Chapter 9: Main Memory

Dr. Shadi Banitaan

Operating System Concepts – 10th Edition Silberschatz, Galvin and Gagne ©2018
Chapter 9: Memory Management
 Background
 Contiguous Memory Allocation
 Paging
 Structure of the Page Table
 Swapping

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Objectives

 To provide a detailed description of various ways of organizing memory

hardware
 To discuss various memory-management techniques

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Background
 Program must be brought (from disk) into memory and placed within a
process for it to be run
 Main memory and registers are only storage CPU can access directly
 Register access is done in one CPU clock (or less)
 Main memory can take many cycles, causing a stall
 Cache sits between main memory and CPU registers
 Protection of memory required to ensure correct operation

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Protection
 Need to ensure that a process can access only those addresses in
its address space.
 We can provide this protection by using a pair of base and limit
registers define the logical address space of a process

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Hardware Address Protection
 CPU must check every memory access generated in user mode to
be sure it is between base and limit for that user

 The instructions to load the base and limit registers are privileged

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Address Binding
 Addresses represented in different ways at different stages of a
program’s life
• Source code addresses usually symbolic
• Compiled code addresses bind to relocatable addresses
 i.e., “14 bytes from beginning of this program”
• Linker or loader will bind relocatable addresses to absolute
addresses
 i.e., 74014
• Each binding maps one address space to another

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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)

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Multistep Processing of a User Program

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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
 Logical address space is the set of all logical addresses generated
by a program
 Physical address space is the set of all physical addresses
generated by a program

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Memory-Management Unit (MMU)
 Hardware device that at run time maps virtual to physical address

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Memory-Management Unit (Cont.)
 Consider simple scheme. which is a generalization of the base-
register scheme.
 The base register now called relocation register
 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
• Execution-time binding occurs when reference is made to location
in memory
• Logical address bound to physical addresses

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Memory-Management Unit (Cont.)
 Consider simple scheme. which is a generalization of the base-
register scheme.
 The base register now called relocation register
 The value in the relocation register is added to every address
generated by a user process at the time it is sent to memory

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Dynamic Loading
 The entire program does need to be in memory to execute
 Routine is not loaded until it is called
 Better memory-space utilization; unused routine is never loaded
 All routines kept on disk in relocatable load format
 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
• OS can help by providing libraries to implement dynamic
loading

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Contiguous Allocation
 Main memory must support both OS and user processes
 Limited resource, must allocate efficiently
 Contiguous allocation is one early method
 Main memory usually into two partitions:
• Resident operating system, usually held in low memory with
interrupt vector
• User processes then held in high memory
• Each process contained in single contiguous section of memory

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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 addresses dynamically

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Hardware Support for Relocation and Limit Registers

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Variable Partition
 Multiple-partition allocation
• Degree of multiprogramming limited by number of partitions
• Variable-partition sizes for efficiency (sized to a given process’ needs)
• Hole – block of available memory; holes of various size are scattered
throughout memory
• When a process arrives, it is allocated memory from a hole large enough to
accommodate it
• Process exiting frees its partition, adjacent free partitions combined
• Operating system maintains information about:
a) allocated partitions b) free partitions (hole)

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Dynamic Storage-Allocation Problem
How to satisfy a request of size n from a list of free holes?

 First-fit: Allocate the first hole that is big enough

 Best-fit: Allocate the smallest hole that is big enough; must
search entire list, unless ordered by size
• Produces the smallest leftover hole
 Worst-fit: Allocate the largest hole; must also search entire list

• Produces the largest leftover hole

First-fit and best-fit better than worst-fit in terms of speed and storage
utilization

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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
 First fit analysis reveals that given N blocks allocated, 0.5 N blocks
lost to fragmentation
• 1/3 may be unusable -> 50-percent rule

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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

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Paging
 Physical address space of a process can be noncontiguous;
process is allocated physical memory whenever the latter is
available
• Avoids external fragmentation
• Avoids problem of varying sized memory chunks
 Divide physical memory into fixed-sized blocks called frames
• Size is power of 2, between 512 bytes and 16 Mbytes
 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
 Still have Internal fragmentation

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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

p d
m -n n
• For given logical address space 2m and page size 2n

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Paging Hardware

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Paging Model of Logical and Physical Memory

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Paging Example
 Logical address: n = 2 and m = 4. Using a page size of 4 bytes and
a physical memory of 32 bytes (8 pages)

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Paging -- Calculating internal fragmentation
 Page size = 2,048 bytes
 Process size = 72,766 bytes
 35 pages + 1,086 bytes
 Internal fragmentation of 2,048 - 1,086 = 962 bytes
 Worst case fragmentation = 1 frame – 1 byte
 On average fragmentation = 1 / 2 frame size
 So small frame sizes desirable?
 But each page table entry takes memory to track
 Page sizes growing over time
• Solaris supports two page sizes – 8 KB and 4 MB

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Free Frames

Before allocation After allocation

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Implementation of Page Table
 Page table is kept in main memory
• Page-table base register (PTBR) points to the page table
• Page-table length register (PTLR) indicates size of the page
table
 In this scheme every data/instruction access requires two memory
accesses
• One for the page table and one for the data / instruction
 The two-memory access problem can be solved by the use of a special
fast-lookup hardware cache called translation look-aside buffers
(TLBs) (also called associative memory).

