Memory Management

Question 1

What is the CPU Instruction Cycle?

Answer 1

1. Fetch: Getting the instruction from memory.
2. Decode: Understanding what the instruction does.
3. Execute: Performing the instruction’s operation.
4. Store: Writing back results to memory if needed.

Question 2

What are registers?

Answer 2

Can be accessed within a single CPU cycle (extremely fast)

Question 3

How is the main memory accessed?

Answer 3

Accessed over a memory bus; typically slower than registers.

Question 4

What is cache memory?

Answer 4

Fast memory between the CPU and main memory to reduce access
times. It stores frequently used data and instructions to avoid slow
main memory access.

Question 5

How is the Memory Hierarchy accessed?

Answer 5

*Helps balance speed, size, and cost, typically organized as:
* Registers > Cache > Main Memory (RAM) > Secondary Storage (Disk)

Question 6

L1 Cache

Answer 6

Fastest and smallest, located inside the CPU core.

Question 7

L2 Cache

Answer 7

Larger but slower, still on the CPU chip.

Question 8

L3 Cache

Answer 8

Shared among cores, significantly larger but
slower than L1 and L2

Question 9

Memory Isolation

Answer 9

Ensures that a process cannot access memory outside its
allocated space, protecting the OS and other processes.

Question 10

Security Enhancement

Answer 10

Prevents malicious or faulty programs from causing memory
corruption.

Question 11

Dynamic Relocation

Answer 11

Allows the OS to move processes in memory by updating only
the base register, without altering the program code.

Question 12

What is address binding?

Answer 12

Address binding transforms logical addresses (generated
by programs) into physical addresses (used by hardware).

Question 13

What is a logical address?

Answer 13

A logical address (also known as a virtual address) is an
abstract address that a process uses to access memory.

Question 14

What is a physical address?

Answer 14

A physical address refers to the actual location in the
physical memory (RAM).

Question 15

What 3 times can address binding of instructions and data to memory addresses happen?

Answer 15

• Compile time
• Load time
• Execution time

Question 16

Load Time Binding

Answer 16

• Used when the load address is not known at compile time.
• The compiler generates relocatable code, meaning addresses are
  relative to a base address.
• The loader calculates the absolute addresses when the program is
  loaded into memory.
• Flexibility: The program can be loaded at different memory
  locations without recompiling

Question 17

Execution Time Binding

Answer 17

• The most dynamic form of address binding.
• Binding occurs when instructions are executed.
• Requires special hardware support, typically through the MMU.
• Allows processes to move freely between memory locations during
  execution, as logical addresses are dynamically translated to
  physical addresses.
• Essential for modern operating systems that use paging or
  segmentation.

Question 18

How is Logical Address (Virtual Address) created?

Answer 18

• Generated by the CPU during program execution
• The address seen by the program

Question 19

Physical Address

Answer 19

• The actual address in the memory unit
• The address used by the memory hardware to access memory
  cells

Question 20

Static Binding

Answer 20

• Address binding is completed before execution.
• Occurs at compile time or load time.
• Suitable for simple systems with predictable memory usage.

Question 21

Dynamic Binding

Answer 21

• Binding occurs during execution.
• Allows for dynamic memory allocation, swapping, and relocation.
• Enables virtual memory systems, allowing processes to use more
  memory than physically available

Question 22

What is dynamic loading?

Answer 22

Dynamic loading is a technique where a program loads a module or
routine into memory only when it is needed.

• Modules can include libraries, functions, or data segments that are
  not immediately required when the program starts.
• Memory Efficiency
• Faster Start-Up

Question 23

Dynamic Linking

Answer 23

• When a program is written, it typically contains reference to
  external libraries
• A program that has these libraries incorporated into its binary at
  compile time is said to be statically linked
• Dynamic linking postpones the linking until runtime
• Reduces the memory footprint of running programs.
• Allows multiple processes to share the same code, particularly
  with shared libraries

Question 24

Standard Swapping

Answer 24

• A process may be swapped out of memory to a backing store, then
  brought back when needed
• Allows total memory used by processes to exceed physical memory size
• Often combined with priority-based scheduling
• Low priority process more likely to be swapped out
• Greatly increases the cost of context switching
• Reading/writing secondary storage is relatively slow
• Time to swap is proportional to memory usage of process

Question 25

Contiguous Allocation

Answer 25

* A process is allocated a single contiguous block of memory in main memory. * The entire memory space required by the process must fit into one continuous segment.

Question 26

Multi-Partition Allocation

Answer 26

* Main memory is divided into multiple fixed or variable-sized partitions, and each partition can hold a single process. * Fixed vs. Variable Partitions: * Fixed-Size Partitions: Equal-sized chunks (partitions) in advance. * Variable-Sized Partitions: Partitions are created dynamically based on process size. * Unused memory referred to as holes

Question 27

Fragmentation

Answer 27

Two types of fragmentation * External fragmentation – when memory is available in holes, but many small holes rather than one contiguous block * Can be reduced 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 * Internal fragmentation – occurs if processes are allocated slightly more than requested (e.g. so that a small hole is left)

Question 28

Dynamic Storage-Allocation Problem

Answer 28

* Different approaches to allocating memory n from a list of 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 utilisation

Question 29

Segmentation

Answer 29

* Memory-management scheme that supports user view of memory * Segmentation involves recognising that a program is a collection of segments, such as: * Main program * Function * Method * Stack * Variables * Symbol table * These do not need to be contiguous in memory

Question 30

Paging – Basic Method

Answer 30

* Physical address space of a process can be noncontiguous; process is allocated physical memory whenever the latter is available * One way of dealing with external fragmentation * Physical memory divided into fixed-size blocks called frames * Logical memory is divided into blocks of the same size, called pages * Program of N pages neds to find N free frames * Page table translates logical to physical addresses * Still have internal fragmentation

Question 31

Address Translation Scheme pages

Answer 31

* 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