Chapter 9: Memory Management
Chapter 9: Memory Management
¡ö Background
¡ö Swapping
¡ö Contiguous Allocation
¡ö Paging
¡ö Segmentation
¡ö Segmentation with Paging
Operating System Concepts
9.1
Silberschatz, Galvin and Gagne ?2002
Background
¡ö Program must be brought into memory and placed within
a process for it to be run.
¡ö Input queue ¨C collection of processes on the disk that are
waiting to be brought into memory to run the program.
¡ö User programs go through several steps before being
run.
Operating System Concepts
9.2
Silberschatz, Galvin and Gagne ?2002
Binding of Instructions and Data to Memory
Address binding of instructions and data to memory addresses can
happen at three different stages.
¡ö Compile time: If memory location known a priori,
absolute code can be generated; must recompile code if
starting location changes.
¡ö Load time: Must generate relocatable code if memory
location is not known at compile time.
¡ö 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).
Operating System Concepts
9.3
Silberschatz, Galvin and Gagne ?2002
Multistep Processing of a User Program
Operating System Concepts
9.4
Silberschatz, Galvin and Gagne ?2002
Logical vs. Physical Address Space
¡ö The concept of a logical address space that is bound to a
separate physical address space is central to proper
memory management.
? Logical address ¨C generated by the CPU; also referred to as
virtual address.
? Physical address ¨C address seen by the memory unit.
¡ö 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.
Operating System Concepts
9.5
Silberschatz, Galvin and Gagne ?2002
Memory-Management Unit (MMU)
¡ö Hardware device that maps virtual to physical address.
¡ö 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.
¡ö The user program deals with logical addresses; it never
sees the real physical addresses.
Operating System Concepts
9.6
Silberschatz, Galvin and Gagne ?2002
Dynamic relocation using a relocation register
Operating System Concepts
9.7
Silberschatz, Galvin and Gagne ?2002
Dynamic Loading
¡ö Routine is not loaded until it is called
¡ö Better memory-space utilization; unused routine is never
loaded.
¡ö Useful when large amounts of code are needed to handle
infrequently occurring cases.
¡ö No special support from the operating system is required
implemented through program design.
Operating System Concepts
9.8
Silberschatz, Galvin and Gagne ?2002
Dynamic Linking
¡ö Linking postponed until execution time.
¡ö Small piece of code, stub, used to locate the appropriate
memory-resident library routine.
¡ö Stub replaces itself with the address of the routine, and
executes the routine.
¡ö Operating system needed to check if routine is in
processes¡¯ memory address.
¡ö Dynamic linking is particularly useful for libraries.
Operating System Concepts
9.9
Silberschatz, Galvin and Gagne ?2002
Overlays
¡ö Keep in memory only those instructions and data that are
needed at any given time.
¡ö Needed when process is larger than amount of memory
allocated to it.
¡ö Implemented by user, no special support needed from
operating system, programming design of overlay
structure is complex
Operating System Concepts
9.10
Silberschatz, Galvin and Gagne ?2002
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