If one of those two expressions couldpossibly generate an error condition or a side effect, this could lead to invalidbehavior. Such is the case for our earlier example

One important feature is that memoryreferences in x86-64 are always given with quad word registers, such as %rax, evenif the operand is a byte, single word, or double word.

logicallybe named movzlq, but this instruction does not exist. Instead, this type of datamovement can be implemented using a movl instruction having a register as thedestination. This technique takes advantage of the property that an instructiongenerating a 4-byte value with a register as the destination will fill the upper 4bytes with zeros.

in memory, to a register destination. Instructions in the movz class fill out theremaining bytes of the destination with zeros, while those in the movs class fillthem out by sign extension, replicating copies of the most significant bit of thesource operand.

The source operand designates a value that is immediate, stored in a register,or stored in memory. The destination operand designates a location that is either aregister or a memory address. x86-64 imposes the restriction that a move instruc-tion cannot have both operands refer to memory locations. Copying a value fromone memory location to another requires two instructions—the first to load thesource value into a register, and the second to write this register value to the des-tination.