In computer architecture, which term describes the elementary operation that is executed directly on data stored in CPU registers?

Introduction / Context:
In computer architecture and organization, registers are the fastest storage locations inside the central processing unit CPU. Operations that act directly on the contents of these registers are given a special name. Understanding this term is important because textbooks often describe how complex instructions are internally broken down into smaller, primitive actions performed on registers.

Given Data / Assumptions:

• We are working in the context of CPU architecture and register operations.
• The question asks for the name of the operation that is executed on data stored in registers.
• Options include bit-operation, macro-operation, micro-operation, byte-operation and word-operation.
• We assume standard terminology as used in computer organization courses.

Concept / Approach:
In this context, the correct term is micro-operation. A micro-operation is a very small and basic operation performed on the information stored in registers, such as loading, clearing, incrementing, shifting or adding register contents. These micro-operations are the building blocks used by the control unit to implement machine instructions. The words bit-operation, byte-operation and word-operation describe the granularity of data, while macro-operation usually refers to a higher level sequence of steps rather than a primitive action.

Step-by-Step Solution:
Step 1: Recall that a register holds binary data within the CPU and that the control unit manipulates this data through very small steps. Step 2: Each primitive step such as transferring data from one register to another, incrementing a register or performing a logical operation on register contents is called a micro-operation. Step 3: Bit-operation refers to operations that act on individual bits, such as bitwise AND or OR, but the standard architectural term for register level primitives is micro-operation. Step 4: Macro-operation suggests a larger or composite operation, often representing a whole instruction or a group of micro-operations, not the fundamental register action itself. Step 5: Byte-operation and word-operation describe operations on bytes or words of data, but they do not specifically refer to the control unit primitives defined in computer organization. Step 6: Therefore, among the options, micro-operation is the correct name for an operation executed on data stored in registers.

Verification / Alternative check:
Standard computer organization textbooks define a micro-operation as an elementary operation performed on the information stored in one or more registers. Examples include register transfer micro-operations, arithmetic micro-operations and logical micro-operations. These definitions match the wording of the question, which asks about operations on data in registers, confirming that micro-operation is the proper term.

Why Other Options Are Wrong:
Bit-operation focuses on the size of the data unit, not on the architectural role of the operation. Macro-operation is the opposite of a micro-operation and usually refers to a higher level sequence of steps. Byte-operation and word-operation describe how large the operands are but not the fundamental control step inside the CPU. None of these alternatives capture the notion of elementary register level actions as accurately as micro-operation.

Common Pitfalls:
Learners sometimes confuse micro-operation with microinstruction or macro-operation. A helpful way to remember is to think of micro- as meaning very small. Micro-operations are the smallest actions the control unit performs on registers, while a microinstruction may specify several micro-operations, and a macro-operation may represent an entire instruction. Keeping this hierarchy in mind reduces confusion in exam questions.

Final Answer:
The operation executed directly on data stored in registers is called a micro-operation.