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21CS208 – Operating Systems Unit-IVVelammal College of Engineering and Technology, Madurai
Department of Information Technology21CS208 – OPERATING SYSTEMS
UNIT IV - STORAGE MANAGEMENT
Memory Management: Main Memory – Swapping, Contiguous Memory Allocation, Paging,
Structure of the Page Tables, Segmentation. Virtual Memory: Demand paging, Page
Replacement,Allocation,Thrashing; Allocating Kernel Memory, OS Examples.
----------------------------------------------************------------------------------------------------
Text Book: Abraham Silberschatz, Peter Baer Galvin and Greg Gagne, “Operating System
Concepts”, 9thEdition,John Wiley and Sons Inc., 2018.
4.1 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
Address binding of instructions and data to memory addresses can happen at three different
stages.
• 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
1
,21CS208 – Operating Systems Unit-IV
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.
o Logical address – generated by the CPU; also referred to as virtual address.
o 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.
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, 21CS208 – Operating Systems Unit-IV
• 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.
• 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
4.2 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;
lower-priority 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.
Schematic View of Swapping
3
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