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The four registers are saved by the called function.
Literal
4 [genitive-の] registers [topic-は], called function side [subject-が] save.
Technical programming documentation. 呼び出された関数側 is a relative clause + noun: 'the called-function side' — referring to the callee function in a call/return convention. ~側 ('side') is a suffix marking which party holds responsibility. This describes a callee-saved register convention from low-level programming. 4つ uses the native Japanese counter つ.