test-and-set.s

.var mutex
.var count

.main
.top	

.acquire
mov  $1, %ax        
xchg %ax, mutex     # atomic swap of 1 and mutex
test $0, %ax        # if we get 0 back: lock is free!
jne  .acquire       # if not, try again

# critical section
mov  count, %ax     # get the value at the address
add  $1, %ax        # increment it
mov  %ax, count     # store it back

# release lock
mov  $0, mutex

# see if we're still looping
sub  $1, %bx
test $0, %bx
jgt .top	

halt