibm.pc:38329 comp.lang.pascal:2688
Path: utzoo!attcan!sobmips!uunet!mcsun!inria!geocub!pelegrin
From: pelegrin@geocub.greco-prog.fr
Newsgroups: comp.sys.ibm.pc,comp.lang.pascal
Subject: Re: chaining interrupts
Message-ID: <1484@geocub.greco-prog.fr>
Date: 15 Nov 89 20:05:35 GMT
References: <4543@solo5.cs.vu.nl>
Sender: news@geocub.greco-prog.fr
Reply-To: pelegrin@goofi.UUCP (Uncle Ben's)
Organization: ENSERB 351 Cours de la Liberation 33405 TALENCE tel 56 84 65 00
Lines: 47


     It is absolutely harmless to link interrupt routines one to
another WITH THE SAME INTERRUPT VECTOR provided that, for interrupts
such as timer ticks, the total amount of time spent in the routines
does not exceed the tick clock rate...
     A general example of a resident interrupt routine installation
program (machine language) is given below :

 Initialization routine :
1) Save the old interrupt vector (INT 21H, FCN 25H) in a double word
   variable named, i.e. OLDVEC (defined with "DD OLDVEC").
2) Set the new interrupt vector (INT 21H, FCN 35H) to the segment:offset
   of your own interrupt routine NEWVEC, located within this program.
3) Make the program resident, including this routine (which will now
   be of no use but we have no way to eliminate it), and the next
   routine :
 Interrupt routine (starting at address NEWVEC) :
1) Push the registers you will use in your routine.
2) Perform whatever you want to... (8^D).
   Since it is an interrupt routine, DS and ES may have any value ; for
   small use of local variables, use the CS: prefix to access them
   within your code segment, else push DS and/or ES and set them equal
   to CS (for example...). If you use a large amount of stack, you can
   also switch to your own local stack.
3) Pop the registers you pushed before, in reverse order.
4) Use the JMP OLDVEC mnemonic (inter-segment jump from a memory operand)
   to link to the old interrupt vector.

 If you want to bypass the old interrupt vector, just perform at step 4)
a single IRET mnemonic for software interrupts, but if it was a hardware
interrupt (issued thru a Programmable Interrupt Timer), NEVER forget to use
the EOI mnemonic to warn the PIT of the end of the interrupt (for such
cases, I *strongly* recommend to link your interrupt to the previous one,
so that you don't have to hold those stinky cases...).

            Have fun.

                   f.p.

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