add ostep homeworks

related_info/ostep/ostep12-threadlock/test-and-test-and-set.s

@@ -0,0 +1,29 @@
+.var mutex
+.var count
+
+.main
+.top	
+
+.acquire
+mov  mutex, %ax
+test $0, %ax
+jne .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