🚀 go-pugleaf

In article <eoq.25.0@vthnw.cvm.msu.edu> eoq@vthnw.cvm.msu.edu (Edward Quillen) writes:
>In article <14339@mindlink.bc.ca> Charlie_Hutchins@mindlink.bc.ca (Charlie Hutchins) writes:
>>Okay, maybe someone could help me here.  I am trying to write a TSR
>>that hooks interrupt 21h but whenever I try and hook that vector I
>>get an invalid command.com error and my system crashes.  

>>void interrupt new21handler(void)
>>{
>>        (old21handler)();
>>}
>>
>Please post replies to this ? because I've always wanted to hook INT 21h
>but have never been able to. Thanks!!!

This is the wrong way to intercept a service interrupt.  Service interrupts 
take arguments in the registers (and sometimes on the stack, but no INT 
21h interrupts do this) and return results in the registers (and sometimes 
on the stack).  

The new21handler above probably changes the values in several registers
before the call, so it'll pass the wrong request to the old handler.  It 
restores the old register values from the stack when it returns; this wipes 
out the result from the service.

Intercepting service interrupts is tricky.  I've no idea how to do it in C, 
but the basic idea is the following:

  1.  On entry, save the registers, and do any preprocessing.
  2.  Restore the registers; if necessary, restore the stack; call the
      old handler.  Note that if parameters are being passed on the stack,
      you need to do some fiddling here to do the call without messing
      up the stack frame.
  3.  When the old handler returns, save the registers, and do any
      postprocessing.
  4.  Restore the returned register values, and return to the original 
      caller.  You have to handle this return carefully:  use RETF 2, not
      IRET.  Otherwise, you'll lose any status information being returned
      in the flags.  

Duncan Murdoch
dmurdoch@mast.queensu.ca