🚀 go-pugleaf

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.  I am using
TC++ 3.0 and I have tryed both using their set vect routine as
well as manually changing the interrupt vector table but in both
cases the same thing happens...crash!  Even if I write an ISR for
INT 21h that simply returns control to the original ISR the system
crahes.  For example, even this crashes my system:

#include <dos.h>
#include <stdio.h>

void interrupt new21handler(void);
void interrupt (*old21handler)();

void main(void)
{
        disable();
        old21handler=getvect(0x21);
        setvect(0x21, new21handler);
        enable();
        keep(0, 4000);
}

void interrupt new21handler(void)
{
        (old21handler)();
}

Anyone know what's wrong here??? As far as I can see this program should
basically do nothing but take up memory but instead, it crahes the system when
run.  Any info would be greatly appreciated.

...Charlie Hutchins