Question

what is the physical ,virtual and logical memory

Answer 1

Physical Memory

The physical memory is the memory available in the system in the form of memory cards in various slots of system. The physical address space is associated with this type of memory. The Physical address space across the all the memory cards is handled by kernel VMM.

Real Memory

The Real Memory is same as Physical memory if the system is non-partitioned. But if the system is partitioned then these two types are different. Now this memory acts as physical memory for that particular partition and the address space will be different for all the partitions with in the same system.

Logical Memory

The Logical Memory comes into picture only when the system is partitioned. The operating systems like AIX, Linux etc have the following requirements:

The address space should start from zero.
The address space should be contiguous.
The Logical memory concept supports both of these. The logical memory represents non-contiguous memory in the form of contiguous memory.

The following benefits are achieved:

The partition is isolated and secured from physical memory address space.
It simplifies the monitoring of partitions by POWER Hypervisor.

Virtual Memory

Sometimes the memory available for operating systems is not enough to carry out all the processes. So, operating system uses the disk space as memory. Thus operating system is tricked to think that it has more memory than it actually has. Some of the virtual memory is mapped to the physical memory and rest is divided by page size (often 4kB). These pages are mapped to the disk blocks. The address translation is handled by kernel VMM.

Answer 2

http://unix.worldiswelcome.com/physical-real-virtual-and-logical-memory