Lecture 3 Memory Management

Memory ManagementMITM 205 Advanced Operating System Concepts

Mr. Alvin R. Malicdem

Professor

Memory Management Primary memory is a prime resource in

computer systems. Its management is an important function of

operating systems. In this area the operating system depends

heavily on hardware support, and the policies and data structures that are used are dictated by the support that is available.

DMMMSU-MLUC College of Information Technology, Center of Development in I.T. 2

Key Players in Memory Mgmt Compiler - structures the address space of an

application OS - maps the compiler’s structures onto the

hardware Hardware - performs the actual accesses to

memory locations


Mapping Memory Addresses Text Segment

this is the code, or machine instructions, of the application being executed, as created by the compiler

Data Segment contains predefined data structures, possibly

initialized before the program starts execution


Mapping Memory Addresses Heap

this is an area in memory used as a pool for dynamic memory allocation

Stack this is the area in memory used to store the

execution frames of functions called by the program

contain stored register values and space for local variables


Early Memory Management Most early systems are single-user

Batch systems One Program loaded in physical memory Runs to completion

Most memory managers implicitly allocate all memory to a user process

The main concern is protection of the OS from unauthorized access of the user


Modern Memory Management Multiprogramming systems - CPU being

transferred from one process to another Allows processes to occupy the memory at the

same time Allows processes whose memory requirement

exceeds that provided by the system Several processes are running and not all of them

will fit in memory


The Memory Hierarchy


Memory Binding (Data & Inst’n) 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


Memory Binding … cont. 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)


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 vs. Physical Address Space 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


Swapping Process currently occupying the memory will

have to give up its space to make way for higher priority process Usually those in I/O operation

Process: Write the core image of this process to the

backing store and the core center of the image of the higher priority process is read in


Schematic View of Swapping


Hierarchy of Storage devices


cachecache

main memorymain memory

secondary storagesecondary storage

Storage access time decreases

storage access speed increases

Storage capacity decreases

Program and data maybe referenced by the CPU directly

Programs and data must be moved to memory before they can be referenced by the CPU

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


Contiguous Allocation… cont. Relocation registers used to protect user

processes from each other, and from changing operating-system code and data Base register contains value of smallest physical

address Limit register contains range of logical addresses

– each logical address must be less than the limit register

MMU maps logical address dynamicallyDMMMSU-MLUC College of Information Technology, Center of Development in I.T. 18

Base and Limit Registers A pair of base and limit registers define the

logical address space


HW address protection with base and limit registers


Memory Allocation Fixed partitions Dynamic/Variable partitions


Partitioning Strategies – Fixed Fixed Partitions – divide memory into equal sized

pieces (except for OS) Degree of multiprogramming = number of partitions Simple policy to implement

All processes must fit into partition space Find any free partition and load the process

Problem – what is the “right” partition size? Process size is limited Internal Fragmentation – unused memory in a partition

that is not available to other processesDMMMSU-MLUC College of Information Technology, Center of Development in I.T. 22

Partitioning Strategies – Variable Idea: remove “wasted” memory that is not needed in

each partition Memory is dynamically divided into partitions based

on process needs Definition:

Hole: a block of free or available memory Holes are scattered throughout physical memory

New process is allocated memory from hole large enough to fit it


Variable Partitions More complex management problem

Must track free and used memory Need data structures to do tracking What holes are used for a process?

External fragmentation memory that is in holes too small to be usable by any process



OS

process 1

process 2

process 3

OS

process 1

process 3

Process 2Terminates

OS

process 1

process 3

Process 4Starts

process 4

Fragmentation External Fragmentation – total memory

space exists to satisfy a request, but it is not contiguous

Internal Fragmentation – allocated memory may be slightly larger than requested memory; this size difference is memory internal to a partition, but not being used


Fragmentation …cont. Reduce external fragmentation by

compaction Shuffle memory contents to place all free

memory together in one large block Compaction is possible only if relocation is

dynamic, and is done at execution time I/O problem

Batch job in memory while it is involved in I/O Do I/O only into OS buffers


Memory Allocation Mechanism MM system maintains data about free and allocated memory

alternatives Bit maps - 1 bit per “allocation unit” Linked Lists - free list updated and coalesced when not allocated to a

process At swap-in or process create

Find free memory that is large enough to hold the process Allocate part (or all) of memory to process and mark remainder as

free Compaction

Moving things around so that holes can be consolidated Expensive in OS time


Memory Management Policies Policy examples

First Fit: scan free list and allocate first hole that is large enough – fast

Next Fit: start search from end of last allocation Best Fit: find smallest hole that is adequate –

slower and lots of fragmentation Worst fit: find largest hole In general, First Fit is the winner


Paging Logical address space of a process can be

noncontiguous; process is allocated physical memory whenever the latter is available

Divide physical memory into fixed-sized blocks called frames (size is power of 2, between 512 bytes and 8,192 bytes)


Paging Divide logical memory into blocks of same

size called pages Keep track of all free frames To run a program of size n pages, need to find

n free frames and load program Set up a page table to translate logical to

physical addresses Internal fragmentation


Address Translation Scheme Address generated by CPU is divided into:

Page number (p) – used as an index into a page table which contains base address of each page in physical memory

Page offset (d) – combined with base address to define the physical memory address that is sent to the memory unit


page number page offset

d

m - n n

Paging Hardware


Paging Model of Logical and Physical Memory


Segmentation Memory-management scheme that supports user

view of memory A program is a collection of segments. A segment is

a logical unit such as:main program,

procedure, function,method,object,local variables, global variables,common block,stack,symbol table, arrays


User’s View of a Program


1

4

2

3

1

3

2

4

user space physical memory space

Segmentation Logical address consists of a pair:

<segment-number, offset> Segment table – maps two-dimensional

physical addresses; each table entry has: Base: contains the starting physical address

where the segments reside in memory. Limit: specifies the length of the segment.


Segment Look-up


segment 0

segment 1

segment 2

segment 3

segment 4

physical memory

segment #

+

virtual address

<?

raise protection fault

no

yes

offset

baselimit

Segment register tableIndex to segment register table

Physical

Address

Segmentation Hardware


Segmentation Example


END OF LECTUREMITM 205 Advanced Operating Systems Concepts


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Lecture 3 Memory Management