COMSCI2101 - OS M3

Memory Management

Refers to how computers manage their main memory.

Main memory

Is where resources that the CPU can

immediately access are kept.

Access Memory

refers to memory that can be accessed by the CPU, can refer to RAM, Cache, or Virtual Memory based on concept.

Random Access Memory (RAM)

A form of volatile memory, storing data and instructions that the CPU needs to access immediately.

Volatile

Is lost upon shutdown or when computers are turned off.

Core Memory

Non-volatile memory used before semiconductors gained prominence, using tiny magnetic rings to store bits of information.

Primary Storage

A broader term including all types of main

memory, directly accessible by the CPU,

primarily referring to RAM and Cache.

Single-user Contiguous Scheme

Requires a minimal amount of work

done by the operating system’s

memory manager compared to the

other schemes.

-kloads entire programs

into memory, allocating as much as

needed.

- It does not support

multiprogramming or networking, as

it processes jobs sequentially.

Fixed Partitions

Makes use of static partitions, permitting the use of multiprogramming.

These partition sizes remain static unless the computer system is shutdown, reconfigured, and restarted.

Dynamic Partitions

Divides memory into variable-sized

partitions based on the size of the

processes or jobs being executed,

ensuring that jobs are given only as

much memory as they request.

Job allocation is done first come, first serve.

- First Fit Memory Allocation

- Best Fit Memory Allocation

Free partitions are allocated using the

following strategies: (2)

First Fit

Loads jobs into the first partition fitting the requirements.

Results in faster allocation

Faster, focused on Speed

Best Fit

Loads jobs into the smallest partition fitting the requirements.

Results in least wasted space

Efficient, focused on Space

Deallocation

Refers to the process of freeing an allocated memory space.

fixed-partition systems

For ___________, deallocation is simply done by resetting the status of a memory block to “free”.

dynamic partitions

For __________, the memory

manager may do one of the following:

- Join two free blocks

- Join three free blocks

- Deallocate an isolated block

Relocatable Dynamic Partitions

Relocates programs to gather all

empty blocks, compacting them to

make a block of memory large

enough to accommodate jobs waiting

to get in.

Compaction

a.k.a Garbage Collection or

Defragmentation

Compaction

Performed by an operating system using the relocatable dynamic partitions scheme to reclaim fragmented sections of memory space.

Virtual Memory

A storage method that considers a portion of secondary memory memory as main memory, giving the user the impression that their main memory is large.

Paging

A storage mechanism used to retrieve processes or jobs from secondary memory to primary memory

Pages

Small equal blocks that are divisions of

processes in memory.

Frame

Small equal pieces that are divisions of

physical memory.

- Each process is contained in pages and are stored into their respective frames of memory.

Demand Paging

Refers to the process of calling the pages to

main memory from secondary memory

upon demand or request.

Page Replacement Algorithm

Is needed to decide which page needs to be replaced when a new page comes in.

Page Fault (*)

Occurs when a program tries to access a

memory page not present in memory.

Page Hit (↓)

Occurs when a program tries to access a

memory page already present in memory.

First In First Out (FIFO) Replacement

Algorithm

The simplest page replacement

algorithm, it removes the pages that

have been in memory the longest.

Least Recently Used (LRU) Replacement

Algorithm

The page replacement algorithm that

swaps out the pages that show the

least amount of recent activity, with

the assumption that these pages are

the least likely to be used again.