add ostep homeworks

related_info/ostep/ostep12-threadlock/peterson.s

@@ -0,0 +1,53 @@
+# array of 2 integers (each size 4 bytes)
+# load address of flag into fx register
+# access flag[] with 0(%fx,%index,4)
+# where %index is a register holding 0 or 1
+# index reg contains 0 -> flag[0], if 1->flag[1]
+.var flag   2     
+
+# global turn variable
+.var turn
+
+# global count
+.var count
+
+.main
+
+# put address of flag into fx
+lea flag, %fx
+
+# assume thread ID is in bx (0 or 1, scale by 4 to get proper flag address)
+mov %bx, %cx   # bx: self, now copies to cx
+neg %cx        # cx: - self
+add $1, %cx    # cx: 1 - self
+
+.acquire
+mov $1, 0(%fx,%bx,4)    # flag[self] = 1
+mov %cx, turn           # turn       = 1 - self
+
+.spin1
+mov 0(%fx,%cx,4), %ax   # flag[1-self]
+test $1, %ax            
+jne .fini               # if flag[1-self] != 1, skip past loop to .fini
+
+.spin2                  # just labeled for fun, not needed
+mov turn, %ax
+test %cx, %ax           # compare 'turn' and '1 - self'
+je .spin1               # if turn==1-self, go back and start spin again
+
+# fall out of spin
+.fini
+
+# do critical section now
+mov count, %ax
+add $1, %ax
+mov %ax, count
+
+.release
+mov $0, 0(%fx,%bx,4)    # flag[self] = 0
+
+
+# end case: make sure it's other's turn
+mov %cx, turn           # turn       = 1 - self
+halt
+