What is the Difference Between Stack Pointer and Program Counter

The main difference between stack pointer and program counter is that the stack pointer is a register that stores the address of the last program request in a stack while the program counter is a register that stores the address of the next instruction to be executed from the memory.

Registers are small storage units built into the CPU. They store data temporarily and help to increase the performance of the computer. The size of the registers can vary depending on the computer architecture. There are various types of registers, and they perform different operations. Two such registers are stack pointer and program counter.

Key Areas Covered

1. What is a Stack Pointer
     – Definition, Functionality
2. What is a Program Counter
     – Definition, Functionality
3. What is the Difference Between Stack Pointer and Program Counter
     – Comparison of Key Differences

Key Terms

CPU, Instruction Pointer, Program Counter, Register, Stack Pointer

What is a Stack Pointer

A stack is a data structure in computers. Inserting new items to the stack is called push whereas removing items from the stack is called pop. For example, assume that there are three elements as A, B and C. The first items pushed to the stack is A. Then B and C are added. B is on top of A while C is on top of A. Now the topmost element is C. When removing the items, C pops out first, then B and finally A. The last inserted items are popped out first. Therefore, a stack operates according to the last in-first out manner. This is the basic operation of a stack.

Figure 1: Basic Operation of a Stack

A stack pointer, or a stack register, is a small register that helps to handle the stack. It stores the address of the last program request. Here, the recently entered request resides at the top of the stack. When inserting a program request to the stack, the stack pointer first increments by one. Then, the request is pushed to the stack. When removing a program request from the stack, the requests first pops out of the stack. Then, the stack pointer decrements by one. Likewise, the stack pointer keeps track of the operations of the stack.

What is a Program Counter

A computer program instructs the CPU to perform a task. Thus, it consists of instructions. These instructions are in a sequence. The CPU fetches these instructions one after the other. A program counter is a register that holds the address of the next instruction to be executed. Instruction pointer, instruction address register, and instruction counter are some of its alternative names.

Each time when the CPU fetches an instruction, the program counter increases by one. After fetching an instruction, it points to the next instruction in the sequence. Resetting the computer will make the program counter value to zero.

Difference Between Stack Pointer and Program Counter

Definition

A stack pointer is a CPU register whose purpose is to keep track of a call stack. In contrast, a program counter is a CPU register that indicates where a computer is in its program sequence. These definitions explain the basic difference between stack pointer and program counter.

Synonyms

Stack pointer is also called a stack register while program counter is also called an instruction pointer, instruction address register, and instruction counter.

Functionality

Functionality is also a major difference between stack pointer and program counter. Stack pointer holds the address of the last program request in a stack while the program counter holds the address of the next instruction that should be executed.

Usage

While stack pointer tracks the operations of the stack, program counter helps to track the current execution point. Hence, this is another difference between stack pointer and program counter.

Conclusion

Stack pointer and program counter are two important registers. In conclusion, the main difference between stack pointer and program counter is that the stack pointer is a register that stores the address of the last program request in a stack while the program counter is a register that stores the address of the next instruction to be executed from the memory.

Reference:

1. “What Is Stack Pointer ? – Definition from WhatIs.com.” WhatIs.com, Available here.
2. “What Is Program Counter? – Definition from WhatIs.com.” WhatIs.com, Available here.

Image Courtesy:

1. “Lifo stack” By Maxtremus – Own work (CC0) via Commons Wikimedia
2. “MC6800 Processor Diagram” By Swtpc6800 en:User:Swtpc6800 Michael Holley – Own work (Public Domain) via Commons Wikimedia