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Paged Memory Allocation: Definition, Purpose & Structure

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  • 0:04 What Is Memory Paging?
  • 0:41 Memory Structure
  • 1:40 Memory Page Allocation
  • 2:28 Internal Fragmentation
  • 3:53 Demand and Page Tables
  • 5:19 Lesson Summary
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Lesson Transcript
Instructor: Lyna Griffin

Lyna has tutored undergraduate Information Management Systems and Database Development. She has a Bachelor's degree in Electrical Engineering and a Masters degree in Information Technology.

In this lesson we will discuss the concept of paged memory allocation. This technique is used by operating systems to share memory among their many executing tasks. We will outline memory paging, internal fragmentation and demand page memory allocation. We will finally discuss how the OS keeps track of pages in memory using page tables.

What Is Memory Paging?

Memory paging is a memory management technique used by the operating system (OS) to manage how a computer's memory resources are shared. Processes being executed are done so within memory.

Paged memory allocation is the process of storing a portion of an executing process on disk or secondary memory. Main memory, or RAM, has fast access times but comparatively low storage capacity. There are some programs that are too big to fit completely into the system's main memory. This is where paging comes into play. Let's look at how a program or task is managed when it's too large to run in main memory all at once.

Memory Structure

Memory is divided up into sections or chunks called frames. A frame is fixed in size and is a unit of physical memory. Frames are the placeholders or storage spaces for pages, the parts or chunks of an executing process or task and represent a logical unit of memory.

Think of memory as an ice tray. The compartments (frames), each hold a specific amount (memory allocation) of water. A full glass of water (the complete process or task) is poured into the compartments to break up its full amount. A frame can be greater or equal in capacity to a page size.

Our computing systems today are built to multitask. Every program or application on a system cannot be executed without the use of the main memory. If memory was just one large single compartment of space, only one task would be executed at a time. To enable multitasking, memory is partitioned or compartmentalized into frames. Processes requesting memory for execution are allocated to suitable frames in memory.

memory structure

Memory Page Allocation

Consider Tasks A, B, and C that need to be loaded into memory for execution. We have a total memory space sized at 32Mb. Memory frames have been partitioned into 4Mb compartments. An area is reserved exclusively for the operating system.

Task A requires 8Mb of memory for execution. This means that task A will have to be allocated two 4Mb frames. This is indicated by pages A0 and A1 in main memory.

Task B requires 11Mb of memory for execution. This means Task B will have to be allocated three 4Mb frames denoted by pages B0, B1, and B2, to accommodate its size.

Task C requires less than 4Mb, but will have to occupy one 4MB frame, shown as page C0

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