> For the complete documentation index, see [llms.txt](https://koshizuow.gitbook.io/compilerbook/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://koshizuow.gitbook.io/compilerbook/calculator_level_language/stack_machine.md).

# 堆疊機

上一章說明了把標記列傳換成抽象語法樹的演算法。在文法中考慮到運算子的優先順序，做出了讓`*`或`/`比`+`或`-`先出現在末梢的抽象語法樹。但是要怎麼把樹轉換成組合語言呢？這章就是要說明這個轉換的方法。

首先，我們來想想為什麼不能用和加減法一樣的方法來轉成組合語言。在可以算加減法的編譯器中，是把結果放在 RAX 暫存器，在該暫存器上進行加法或減法的計算。也就是說編譯的程式中途能暫時紀錄的答案只有1個。

但是，包含乘除法的話，中途要紀錄的答案就不僅限1個了。舉例來說，考慮`2*3+4*5`這個算式，為了計算加法必須要有左右兩邊的計算結果，在算加法前要先算`2*3`和`4*5`。也就是說如果不能紀錄2個結果，就沒辦法算出全部的結果。

有種可以簡單進行這樣的計算的電腦叫作「堆疊機」（stack machine）。接下來我們暫時放下抽象語法樹，來學一下堆疊機。


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