Operating System Memory Management

Podcast

Play an AI-generated podcast conversation about this lesson

Questions and Answers

What is a primary purpose of memory management in an operating system?

To improve disk access speed

To maximize CPU processing power

To ensure a process can only access its specific address space (correct)

To decrease the number of CPU registers

Which system component directly interfaces with memory and registers?

Disk management unit

CPU (correct)

Cache memory

Input/Output controllers

In memory architecture, what is the role of cache memory?

To act as a replacement for main memory

To reduce the number of clock cycles needed for access (correct)

To store unused programs until needed

To directly load data from the disk into the CPU

What is one of the challenges posed by the use of main memory?

Accessing it can take many cycles, potentially causing a stall Signup and view all the answers

What does the term 'contiguous memory allocation' refer to?

Allocating a single block of memory for a process without fragmentation Signup and view all the answers

What is the calculation for internal fragmentation when a process size is 72,766 bytes and uses a page size of 2,048 bytes?

962 bytes Signup and view all the answers

Why are smaller frame sizes generally considered desirable?

They lead to less internal fragmentation. Signup and view all the answers

What does the page-table length register (PTLR) indicate?

The size of the page table. Signup and view all the answers

What is the role of the translation lookaside buffer (TLB) in a paging system?

To speed up the retrieval of page table entries. Signup and view all the answers

What is the worst-case fragmentation scenario in a paging system?

One frame minus one byte wasted. Signup and view all the answers

What is the primary function of the Memory-Management Unit (MMU)?

To map virtual addresses to physical addresses at runtime Signup and view all the answers

How does the logical address differ from the physical address during execution-time binding?

Logical addresses are generated by the CPU, whereas physical addresses are viewed by the memory unit. Signup and view all the answers

What happens when a user process generates an address?

The logical address is modified by the relocation register before use. Signup and view all the answers

In which address-binding scheme are logical and physical addresses equivalent?

Compile-time binding Signup and view all the answers

Which statement is true about logical address space?

It refers to addresses that are visible to user processes. Signup and view all the answers

What is the role of the relocation register in memory management?

To add a base address to user-generated logical addresses before accessing memory Signup and view all the answers

Which of the following best describes the physical address space?

The set of all physical addresses that can be utilized by a program Signup and view all the answers

Which statement best describes logical addresses?

They represent an abstract view of memory for user processes. Signup and view all the answers

What is the purpose of the relocation register in a computer system?

To add a constant value to user-generated addresses before accessing memory Signup and view all the answers

What advantage does dynamic loading provide in memory management?

It optimizes memory usage by only loading routines when needed. Signup and view all the answers

Which characteristic defines dynamic linking as opposed to static linking?

Dynamic linking allows libraries to be linked during execution. Signup and view all the answers

In contiguous allocation, what is the role of the limit register?

To restrict logical addresses to a set range for safety. Signup and view all the answers

How does the operating system maintain efficient memory allocation in a variable partition system?

By tracking both allocated and free partitions carefully. Signup and view all the answers

What is a characteristic of a hole in the context of variable partition allocation?

It is a block of available memory varying in size throughout memory. Signup and view all the answers

What occurs when a process exits in a variable-partition system?

The freed partition may be combined with adjacent free partitions. Signup and view all the answers

Which of the following statements about dynamic storage-allocation problem is true?

It addresses how to allocate a size $n$ from free holes efficiently. Signup and view all the answers

What role does the stub play in the dynamic linking process?

It locates the appropriate memory-resident library routine during execution. Signup and view all the answers

Which of the following is true about memory management through contiguous allocation?

User processes require contiguous sections of memory for efficient execution. Signup and view all the answers

What is the purpose of the TLB in the context of virtual memory management?

To hold translation table entries for fast hardware lookups. Signup and view all the answers

Which statement accurately describes the functionality of the hash tables in the inverted page table architecture?

They represent contiguous areas of mapped virtual memory. Signup and view all the answers

What happens when a virtual address reference results in a TLB miss?

The kernel is interrupted to search the hash table. Signup and view all the answers

What structural components make up the inverted page table architecture in the Oracle SPARC Solaris?

Two hash tables: one for the kernel and one for user processes. Signup and view all the answers

What role does the translation storage buffer (TSB) play in the memory address translation process?

