Unit-3: Operating system lecture notes (Memeory Management) P.R.Engineering college

Unit-3: Operating system lecture notes (Memeory Management) in Operating System uploaded at P.R.Engineering college

Unit-3: Operating system lecture notes (Memeory Management) in Operating System at P.R.Engineering college P.R.Engineering college Staff uploaded in lecturenotes.net

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UNIT III
Memory Management
Background
 Program must be brought into memory and placed within a process for it to be run.
 Input queue – collection of processes on the disk that are waiting to be brought into memory 
to run the program.
 User programs go through several steps before being run. 
Binding of Instructions and Data to Memory
 Compile time: If memory location known a priori, absolute code can be generated; must 
recompile code if starting location changes.
 Load time: Must generate relocatable code if memory location is not known at compile time.
 Execution time: Binding delayed until run time if the process can be moved during its 
execution from one memory segment to another. Need hardware support for address maps 
(e.g., base and limit registers). 
Multistep Processing of a User Program  Memeory Management UNIT III
Operating systems
Page 2
Logical vs. Physical Address Space
 The concept of a logical address space that is bound to a separate physical address space is 
central to proper memory management.
 Logical address – generated by the CPU; also referred to as virtual address.
 Physical address – address seen by the memory unit.
 Logical and physical addresses are the same in compile-time and load-time address-binding 
schemes; logical (virtual) and physical addresses differ in execution-time address-binding 
scheme.
Memory-Management Unit (MMU)
 Hardware device that maps virtual to physical address.
 In MMU scheme, the value in the relocation register is added to every address generated by a 
user process at the time it is sent to memory.
 The user program deals with logical addresses; it never sees the real physical addresses.
Dynamic relocation using a relocation register
Dynamic Loading
 Routine is not loaded until it is called
 Better memory-space utilization; unused routine is never loaded.
 Useful when large amounts of code are needed to handle infrequently occurring cases.
 No special support from the operating system is required implemented through program 
design.
Dynamic Linking
 Linking postponed until execution time.
 Small piece of code, stub, used to locate the appropriate memory-resident library routine.
 Stub replaces itself with the address of the routine, and executes the routine.
 Operating system needed to check if routine is in processes’ memory address. Memeory Management UNIT III
Operating systems
Page 3
 Dynamic linking is particularly useful for libraries.
Overlays
 Keep in memory only those instructions and data that are needed at any given time.
 Needed when process is larger than amount of memory allocated to it.
 Implemented by user, no special support needed from operating system, programming design 
of overlay structure is complex
Overlays for a Two-Pass Assembler
Swapping
 A process can be swapped temporarily out of memory to a backing store, and then brought 
back into memory for continued execution.
 Backing store – fast disk large enough to accommodate copies of all memory images for all 
users; must provide direct access to these memory images.
 Roll out, roll in – swapping variant used for priority-based scheduling algorithms; lowerpriority process is swapped out so higher-priority process can be loaded and executed.
 Major part of swap time is transfer time; total transfer time is directly proportional to the 
amount of memory swapped.
 Modified versions of swapping are found on many systems, i.e., UNIX, Linux, and Windows. Memeory Management UNIT III
Operating systems
Page 4
Schematic View of Swapping
Contiguous Allocation
 Main memory usually into two partitions:
 Resident operating system, usually held in low memory with interrupt vector.
 User processes then held in high memory.
 Single-partition allocation
 Relocation-register scheme used to protect user processes from each other, and from 
changing operating-system code and data.
 Relocation register contains value of smallest physical address; limit register contains 
range of logical addresses – each logical address must be less than the limit register. 
Hardware Support for Relocation and Limit Registers

Title:
Unit-3: Operating system lecture notes (Memeory Management)

Subject:
Operating System

Institute:
P.R.Engineering college

Unit-3: Operating system lecture notes (Memeory Management) P.R.Engineering college

Unit-3: Operating system lecture notes (Memeory Management) in Operating System uploaded at P.R.Engineering college

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