1.4 Memory Management

Question 1

Dynamic Memory Allocation

Answer 1

OS allows provides mechanism for programms to Dynamically allocate memory via system libraries and special functions

Question 2

Memory Fragmentation

Answer 2

Inefficient splitting of memory space into small chunks in the main memory or on a hard disk.

Question 3

Allocation Strategy

Answer 3

This is an algorithm that decides which free resource (here, memory region) is allocated to which request.

Question 4

First Fit

Answer 4

• Allocate first free matching Block
• Simple and fast
  But
• Small free chunks accumulate at small adresses

Question 5

Next fit

Answer 5

Same as first, but continue search at last allocation

Question 6

Best fit

Answer 6

• Allocate the block that leaves the smallest reminder
  But
• Allocation of many small residual gaps

Question 7

Worst fit

Answer 7

• Allocate the block that leaves the largest reminder
• Remaining gaps remain large and therefore easy to allocate

Question 8

What is memory protection and why is it important?

Answer 8

Ensures that multiple processes are only accessing designated address space.

Process isolation
* Malfuntioning programs can only impact own address space

Data security
* Prevents unauthorized access to sensitive data

Question 9

How is memory protection archieved?

Answer 9

Hardware support of the CPU
* Detects attempts
* Raises exception
* Triggers OS to stop process

Question 10

Boundary registers

Answer 10

• Part of the CPU Check validity of memory address accessed by process
• Uses start and end information of process control block

Question 11

Recap: What happens if process violates memory boundaries?

Answer 11

1. Memory Access violation detected by Boundary registers via process control block
2. Boundary registers promption CPU to hand over CPU to hand over control to OS
3. Corrective action

Question 12

Which part of the computer uses logical and which part physical addresses to acces memory?

Answer 12

• Address bus, physical
• CPU and programs, logical

Question 13

Which part is doing the conversion of logical to physical addresses?

Answer 13

MMU

Question 14

MMU

Answer 14

• MMU is part of the CPU
• Conversion of virtual to physical addresses
• Consists of two registers: Base register and Limit register

Question 15

Base Register

Answer 15

• Stores starting physical address of programm memory space
• When programs logical address is generated, it assumes 0
• MMU adds value in base register to logical address to determine physical address

Question 16

Limit register

Answer 16

• Contains size of programms address space
• Checks allowed memory range
• Memory protection

Question 17

Benefits of Logical addresses

Answer 17

• Allow programms to be written and stored as if running and stored as dedicated block, starting from 0
• simplifies programming
• flexible memory management
• Secure Multitasking

Question 18

What is the Disadvantage of Virtual Memory Addresses and how can it be mitigated?

Answer 18

• Contingious memory space is required
• This can only be achieved if there is enough space in immediate physical range

Mitigation:
Virtual memory Addresses by way of paging

Question 19

Paging

Answer 19

With this method of memory management, the main memory is divided into pages (in the form of tables) and managed by the operating system in cooperation with the MMU.

Question 20

Memory Page

Answer 20

• Blocks of a fixed length in the virtual address space used by processes, managed through virtual memory management techniques like paging.

Question 21

Page Frames

Answer 21

Physical memory block in Main meory

Question 22

Page Tables

Answer 22

In virtual memory management with paging, address translation is performed via page tables, which store, for each process, which of its memory pages are located on which page frame within the main memory.

• Every Process has its own Page Table

Question 23

TLB

Answer 23

Translation Lookaside Buffer
* Temporary storage of recent table entries
* Faster Address Translation

Question 24

What are the three steps for converting virtual addresses to physical ones?

Answer 24

1. Identify Memory Page
2. Locate Page frame via MMU and page tables for Mapping
3. Calculate Physical Addresses

Question 25

Demand Paging

Answer 25

• Links virtual memory addresses with lager hard disk storage
• Pages only read into memory if needed
• Moved if no longer in use

Question 26

Page fault

Answer 26

• MMU does not find match in page tables
• OS steps in:
  1. Checks problem
  2. Loads Data if needed
  3. Updates and continues

Question 27

Swap Space

Answer 27

When no more free space in memory, OS makes space by moving unused data back to Hard Disk to:

SWAP SPACE
* part of hard Disk used for temporarily storing Data

Question 28

FIFO

Answer 28

First In / First Out
* Page that is longest in main memory

Question 29

LFU

Answer 29

Least frequently used
* Page that is least frequently used

Question 30

LRU

Answer 30

Least Recently Used
* Page not Accessed for longest

Question 31

What is the problem with the main Replacement strategies and how to mitigate?

Answer 31

• Requires recording of information
• Massive administrative effort by the OS

Mitigation
* NRU

Question 32

NRU

Answer 32

Not Recently used
* MMU sets bit as flag in page table if page address has been written to or read by process (Accessed)
* If content was changed by write access (Dirty)

• OS periodically deletes bits every few time slices
• Page eradication based on priority

Question 33

Describe the role of a Memory Management Unit (MMU) in the process of converting logical addresses to physical addresses.

Answer 33

1. The Memory Management Unit (MMU) plays a crucial role in converting logical addresses to physical addresses.
2. It has two registers that store the start address of the program (base register) and the length of the address range of the process (limit register).
3. The MMU continuously converts logical addresses, which always start at address zero, to physical addresses by adding the content of the base register.
4. Thus, it ensures the successful conversion of addresses before they are put on the address bus.

Question 34

What is the role of the interrupt controller in the mechanism for interrupt handling in a preemptive multitasking system?

Answer 34

It triggers an interrupt when the hardware alarm time expires, thus allowing the OS to regain control over the CPU?

Question 35

Apply the concept of Virtual Memory Addresses and briefly discuss the significance of the MMU in mapping the logical addresses to physical addresses in the computer memory.

Answer 35

1. The concept of Virtual Memory Addresses enables the use of memory space that exceeds the actual physical memory of a computer by using a method called paging.
2. The Memory Management Unit (MMU) plays a crucial role in this context. It is responsible for the conversion of logical addresses (sometimes called virtual addresses) into physical addresses.
3. It does this in a granular manner, using fixed-length blocks known as memory pages. Using tables, the MMU maps these pages to the physical memory of the computer.
4. The importance of MMU therefore lies in its ability to effectively manage and utilize the computer’s memory, ensuring an efficient use of the limited physical memory and allowing processes to run smoothly even when they require memory space that exceeds the physical memory.

Question 37

Describe the role of the Translation Lookaside Buffer in the memory management system and discuss its importance in the address conversion process.

Answer 37

• The Translation Lookaside Buffer (TLB) is essentially a fast associative cache that stores recently used table entries for the MMU, facilitating a faster address conversion process.
• The table entries stored in the TLB allow the MMU to translate addresses without needing to search through the actual page tables, given that the addresses are moving within a page. This is particularly useful because it speeds up the operation of the effects of the MMU in translating virtual addresses into physical addresses.
• By storing table entries in the TLB, the system avoids having to access the main memory every time an address translation is needed, saving time and enhancing the overall efficiency of the system. Therefore, the TLB plays a vital role in optimizing the process of address conversion in a system that employs virtual memory addresses.

Question 38

Which one of the following allocation strategies tends to leave many small, residual gaps?

Answer 38

Best Fit