Eighty One Not Eighty Two

counter starts at 80 and two threads each run counter = counter + 1. On x86 that compiles to three instructions:

mov  0x8049a1c, %eax
add  $0x1, %eax
mov  %eax, 0x8049a1c

The scheduler produces this interleaving:

step	thread	instruction	T1 %eax	T2 %eax	counter
1	T1	mov (load)	80	-	80
2	T1	add	81	-	80
3	-	interrupt, switch to T2	81	-	80
4	T2	mov (load)	81	80	80
5	T2	add	81	81	80
6	T2	mov (store)	81	81	81
7	-	switch back to T1	81	81	81
8	T1	mov (store)	81	81	?

What is counter at step 8, and why?