os_kernel_lab/related_info/ostep/ostep12-threadlock/yield.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!
je .acquire_done    
yield               # if not, yield and try again
j .acquire
.acquire_done

# 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