CPU架构中按位布尔及移位操作为何被称为逻辑操作？——关于《Computer Organization and Design》术语定义的技术问询

Great questions—this is one of those cases where terminology shifts (pun totally intended!) between programming languages and CPU architecture, so it’s totally normal to feel confused about the overlap. Let’s break down each question clearly:

1. Why are bitwise Boolean operations and shifts categorized as "logical operations" in CPU architecture? #

In hardware design, CPU functionality is grouped around what the arithmetic logic unit (ALU) handles, and the ALU splits its work into two core buckets:

• Arithmetic operations: Tasks like addition, subtraction, multiplication, and division that treat data as whole numerical values.
• Logical operations: Operations rooted in Boolean logic (AND, OR, NOT, XOR) that manipulate individual bits within a word.

Bitwise Boolean operations fit directly here because they apply Boolean logic rules to every bit position independently. Shifts get lumped in for two key reasons:

• Hardware-wise, shifts are almost always implemented in the ALU alongside bitwise operations, sharing circuitry and control signals—so they’re part of the same functional block.
• Shifts are deeply tied to the same low-level bit-handling tasks (like packing/unpacking bits, aligning data, or speeding up arithmetic by shifting left to multiply by powers of two) as bitwise Boolean ops. Since they serve the same category of work, they’re grouped under the "logical operations" umbrella in architecture terminology.

This is a sharp contrast to programming languages, where "logical operators" (like && and ||) refer to short-circuiting boolean checks that return a single true/false value.

2. Why does Computer Organization and Design refer to bitwise operators like & and | as logical operators? #

The book is leaning into CPU architecture terminology rather than the programming language terminology you’re likely familiar with.

In nearly all instruction sets (including RISC-V, which the book covers extensively), the instructions that perform bitwise AND/OR/XOR are explicitly called logical instructions (e.g., RISC-V’s and, or, xor instructions). The book is connecting the programming language operators (&, |) directly to their corresponding hardware instructions.

The context from the book makes this clear: it’s talking about operations used for packing/unpacking bits and words—exactly the use case for these bitwise operators, which map one-to-one to the CPU’s logical instruction set. The book isn’t contradicting programming language terminology; it’s using the standard vocabulary for how the CPU executes these operations under the hood.

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