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Translation Look-Aside Buffer
 Some TLBs store address-space identifiers (ASIDs) in each
TLB entry – uniquely identifies each process to provide address-
space protection for that process
• Otherwise need to flush at every context switch
 TLBs typically small (64 to 1,024 entries)
 On a TLB miss, value is loaded into the TLB for faster access
next time
• Replacement policies must be considered
• Some entries can be wired down for permanent fast access

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Hardware
 Associative memory – parallel search
P a ge # F ra m e #

 Address translation (p, d)

• If p is in associative register, get frame # out
• Otherwise get frame # from page table in memory

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Paging Hardware With TLB

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Effective Access Time
 Hit ratio – percentage of times that a page number is found in the TLB
 An 80% hit ratio means that we find the desired page number in the
TLB 80% of the time.
 Suppose that 10 nanoseconds to access memory.
• If we find the desired page in TLB then a mapped-memory access
take 10 ns
• Otherwise we need two memory access so it is 20 ns
 Effective Access Time (EAT)
EAT = 0.80 x 10 + 0.20 x 20 = 12 nanoseconds
implying 20% slowdown in access time
 Consider amore realistic hit ratio of 99%,
EAT = 0.99 x 10 + 0.01 x 20 = 10.1ns
implying only 1% slowdown in access time.

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Memory Protection
 Memory protection implemented by associating protection bit with
each frame to indicate if read-only or read-write access is allowed
• Can also add more bits to indicate page execute-only, and so on
 Valid-invalid bit attached to each entry in the page table:
• “valid” indicates that the associated page is in the process’ logical
address space, and is thus a legal page
• “invalid” indicates that the page is not in the process’ logical
address space
• Or use page-table length register (PTLR)
 Any violations result in a trap to the kernel

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Valid (v) or Invalid (i) Bit In A Page Table

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Shared Pages
 Shared code
• One copy of read-only (reentrant) code shared among processes
(i.e., text editors, compilers, window systems)
• Similar to multiple threads sharing the same process space
• Also useful for interprocess communication if sharing of read-write
pages is allowed
 Private code and data
• Each process keeps a separate copy of the code and data
• The pages for the private code and data can appear anywhere in
the logical address space

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Shared Pages Example

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Structure of the Page Table
 Memory structures for paging can get huge using straight-forward
methods
• Consider a 32-bit logical address space as on modern computers
• Page size of 4 KB (212)
• Page table would have 1 million entries (232 / 212)
• If each entry is 4 bytes  each process 4 MB of physical address
space for the page table alone
 Don’t want to allocate that contiguously in main memory
• One simple solution is to divide the page table into smaller units
 Hierarchical Paging
 Hashed Page Tables
 Inverted Page Tables

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Hierarchical Page Tables
 Break up the logical address space into multiple page tables
 A simple technique is a two-level page table
 We then page the page table

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Two-Level Paging Example
 A logical address (on 32-bit machine with 4K page size) is divided into:
• a page number consisting of 20 bits
• a page offset consisting of 12 bits
 Since the page table is paged, the page number is further divided into:
• a 10-bit page number
• a 10-bit page offset
 Thus, a logical address is as follows:

 where p1 is an index into the outer page table, and p2 is the

displacement within the page of the inner page table
 Known as forward-mapped page table

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Address-Translation Scheme

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64-bit Logical Address Space
 Even two-level paging scheme not sufficient
 If page size is 4 KB (212)
• Then page table has 252 entries
• If two level scheme, inner page tables could be 210 4-byte entries
• Address would look like

• Outer page table has 242 entries or 244 bytes

• One solution is to add a 2nd outer page table
• But in the following example the 2nd outer page table is still 234
bytes in size
 And possibly 4 memory access to get to one physical memory
location

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Three-level Paging Scheme

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Hashed Page Tables
 Common in address spaces > 32 bits
 The virtual page number is hashed into a page table
• This page table contains a chain of elements hashing to the same
location
 Each element contains (1) the virtual page number (2) the value of the
mapped page frame (3) a pointer to the next element
 Virtual page numbers are compared in this chain searching for a
match
• If a match is found, the corresponding physical frame is extracted
 Variation for 64-bit addresses is clustered page tables
• Similar to hashed but each entry refers to several pages (such as
16) rather than 1
• Especially useful for sparse address spaces (where memory
references are non-contiguous and scattered)

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Hashed Page Table

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Inverted Page Table
 Rather than each process having a page table and keeping track of all
possible logical pages, track all physical pages
 One entry for each real page of memory
 Entry consists of the virtual address of the page stored in that real
memory location, with information about the process that owns that
page
 Decreases memory needed to store each page table, but increases
time needed to search the table when a page reference occurs
 Use hash table to limit the search to one — or at most a few — page-
table entries
• TLB can accelerate access
 But how to implement shared memory?
• One mapping of a virtual address to the shared physical address

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Inverted Page Table Architecture

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End of Chapter 9

Dr. Shadi Banitaan

Operating System Concepts – 10th Edition Silberschatz, Galvin and Gagne ©2018