Use of memory operand constraint.
内存操作数约束的使用。
"M" : a memory operand is allowed, with any kind of address that the machine supports in general.
对内存的操作被允许，用一个合法内存空间来做操作数。
"o" : A memory operand is allowed, but only if the address is offsettable. ie, adding a small offset to the address gives a valid address.
对内存的操作被允许，但是必须支持地址偏移值，即，对于给出的地址，加上一个偏移量，此时也是一个合法的地址。
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