Stuck in infinite loop in QEmu - c

Question

I have the following code, where I am trying to add a new interrupt (int 0x21) for 16-bit mode of x86:
void main()
{
char* str = "Enter the string!\n\r";
makeInterrupt21();
interrupt(0x21,0,str,0,0);
}
The interrupt and the makeInterrupt21 functions are written in assembly. The interrupt function simply calls the interrupt number as given by the first argument with rest of the arguments determining the valuses of ax, bx, cx and dx registers respectively.
The makeInterrupt21 function sets the entry for the jump address of interrupt 0x21.
.global _interrupt
_interrupt:
push bp
mov bp, sp
push si
push ds
mov ax, #0x100
mov ds, ax
mov ax, [bp + 0x4]
mov si, #intr
mov [si + 1], al
pop ds
mov ax, [bp + 0x6]
mov bx, [bp + 0x8]
mov cx, [bp + 0xa]
mov dx, [bp + 0xc]
intr: int #0x0
pop si
pop bp
ret
.global _makeInterrupt21
_makeInterrupt21:
push bp
mov bp, sp
push es
mov ax, #0x0
mov es, ax
mov bx, #0x84
mov ax, #interrupt21ServiceHandler
eseg
mov [bx], ax
eseg
mov [bx + 0x2], ds
pop es
pop bp
ret
.extern _handleInterrupt21
interrupt21ServiceHandler:
push dx
push cx
push bx
push ax
call _handleInterrupt21
ret
The handleInterrupt21 is the interrupt handler function for the interrupt 0x21 and is written in C:
void handleInterrupt21(int ax, int bx, int cx, int dx)
{
char* line;
line = bx;
switch (ax)
{
case 0x0:
printString(line);
break;
default:
printString("Error invalid arguments!");
break;
}
}
The printString function prints the string starting at the address given by the argument given to it:
int printString(char* string)
{
int i = 0;
while (*(string + i) != '\0')
{
char al = *(string + i);
char ah = 0xe;
int ax = ah * 256 + al;
interrupt(0x10,ax,0,0,0);
i++;
}
return i;
}
When I run this code on QEmu, I get into an infinite loop which prints:
Enter the string!
Enter the string!
Enter the string!
When I ran this on QEmu, I got the following error:
Program received signal SIGTRAP, Trace/breakpoint trap.
Could anyone explain why am I stuck in a loop?
EDIT:
Also please explain the meaning of breakpoint trap and why am I getting this error. I read a related question on this but I couldn't understand the answer.