《操作系统》的实验代码。
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1.2 KiB

# 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