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How do I write INT 21h extenders?

maartenp
maartenp asked
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Last Modified: 2012-05-04
I want to write a TSR to capture DOS interrupt 21h.  When INT 21h is called, my program should call BooleanFunction, and if BooleanFunction returns True, to call the old int 21h handler as if nothing happened, but if BooleanFunction returns False, to send the "File not found" code back to the application and return from the handler.  Can anyone offer some skeleton code for this?
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Commented:
program test;
func true/false
begin
    check something and return true/false
end;

procedure proc: interrupt;
begin
  if true/false then do old handler
else  do my handler
end;

begin
  getintvec(21h,x)
   setintvec(@proc,x)

   keep
end.

I relize that this is very bare code, but I think it should get you going in the right direction

Author

Commented:
I know how to do that.  I wanted to know about specifics of
capturing interrupt 21.  It has many syntax errors, and from
what I can see, it doesn't give me any information at all!
I am very disappointed with this site.

Author

Commented:
I just found a better answer on the Usenet.  I can't find
any way to delete this, so I have to leave it here.
NOBODY ANSWER THIS!
Maybe my message will get deleted eventually.
So send a massage to experts-exchange so they will delete your question.

BTW, the best way to capture INT21 is to use some assembler.
With pascal you will have many problem transfering all the registers.

Commented:
I would not advise writing capturing 21H in pascal because of runtime left in the memory.  It would dramaticaly decrease available memory.
Commented:
{here it is:}

{$M 1400,0,0}

var oldproc : procedure;

procedure newhandler(ax,bx,cx,dx : word}; Interrupt;

function yourfunction : Boolean;
Begin

{....
Your function here}

end;

begin
  if yourfunction then oldproc;  {return to old interrupt}
  asm
    mov ah,2h    {file not found error code}
    retf 0002    {return without doing the old int 21h
  end;
end;

Begin

getintvec($21,@oldproc);
setintvec($21,addr(newhandler));
keep(0);
end.


{thats all.}

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