It is a cache that includes entries for recently accessed pages. Signup and view all the answers

What does an entry in the hash table for virtual memory translate to?

A base address and span indicating the number of pages. Signup and view all the answers

What is the main goal of the architecture used in modern, tightly integrated hardware operating systems?

Achieving efficiency with low overhead. Signup and view all the answers

In the case of a TLB hit, what is the result of the lookup?

The CPU retrieves the physical memory address directly. Signup and view all the answers

What is a condition that typically triggers the process of swapping in an operating system?

When free memory is extremely low. Signup and view all the answers

How can context switch time be affected by swapping?

It can become very high if processes are not in memory. Signup and view all the answers

Which of the following methods do mobile operating systems typically use to manage low memory instead of swapping?

Voluntary memory relinquishment by apps. Signup and view all the answers

What can happen if a process is swapped out while pending I/O operations are occurring?

I/O would trigger on the wrong process. Signup and view all the answers

What does Intel's Page Address Extension (PAE) allow 32-bit applications to do?

Access more than 4GB of memory space. Signup and view all the answers

What is a common memory management method supported by both iOS and Android?

Paging. Signup and view all the answers

In the Intel IA-32 architecture, how many segments can a process have?

Up to 16K segments. Signup and view all the answers

Which component primarily handles the translation of logical addresses to physical addresses in the IA-32 architecture?

Memory Management Unit (MMU). Signup and view all the answers

What is a characteristic of modern operating systems regarding standard swapping?

Standard swapping is rarely used, and modified versions are common. Signup and view all the answers

In the context of context switch time, what is one method to potentially reduce the swapping component time?

Reduce the size of memory that is swapped. Signup and view all the answers

What is the practical limit of addressing implemented in contemporary Intel x86-64 systems?

48 bits for virtual addresses. Signup and view all the answers

What major advantage does a two-level translation lookaside buffer (TLB) provide in the ARM architecture?

Faster address translation. Signup and view all the answers

What is the primary storage problem for mobile systems that limits their use of swapping?

Limited write cycles in flash memory. Signup and view all the answers

Study Notes

Main Memory

Main memory supports both operating system and user processes
Memory is a limited resource, so efficient allocation is crucial
Contiguous allocation is one early approach
Memory is divided into partitions: resident OS (low memory, interrupt vector), user processes (high memory)
Each process is in its own contiguous section

Protection

Processes need protection from each other and the OS
Base and limit registers define a process' logical address space
The CPU checks every memory access to ensure it's within the base/limit range
Instructions to load those registers are usually privileged

Address Binding

Programs can be bound to memory addresses at different stages: compile time, load time, or execution time
- Compile time: Addresses known at compile time.
- Load time: Addresses known at load time.
- Execution time: Addresses are determined at run time.
Binding methods map symbolic addresses to absolute addresses

Multistep Processing

Source program/code is first compiled, or assembled
Then object modules are written
Object modules are linked together
The resulting executable load module is loaded by the loader

Logical vs Physical Address Space

Logical (virtual) address is generated by the CPU
Physical address is used by the memory unit
In compile-time and load-time, the logical and physical addresses are the same.
They differ in execution time.
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

Memory-Management Unit (MMU)

Hardware that maps logical to physical addresses at runtime
This is a generalization of the base-register scheme (the relocation register)
Used to protect user processes from each other

Dynamic Loading

The entire program isn't loaded into memory at once
Routines loaded only when needed
Improves memory usage

Dynamic Linking

Linking of libraries is postponed until execution time
Uses stubs to locate routines
Implementation of shared libraries

Contiguous Allocation (cont.)

Relocation registers needed to protect user processes from each other and from changing operating system code/data
Base register contains smallest physical address
Limit register contains the range of logical addresses
Allows for dynamic mapping of logical to physical addresses

Variable Partition

Multiple partitions of variable sizes
Holds OS and user programs
Free memory areas (holes)
The OS maintains a record of free blocks of memory

Dynamic Storage Allocation

Methods (First-fit, best-fit, worst-fit) to satisfy requests for memory allocation sized 'n' bytes
First-fit: fastest, but doesn't necessarily minimize leftover space
Best-fit: minimizes leftover space, but slowest.
Worst-fit: allocates the largest hole, producing the largest leftover hole.

