OS Memory Management: Logical vs. Physical Address Space

 Memory Management Concepts:

In a mono programming or uni-programming system, the main memory is divided into two parts, one part for OS and another for a job that is currently executing. Consider the following figure:

Fig: Main Memory Partition

Partition 2 is allowed for user processes. But, some part of the partition 2 is wasted. It is indicated by Black lines in figure. In multiprogramming environment, the user space is divided into number of partitions. Each partition is for one process. The task of subdivision is carried out dynamically by the operating system. This task is known as “Memory Management”. The efficient memory management is possible with multiprogramming. As few processes are in main memory, the remaining processes are waiting for an IO and the processor will be idle. So, memory needs to be allocated efficiently to pack as many processes into the memory as possible.

 

 Logical vs. Physical Address Space:

 

An address generated by CPU is called a logical address. Whereas, an address generated by Memory Management Unit (MMU) is called physical address. For ex:  J1 is a program written by a user. The size of program is 100 KB. But, program is loaded in the main memory from 2100 to 2200 KB. This actual loaded address in main memory is called the physical address. The set of all logical addresses generated by a program referred to as a Logical Address Space. The set of physical address corresponding these logical addresses referred to as a Physical Address Space. In our ex: from 0 to 100 KB is the logical address space and 2100 to 2200 KB is the physical address space.

 

Physical address space = Logical address space + Contents of Relocation Register

2200 = 100 + 2100

Consider the following fig:

The runtime mapping from logical to physical address is done by the Memory Management Unit (MMU), which is not a hardware device. The base register is also called the Relocation Register. The value in the relocation register is added to every address generated by a user process at the time it is sent to the memory. For ex: the base is at 2100, then an attempt by the user to access location 0 is dynamically related at location 2100. An access to location 100 is mapped to location 2200.