MIPS移位量位数技术咨询：64位寄存器最大可移位位数是否为63位？

Hey there! Great question about MIPS64 shift operations—let's break this down clearly so you can be confident in the answer.

First off, your initial guess is spot-on: in MIPS64 architecture, the maximum valid shift amount for a 64-bit register is 63 bits. Here's why that's the case, and how MIPS handles shift operations:

1. Immediate Shift Instructions #

For instructions like SLL (logical left shift), SRL (logical right shift), and SRA (arithmetic right shift)—which take a fixed shift amount directly in the instruction—MIPS64 sets aside a 6-bit field to store that value. A 6-bit binary number can range from 0 to 63, so that's the upper limit for these immediate shift commands.

2. Register-Based Shift Instructions #

For instructions that use a register to hold the shift amount (like SLLV, SRLV, SRALV), MIPS64 only looks at the lower 6 bits of that register. Again, this gives a maximum value of 63, so the same upper limit applies here.

3. Why Not 64 Bits? #

You might wonder why 64-bit shifts aren't supported. Logically, shifting a 64-bit register by 64 positions would be the same as shifting it by 0 (it's a full loop, so no actual change to the register value). MIPS avoids this redundant operation by limiting valid shift amounts to 0-63, which also simplifies the hardware design for shift circuits.

As a quick side note: if you ever try to use a shift amount of 64 or higher, MIPS will automatically take that value modulo 64 (so 64 becomes 0, 65 becomes 1, etc.). But the maximum meaningful shift you can explicitly specify is still 63 bits.

内容的提问来源于stack exchange，提问作者user16438868