Page 130 - DCAP403_Operating System

Unit 7: Memory Management

operating system which reprogram the MMU to map the address to some physical memory, Notes
perhaps writing the old contents of that memory to disk and reading back from disk what the
memory should contain at the new logical address. In this case, the logical address may be
referred to as a virtual address.
Logical vs. Physical Address Space

An address generated by the CPU is commonly referred to as a logical address, whereas an
address seen by the memory unit – that is, the one loaded into the memory-address register of
the memory – is commonly referred to as a physical address.

The compile-time and load-time address-binding methods generate identical logical and physical
addresses. However, the execution-time address-binding scheme results in differing logical and
physical addresses. In this case, you usually refer to the logical address as a virtual address.
The set of all logical addresses generated by a program is a logical-address space; the set of all
physical addresses corresponding to these logical addresses is a physical-address space. Thus, in
the execution-time address-binding scheme, the logical- and physical-address spaces differ.

7.3 Swapping

Any operating system has a fixed amount of physical memory available. Usually, application
need more than the physical memory installed on your system, for that purpose the operating
system uses a swap mechanism: instead of storing data in physical memory, it uses a disk fi le.
Swapping is the act of moving processes between memory and a backing store. This is done to
free up available memory. Swapping is necessary when there are more processes than available
memory. At the coarsest level, swapping is done a process at a time.
To move a program from fast-access memory to a slow-access memory is known as “swap out”,

and the reverse operation is known as “swap in”. The term often refers specifically to the use of
a hard disk (or a swap file) as virtual memory or “swap space”.

When a program is to be executed, possibly as determined by a scheduler, it is swapped into
core for processing; when it can no longer continue executing for some reason, or the scheduler
decides its time slice has expired, it is swapped out again.

Figure 7.3: Memory Swapping

SWAP-IN

SWAP-OUT Operating
Swapping device system
(usually, a hard disk) Memory

Task Differentiate PROM and EPROM type memory.

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