浮点数据转换指令包括不同的浮点精度数之间的转换，还包括整型和浮点数之间的转化。
在了解数据转换指令前，必须学习 IEEE 754 定义的五种舍入规则。前两条规则舍入到最接近的值，其他的称为定向舍入：
舍入到最接近的值
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Round to nearest, ties to even – rounds to the nearest value; if the number falls midway, it is rounded to the nearest value with an even least significant digit.（四舍五入到最接近的值；如果数字落在中间，则会四舍五入到最接近的具有偶数最低有效数字的值）
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Round to nearest, ties away from zero (or ties to away) – rounds to the nearest value; if the number falls midway, it is rounded to the nearest value above (for positive numbers) or below (for negative numbers).（舍入到最接近的值；如果数字落在中间，则四舍五入到上方（对于正数）或下方（对于负数）最接近的值）
定向舍入
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Round toward 0 – directed rounding towards zero (also known as truncation).（向零定向舍入（也称为截断））