为什么需要额外的`CMP`命令来实现测试和设置(TSL)？

Question

以下代码来自维基百科(http://en.wikipedia.org/wiki/Test-and-set)

enter_region:        ; A "jump to" tag; function entry point.

  tsl reg, flag      ; Test and Set Lock; flag is the
                     ; shared variable; it is copied
                     ; into the register reg and flag
                     ; then atomically set to 1.

  cmp reg, #0        ; Was flag zero on entry_region?

  jnz enter_region   ; Jump to enter_region if
                     ; reg is non-zero; i.e.,
                     ; flag was non-zero on entry.

  ret                ; Exit; i.e., flag was zero on
                     ; entry. If we get here, tsl
                     ; will have set it non-zero; thus,
                     ; we have claimed the resource as-
                     ; sociated with flag.

leave_region:
  move flag, #0      ;store 0 in flag
  ret                ;return to caller

据我所知，这两个命令tsl reg, flag和cmp reg, #0可以合并成一个命令tsl reg, flag，它可以一起做三件事：(1)将flag复制到reg中(2)将flag设置为1 (3)测试reg是否为零。将(3)命令与tsl命令分开有什么好处或必要性？有人对此有什么想法吗？

Answer 1

tsl实际上并不测试flag的内容(除了它的名字之外)；它只是自动地将flag复制到reg。在x86中，它通常使用xchg或比较并交换指令实现，如果后者可用的话。