TSR in assembly save in memory

Question

I have problem with TSR, trying to save in memory and when start "program" to keep writing something else, and in behind to keep working (stopwatch).

This is the code:

cli
xor ax, ax
mov es, ax
mov bx, [es:1Ch*4]
mov [old_int_off], bx
mov bx, [es:1Ch*4+2]
mov [old_int_seg], bx

mov dx, timer
mov [es:1Ch*4], dx
mov ax, cs
mov [es:1Ch*4+2], ax
push ds
pop gs
sti

mov ax,3100h
mov dx,500
int 21h

When I start program, I can writing something else. When I pressF1 to start stopwatch, nothing happened.

But without this:

mov ax, 3100h
mov dx, 500
int 21h

F1 work, but i can't writing.