Lab Manual Operating System
Lab Manual for Operating System
Lab No. 11
Memory Management using Paging and
Segmentation
Objectives
To understand basic concept, working and usage of memory management
using paging and segmentation.
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Department of Software Engineering
Bahria University (Karachi Campus)
Lab Manual Operating System
LAB # 11
Memory Management using Paging and Segmentation
Paging
In computer operating systems, paging is one of the memory management schemes by which a
computer stores and retrieves data from the secondary storage for use in main memory. In the paging
memory-management scheme, the operating system retrieves data from secondary storage in samesize blocks called pages. Paging permits the physical address space of a process to be noncontiguous.
The basic method for implementing paging involves breaking physical memory into fixed-sized blocks
called frames and breaking logical memory into blocks of the same size called pages. When a process
is to be executed, its pages are loaded into any available memory frames from their source.
Algorithm
1. Input memory limit, page size, and number of processes.
2. Calculate the number of pages as memory limit/page size.
a. Output number of pages.
b. Initialize remaining pages with number of pages.
3. For every process
a. Input the number of pages required by the process.
b. If the requirement exceeds remaining pages, output “Not enough memory available.”
c. Decrement remaining pages by number of pages required.
d. Input page table for the process.
4. Input logical address (to find physical address), process number, page number, and offset.
5. If the process number exceeds number of processes, or the page number is greater than or equal
to the allocated pages, or the offset is greater than or equal to the page size, output “Invalid
entry.”
6. Else, calculate physical address by using the reference of the entry in page table and adding the
offset to it, and output the physical address.
Segmentation
Segmentation is another memory management technique in which the logical memory is broken
down into unequal sized blocks called segments. Similar to paging mechanism, segmentation allows
allocation of non-contiguous memory blocks to a program. A consequence of using unequal sized
blocks is that the relationship between logical and physical addresses no longer remains simple.
Algorithm
1.
Input memory limit, number of segments, limit and base address of each segment, and logical
address.
2. If memory limit is less than base address, then output “Invalid memory limit or address.”
3. For each segment
a. Input segment number and offset.
b. Input values for segment table and create it.
4. If the segment number and offset are valid, computer the physical address, and output it.
64
Department of Software Engineering
Bahria University (Karachi Campus)
Lab Manual Operating System
Memory Partitioning
One of the simplest methods for memory allocation is to divide memory into several partitions. Each
partition may contain exactly one process. In this multiple-partition method, when a partition is free, a
process is loaded into the free partition. When the process terminates, the partition becomes available
for another process. The operating system keeps a table indicating which parts of memory are
available and which are occupied. When a process arrives, and needs memory, a memory section large
enough for this process is provided.
Placement Algorithms
The OS designer must be clever in deciding how to assign processes to memory. When it is time to
load a process into main memory, and if there is more than one free block of memory of sufficient
size, then the operating system must decide which free block to allocate.
Best-Fit Algorithm
Best-fit chooses the block that is closest in size to the request.
Sample Code
#include<stdio.h>
#define max 20
void main()
{
int f[max],p[max],i,j,nf,np,temp,lowest=10000;
static int ff[max],pf[max];
printf("\nEnter the number of partitions:");
scanf("%d",&nf);
printf("Enter the number of processes:");
scanf("%d",&np);
printf("\nEnter the size of the partitions:-\n");
for(i=1;i<=nf;i++)
{
printf("Partition %d:",i);
scanf("%d",&f[i]);
}
printf("Enter the size of the processes :-\n");
for(i=1;i<=np;i++)
{
printf("Process %d:",i);
scanf("%d",&p[i]);
}
for(i=1;i<=np;i++)
{
for(j=1;j<=nf;j++)
{
if(ff[j]!=1)
{
temp=f[j]-p[i];
65
Department of Software Engineering
Bahria University (Karachi Campus)
Lab Manual Operating System
if(temp>=0)
{
if(lowest>temp)
{
pf[i]=j;
lowest=temp;
}
}
}
}
ff[pf[i]]=1;
lowest=10000;
}
printf("\nProcess No\tProcess Size \tPartition Size");
for(i=1;i<=np && pf[i]!=0;i++)
printf("\n%d\t\t%d\t\t%d \n",i,p[i],f[pf[i]]);
}
First-Fit Algorithm
First-fit begins to scan memory from the beginning and chooses the first available block that is large
enough.
Next-Fit Algorithm
Next-fit begins to scan memory from the location of the last placement and chooses the next available
block that is large enough.
Time Boxing
Activity Name
Login Systems + Setting up Linux Environment
Walk through Theory & Tasks
Implement Tasks
Evaluation Time
Activity Time
3 mints + 7 mints
60 mints
80 mints
30 mints
Total Duration
Total Time
10 mints
60 mints
80 mints
30 mints
180 mints
Objectives/Outcomes
•
To learn segmentation and how to implement it.
•
To learn paging and how to implement it.
•
To learn page replacement algorithms.
Lab Tasks/Practical Work
1. Following the code guideline of Best-Fit Placement algorithm, write the C language program for
First-Fit and Next-Fit Placement algorithms.
66
Department of Software Engineering
Bahria University (Karachi Campus)