c - 了解 PowerPC rlwinm 指令

Question

所以我终于说服自己尝试学习/使用 PowerPC (PPC)。一切进展顺利，大部分信息都是在网上找到的。但是，在查看一些示例时，我发现了这一点:

rlwinm    r3, r3, 0,1,1

我如何在 C 中执行此操作？我尝试做一些研究，但找不到任何对我有帮助的东西。提前致谢!

Answer 1

rlwinm 代表“Rotate Left Word Immediate then aNd with Mask，正确的用法是

rlwinm  RA, RS, SH, MB, ME

根据 description page :

• RA Specifies target general-purpose register where result of operation is stored.
• RS Specifies source general-purpose register for operation.
• SH Specifies shift value for operation.
• MB Specifies begin value of mask for operation.
• ME Specifies end value of mask for operation.
• BM Specifies value of 32-bit mask.

和

• If the MB value is less than the ME value + 1, then the mask bits between and including the starting point and the end point are set to ones. All other bits are set to zeros.
• If the MB value is the same as the ME value + 1, then all 32 mask bits are set to ones.
• If the MB value is greater than the ME value + 1, then all of the mask bits between and including the ME value +1 and the MB value -1 are set to zeros. All other bits are set to ones.

因此在您的示例中，源和目标是相同的。移位量是0，所以没有移位。并且 MB=ME=1，因此适用第一种情况，这样掩码变为全零，位号 1 为 1，同时从 MSB=0 开始编号:0x40000000。

在C中我们可以这么简单的写

a &= 0x40000000;

假设 a 是 32 位变量。