assembly - 不同 CPU 上未对齐的 MOVSD 会发生什么情况？

Question

基本上问题说的是，如果我在各种 CPU 上执行不是 8 字节(甚至 4 字节)对齐的 MOVSD，会发生什么？它是否会对性能产生影响，是否会出现段错误等？

Answer 1

摘自英特尔规范第 1 卷第 4.1 节

4.1.1 Alignment of Words, Doublewords, Quadwords, and Double Quadwords

Words, doublewords, and quadwords do not need to be aligned in memory on natural boundaries. The natural boundaries for words, double words, and quadwords are even-numbered addresses, addresses evenly divisible by four, and addresses evenly divisible by eight, respectively. However, to improve the performance of programs, data structures (especially stacks) should be aligned on natural boundaries when- ever possible. The reason for this is that the processor requires two memory accesses to make an unaligned memory access; aligned accesses require only one memory access. A word or doubleword operand that crosses a 4-byte boundary or a quadword operand that crosses an 8-byte boundary is considered unaligned and requires two separate memory bus cycles for access.

在“正常”情况下，未处理的 movsd 将需要 2 个周期才能完成。

如果您打开对齐检查(在 EFLAGS 中)，则 CPU 将发出 AC 信号。它的主要目的是帮助您检测未对齐的访问。