Question

For Exercises 1-15, mark the answers true or false as follows: A. True B. False The program counter and the instruction register are two names for the same place.

Short answer

B. False

Step-by-step solution

Identify Key Components

First, let's identify what a program counter and an instruction register are. The program counter is a register in the CPU that contains the address of the next instruction to be executed. The instruction register, on the other hand, holds the currently executing instruction.

Compare the Definitions

Now, compare the definitions: The program counter only deals with addresses and instruction sequencing, while the instruction register holds the actual instruction being executed. These components serve different purposes.

Determine the Truth Value

Since the program counter and the instruction register have different functions and purposes in a CPU, they are not the same. Therefore, the statement claiming they are two names for the same place is false.

Key concepts

Understanding the Program Counter

The program counter (PC) is a vital component in a computer's CPU that helps it to keep track of the execution sequence of instructions. It acts like a roadmap for the CPU, telling it which instruction it needs to execute next. This register only contains the address of these instructions, not the instructions themselves.
When the CPU executes a program, the program counter points to the memory location of the current instruction that is being executed. After the instruction is completed, the program counter is updated to point to the next instruction. This update happens automatically, ensuring that the CPU executes instructions in the correct order.

• Holds the memory address of the next instruction.
• Automatically updates after each instruction execution.
• Ensures the CPU knows what instruction to fetch next.

Without the program counter, the CPU could not function efficiently, as it would not know the order or location of instructions, leading to execution errors.

Decoding the Instruction Register

The instruction register (IR) is another crucial register located within the CPU, playing a distinct but equally important role in instruction processing. It serves as the temporary home for the instruction that is currently being executed by the CPU.
When the CPU fetches an instruction from memory, it loads it into the instruction register, where the instruction decoding process occurs. This register temporarily holds the instruction so that the CPU can analyze and understand what action it needs to perform.

• Holds the instruction being executed at any given moment.
• Facilitates the decoding of instructions.
• Temporary storage during the instruction execution process.

Understanding the instruction register's role makes it clear that it works closely with the program counter, although they have different purposes.

The Role of CPU Registers

CPU registers are small, fast storage locations found within the CPU. Besides the program counter and instruction register, CPUs typically have several other registers to enhance processing efficiency. These registers serve as the working memory of the CPU, temporarily storing data and instructions needed during computation.
Registers come in various types, each serving a specific function:

• Accumulator Register: Typically stores intermediate results of calculations.
• General Purpose Registers: Used to hold data and addresses being processed currently.
• Status Register: Keeps track of conditions, such as carry or overflow during arithmetic operations.

These registers speed up computation by minimizing the time the CPU spends accessing slower memory like RAM. They are unique due to their ability to read and write data incredibly quickly.

The Process of Instruction Execution

Instruction execution is the heart of computer operations, involving several steps that the CPU carries out to perform tasks. Execution involves fetching, decoding, and executing the instruction, which takes place in cycles known as the fetch-decode-execute cycle.
Here's a simple breakdown:

• Fetch: The CPU reads the next instruction from memory using the program counter.
• Decode: The fetched instruction is placed into the instruction register, where it's decoded to determine what must be done.
• Execute: The CPU performs the instruction, such as arithmetic operations, moving data, or branching operations.

After execution, the cycle repeats with the program counter moving to the next instruction address. This repetitive cycle allows the CPU to perform complex operations seamlessly, which might appear instantaneous to users.