Fragmentation

External fragmentation: Memory space exists, but it is not contiguous.
Internal fragmentation: Assigned memory is larger than needed.
Compaction: Attempts to reduce external fragmentation by moving memory blocks

Paging

Physical address space of a process can be noncontiguous
Improves external fragmentation
Divides physical memory into fixed-sized blocks
Divides logical memory into blocks of same size

Address Translation

CPU-generated address divided into page number and page offset
Page number is used as an index to a page table
Page offset is combined with the base address to define the physical memory address

Paging Hardware

Logical to physical address mapping
Page table hardware

Paging Model

Logical memory and physical memory are divided into pages
Logical page number corresponds to a specific physical frame

Paging Example

Showcases logical and physical address mapping with page table lookup

Paging - Calculating internal fragmentation

Calculates internal memory wasted due to paging.

Free Frames

Free frames stored in a free-frame list
Allocation occurs using the free-frame list
Free frames are added back to free-frame list upon deallocation

Implementation of Page Table

Page table kept in main memory
Page-table base register (PTBR) and length register (PTLR)

Translation Look-Aside Buffer (TLB)

Special fast-lookup hardware cache
Maps logical to physical addresses

Hardware

Associative memory is used for parallel search
Address translation uses page number and the associative register
Page number in register, frame number returned
Otherwise, search in memory page table

Paging Hardware with TLB

A diagram showing the steps involved in the TLB lookup

Effective Access Time

Calculating effective access time considering hit ratio.

Memory Protection

Mechanisms used for memory protection
Valid-invalid bit in the page table

Valid or Invalid Bit

Valid: indicates the page is in the process' logical address space
Invalid: indicates the page isn't in process' logical address space

Shared Pages

One copy of read-only code shared amongst multiple processes
Improves efficiency and reduces memory usage
Multiple processes can access the shared memory

Shared Pages Example

Example of how page tables for shared code/data are organized

Structure of the Page Table

Memory structures for paging
Dividing page tables into smaller units
Hierarchical paging, hashed page tables, inverted page tables

Two-Level Paging

Dividing a 32-bit logical address into two page numbers and an offset
Mapping to page tables

64-bit Logical Address Space

When 64-bit logical addresses are involved

Three-Level Paging

A variant of multilevel page table structure
Addresses are broken down into three page numbers

Hashed Page Tables

Hashing virtual page numbers to virtual page entries in a hash table.

Inverted Page Table

Only one entry per physical page instead of per virtual page for a process.

Oracle SPARC Solaris

Example of a modern 64-bit operating system

Swapping

Moving processes between memory and a backing store.
Total physical memory space needed for processes may exceed physical memory.
Methods to accommodate processes

Swapping (cont.)

Swapping issues (address binding).
Swapping variants found in operating systems like UNIX, Linux, and Windows

Context Switch Time

Time for switching a process to another.
Including the swap in/out times if necessary
Increased context switching time

Context Switch time and Swapping (cont.)

Constraints on swapping
Considerations for pending I/O
Use of double buffering

Swapping on Mobile Systems

Differences in memory management to accommodate flash-based memory, limited cycles, low throughput

Swapping with Paging

Swapping processes that require paging.

Example: The Intel 32 and 64-bit Architectures

Dominant processors used in modern computing.
Typical architectures used for 32-bit and 64-bit systems

Example: The Intel IA-32 Architecture

Supports both segmentation and segmentation with paging.
Details about how segments are handled in the architecture

Example: The Intel IA-32 Architecture (cont.)

CPU generating logical addresses.

Logical To Physical Address Translation

The transformation steps from a logical address to a physical address in IA-32 architecture.

Intel IA-32 Segmentation

The segmentation unit in a 32-bit Intel processor.

Intel IA-32 Paging Architecture

Page directory, page table layout.

Intel IA-32 Page Address Extensions (PAE)

Increasing address space.

Intel x86-64

Typical architecture for the 64-bit systems.

Example: ARM Architecture

Describes the mobile platform chip architecture.
Methods for paging, TLBs, and memory allocations.

End of Chapter 9

Studying That Suits You

Use AI to generate personalized quizzes and flashcards to suit your learning preferences.

Description

This quiz covers key concepts related to main memory management in operating systems, including memory allocation, protection, and address binding. Understand how processes interact with memory and the importance of efficient memory usage. Test your knowledge on how operating systems ensure process protection and handle memory addresses.