Module IV Memory Organization Virtual Memory In a

Module IV Memory Organization

Virtual Memory • In a virtual memory system, the OS manages the long pro grams. • User programs can be larger than the physical main memory (2 GB). • OS stores the entire program on a hard disk whose capacity is much larger than the physical memory size.

Virtual Memory • At a given time, only some portions of the program are brought into the main memory from the hard disk. • As and when needed, a portion which is not in main memory is taken from the hard disk, and at the same time, a portion of the program which is present is released out of the main memory and stored on the hard disk. This process is known as swapping. • In a virtual memory system, when a program is executed, swapping is performed as per the require ment on a continuous basis.

Virtual Memory

Virtual Memory • The CPU fetches instruction and data from the main memory. • Whenever the required instruction or data is not present in main memory, the hardware raises an interrupt known as virtual memory interrupt or page fault. • In response, the operating system loads a section of the program (containing the required instruction or data) from the hard disk drive to the main memory. • The page fault occur within an instruction cycle. • After the page fault interrupt is serviced, the CPU will continue processing the partially executed instruction.

Virtual Memory

Advantages of Virtual Memory • Program size is not limited by physical memory size. • User do not have to do memory allocation. Main memory allocation is done automatically according to the demands of the program. • Program can be loaded in any area of physical memory • It allows processes to share files easily.

Virtual Memory • It is implemented using two techniques: – Segmentation – Paging

Segmentation • This memory management technique supports user’s view of memory.

User’s view of a program

Segmentation • Users prefer to view memory as a collection of variable sized segments : – Code : to store program – Data : variables defined by the code are stored – Stack : Used by sub routines and interrupt service routines to hold temporary data and addresses.

Segmentation • All applications need to be loaded into main memory before they can be executed. • In order to do so, a temporary 'segment' is created by the memory manager

A typical segment

Segmentation • There can be many segments in memory at the same time. • Each one is a separate process or application and each may be a different size

Segmentation • Segmentation allows the OS to do is to place each one of those segments in different parts of physical memory.

Segmentation

Segmentation • In segmentation, a memory reference includes a value that identifies a segment (segment number) and an offset within that segment. • A segment is associated with – a flag indicating whether it is present in main memory or not – information indicating where the segment is located in memory(the address of the first location in the segment)(base address) – has a set of permissions – Length

Segmentation • If the segment is not present in main memory, an exception is raised and the OS will read the segment into memory from secondary storage.

Segmentation • If a reference to a location within a segment is made, – Physical address = base address +offset • A memory management unit (MMU) is responsible for – translating a segment and offset (logical address) into a memory address, – checking to make sure the translation can be done and that the reference to that segment and offset is permitted.

Segmentation • The mapping is done with help of segment table. • Logical address consists of two parts: a segment number s, and offset d • s is used as an index to the segment table. • Each entry of segment table has base and limits. – base contains starting physical address where resides in memory – limit specifies length of the segments • d must be between 0 and the segment limit. If not, it traps to addressing error • If offset is valid, it is added to base to produce physical address

Segmentation: Hardware

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