## push

    rsp = rsp - 8       ; normal case
    *rsp = dest

The push instruction decrements the stack pointer (rsp) and then places the operand at the memory address contained in rsp. The destination can be a 64, 32, or 16 bit register or memory location or a 32, 16 or 8 bit immediate value. While it is possible to push quantities less than 64 bits, I suggest exercising care when pushing smaller quantities. In general it is important to maintain the stack 16 byte boundaries, so if you wish to push something and then call a function which you did not write (like printf), then push an even number of quadwords. It is possible to limit the use of push to the first instruction in the beginning of a function as part of establishing a stack frame.

            push    rbp             ; push register rbp onto the stack
            push    qword [x]       ; push 64 bit variable x
            push    eax             ; push 32 bit register

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