前面讲了x10-x17，一共8个寄存器用于存储变量。而如果超过了8个，就需要用到“栈”（stack）这个概念了：
stack A data structure for spilling registers organized as a last-in�first-out queue.
对于“栈”（stack），几个相关的概念是：
stack pointer A value denoting the most recently allocated address in a stack that shows where registers should be spilled or where old register values can be found. In RISC-V, it is register sp, or x2.
push Add element to stack.
pop Remove element from stack.
当你进行压栈（push）的时候，栈是变大还是变小呢？
By historical precedent, stacks “grow” from higher addresses to lower addresses.
好像真的在“压”，所以从大到小了。
示例：
答案：
The parameter variables g, h, i, and j correspond to the argument registers x10, x11, x12, and x13, and f corresponds to x20.
首先，定义label：
leaf_example:
随后，压栈并存储：
addi sp, sp, -24 // adjust stack to make room for 3 items
sd x5, 16(sp) // save register x5 for use afterwards
sd x6, 8(sp) // save register x6 for use afterwards
sd x20, 0(sp) // save register x20 for use afterwards
然后是进行相应的运算，实现C代码：
add x5, x10, x11 // register x5 contains g + h
add x6, x12, x13 // register x6 contains i + j
sub x20, x5, x6 // f = x5 − x6, which is (g + h) − (i + j)
再得到f的数值：
addi x10, x20, 0 // returns f (x10 = x20 + 0)
最后是弹栈：
ld x20, 0(sp) // restore register x20 for caller
ld x6, 8(sp) // restore register x6 for caller
ld x5, 16(sp) // restore register x5 for caller
addi sp, sp, 24 // adjust stack to delete 3 items
并返回：
jalr x0, 0(x1) // branch back to calling routine
对应的图示：
RISC-V将寄存器分成了两组：
x5−x7 and x28−x31: temporary registers that are not preserved by the callee (called procedure) on a procedure call n
x8−x9 and x18−x27: saved registers that must be preserved on a procedure call (if used, the callee saves and restores them)
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