Physical vs. Logical Address Space in Memory Management: Understanding Memory Addressing in Operating Systems

Compare and contrast physical and logical address spaces in memory management. This guide clarifies how these address spaces represent memory locations (actual RAM vs. program's view), their relationship in virtual memory systems, and the role of address translation (mapping logical addresses to physical addresses).

Physical vs. Logical Address Space in Memory Management

Understanding Address Spaces

In computer systems, the terms "physical address space" and "logical address space" refer to different ways of representing memory locations. The physical address space represents the actual RAM (Random Access Memory) installed in the system, while the logical address space represents the memory space as seen by a program. Understanding the difference between these address spaces is critical for comprehending how operating systems manage memory, especially in systems that implement virtual memory.

Physical Address Space

The physical address space is the total amount of RAM (main memory) available in the system. A program's instructions can only access data in physical memory. A process's size (its memory requirement) must be less than or equal to the available physical memory. The number of bits needed to represent a physical address depends on the size of the physical memory.

Calculating Physical Address Size

Let's consider an example:

  • Physical address space = 64KB = 26 KB = 26 * 210 bytes = 216 bytes
  • Word size = 8 bytes = 23 bytes

Therefore, the physical address space in words is (216) / (23) = 213 words. The number of bits needed for a physical address is log2(213) = 13 bits.

Logical Address Space

The logical address space is the amount of memory a process *believes* it has. This might be larger than the physical RAM available because of virtual memory. The process uses logical addresses to refer to memory locations. The size of the logical address space is determined by the size of the process.

Calculating Logical Address Size

Here's an example:

  • Logical address space = 128MB = 27 * 220 bytes = 227 bytes
  • Word size = 4 bytes = 22 bytes

Therefore, the logical address space in words is (227) / (22) = 225 words. The number of bits for a logical address is log2(225) = 25 bits.

What is a Word?

A word is the basic unit of data that a CPU can process in a single operation. It's a fixed-size collection of bits. The word size is defined by the system's architecture. It is dependent on the number of bits in the address input to the decoder and the number of memory locations that result from the decoding process (2n for an n-bit address).