Difference between Logical Address and Physical Address

What is Physical address? 

A physical address, in computing, refers to the actual location of data in the computer's physical memory or RAM (Random Access Memory). Unlike logical addresses, which are used by programs to reference memory locations in an abstract manner, physical addresses directly correspond to specific locations in the computer's physical memory modules.

When a program executes and accesses memory, the CPU generates logical addresses that are translated by the memory management unit (MMU) into physical addresses before data can be read from or written to memory. This translation process involves mapping logical addresses to their corresponding physical addresses based on the memory management scheme in use, such as paging or segmentation.

Important Points to Remember

Here are some important points to remember about logical and physical addresses in computing:

Logical Address:

1. Virtual Address Space: Logical addresses represent the virtual address space that a process sees. They are generated by the CPU during program execution.
2. Abstraction: Logical addresses provide an abstraction layer, allowing programs to reference memory locations independently of the underlying physical memory layout.
3. Used by Programs: Programs use logical addresses to access memory, assuming a contiguous and uniform memory space.
4. Managed by OS: Logical address translation is managed by the operating system and the memory management unit (MMU) hardware.
5. Allows Virtual Memory: Logical addresses facilitate the implementation of virtual memory, allowing for efficient memory management and multitasking.
    

Physical Address:

1. Actual Memory Location: Physical addresses represent the actual location of data in the physical memory or RAM.
2. Direct Access: Hardware components access data in memory using physical addresses.
3. Managed by Hardware: The translation of logical addresses to physical addresses is handled by the memory management unit (MMU) hardware in conjunction with the operating system.
4. Dependent on Hardware Configuration: Physical addresses depend on the installed hardware configuration, including the amount and layout of physical memory modules.
5. Limited by Hardware: Physical addresses are limited by the amount of physical memory installed in the system.

Differences between Logical Address and Physical Address

The key differences between logical address and physical address are:

What is a Memory Management Unit(MMU)? 

MMU or the memory management unit is a hardware device that maps a logical address to its corresponding physical address. Many memory mapping methods can be used for this purpose of mapping.

Dynamic relocation using relocation register

In the above example, the mapping of logical to physical address is achieved using a simple MMU scheme known as base-register scheme.

The value of the base register or ‘relocation register’ is added with the logical address generated by the CPU. The sum of these two values is the physical address located in memory. Here the logical address ‘125’ is added with relocation register value ‘14000’ to obtain the physical address of ‘14125’. 

Therefore we have two types of addresses, logical and physical addresses.

The range of logical addresses is  0 to MAX. 

While the physical address range is (R+0) to (R+MAX) for a particular base value R.
You can also read about - interleave memory.

Mapping Logical/Virtual Addresses to Physical Addresses

MMU or the memory-management unit, is a hardware device that maps a logical or virtual address to its corresponding physical address.

The mapping can be done in three ways which are discussed below:

1. Compile time: At the compile time, if it is known where the program will reside, it helps you generate the physical address. The compilation during the program execution generates the physical address
    
2. Load time: At the compile time, if it is not known where the program will reside, it helps you generate the reallocated address, which is when the compiler converts the symbolic addresses into the relative addresses
    
3. Run time binding: If the process can be moved from one memory to another memory during the execution, then is the runtime binding

MMU is essential for effectively managing memory addresses, which improves computer systems' flexibility and performance. Compile-time, load-time, and run-time binding are three techniques that enable dynamic adaptation to different program execution conditions, assuring optimal memory consumption.

Frequently Asked Questions 

What is a physical address example?

Physical address example: 0x0000FFFF (hexadecimal) or 65535 (decimal) - Represents a specific location in the computer's physical memory.

What is an example of a logical address?

Logical address example: 0xABCD1234 - Represents a virtual memory address generated by the CPU during program execution.

What are the 3 types of addresses in a network?

The network has three types of physical addresses: Unicast, Multicast, and Broadcast. Unicast is from one system to another system, Multicast is from one system to many systems, and the broadcast has to be received by every system in the network.

Is IP address physical or logical?

The IP address is the logical address. It is given by the software in the servers and can be changed from time to time. For Example, every time a new IP address is assigned to your laptop when you use a different hotspot.

Conclusion

In this blog, we discussed logical and physical addresses in Operating Systems. The CPU generates the logical address during program execution, and the physical address refers to an actual physical location located in the memory unit. 

If you are preparing for interviews, don’t forget to study Operating Systems - as it is one of the most asked topics in interviews.

Recommended Reading:

• Free Space Management in OS
• Secondary Memory
• Paging
• IEnumerable vs IQueryable

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