Memory and storage: RAM, ROM and secondary storage

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课程笔记

What is Computer Memory?

Computer memory stores data and programs for immediate use by the computer.
Memory is also called main memory or primary storage.
RAM (Random-Access Memory) is the most common type of main memory.
Main memory is fast but more expensive per bit and has lower capacity than mass storage.
Memory holds open programs and data being actively processed.
Operating systems use spare RAM as a cache to speed up reading and writing.

RAM: Volatile Working Memory

RAM stands for Random-Access Memory.
RAM is volatile: it loses its contents when power is off.
RAM is used for temporary storage of running programs and data.
The two main types of RAM are DRAM (dynamic) and SRAM (static).
DRAM is used for primary storage (main memory).
SRAM is used mainly for CPU cache because it is faster but more expensive.

ROM: Non-Volatile Start-Up Memory

ROM stands for Read-Only Memory.
ROM is non-volatile: it keeps its data even when power is off.
ROM contains start-up instructions (firmware) that the computer uses when it boots.
ROM is usually read-only, meaning data cannot be easily changed.
Other non-volatile memories include PROM, EPROM, and EEPROM.

Secondary Storage

Secondary storage is non-volatile storage for keeping data long-term.
It is slower than main memory but cheaper per bit and has higher capacity.
Common types: magnetic, optical, and solid-state.
Magnetic storage (e.g., hard disk drives) uses magnetised spots on spinning platters.
Optical storage (e.g., CDs, DVDs) uses lasers to read/write pits on discs.
Solid-state storage (e.g., SSDs, USB flash drives) uses flash memory with no moving parts.

Hard disk drive mechanism

Comparing Storage Types

Speed: RAM is fastest, then solid-state, then magnetic, then optical.
Cost: RAM is most expensive per GB, then solid-state, then magnetic, then optical.
Capacity: Secondary storage typically has much larger capacity than RAM.
Portability: Optical discs and USB flash drives are highly portable; internal HDDs/SSDs are less so.
Volatility: RAM is volatile; ROM and secondary storage are non-volatile.

Solid-state drive architecture

Why a Computer Needs All Three

RAM provides fast, temporary workspace for running programs.
ROM ensures the computer can start up even when no other software is loaded.
Secondary storage saves data permanently for future use.
Without RAM, the CPU would have to use slow storage, making the computer very slow.
Without ROM, the computer wouldn't know how to boot.
Without secondary storage, you couldn't save files or install programs permanently.

How Memory is Organised

Memory is made of memory cells, each storing one bit (0 or 1).
Cells are grouped into words of fixed length (e.g., 8, 16, 32, 64 bits).
Each word has a unique binary address of N bits, allowing 2^N words to be stored.
Modern memory uses semiconductor technology on integrated circuits.
Flash memory can store multiple bits per cell (multi-level cells).

Cache holds frequently needed data close to the CPU, so a hit avoids the slower trip to main memory.

Historical Context (Brief)

Early computers used vacuum tubes (e.g., ENIAC) with tiny capacity.
Delay-line memory used sound waves in mercury to store bits.
Magnetic-core memory was the first non-volatile memory and dominated until the 1970s.
MOS semiconductor memory (invented in the 1960s) was cheaper and faster, replacing magnetic core.

Closer to the CPU means faster and smaller; further away means larger, slower and cheaper per bit.

Main memory holds work in progress and is lost when power goes off; secondary storage keeps files permanently.

幻灯片

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练习题

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1.What does RAM stand for?
Easy
- ARead-Only Memory
- BRandom-Access Memory
- CRapid-Access Memory
- DRandom-Access Module
2.RAM is volatile memory, meaning it loses its contents when the power is turned off.
Easy
True or false?
3.Which type of memory is used to store the start-up instructions that a computer needs when it is first turned on?
Easy
- ARAM
- BROM
- CSecondary storage
- DCache
4.ROM is a type of non-volatile memory.
Easy
True or false?
5.Which of the following are characteristics of secondary storage compared to RAM? (Select all that apply)
Medium
- AIt is slower
- BIt is less expensive per bit
- CIt has higher capacity
- DIt is volatile
- EIt is used for long-term storage
6.Which of the following is an example of a solid-state storage device?
Easy
- AHard disk drive
- BCD-ROM
- CUSB flash drive
- DFloppy disk
7.Magnetic storage is generally more portable than solid-state storage.
Easy
True or false?
8.Match each type of memory/storage with its correct description.
Medium
- RAM
- ROM
- SSD
- Volatile main memory
- Non-volatile start-up memory
- Solid-state secondary storage

复习笔记

## Memory and Storage: RAM, ROM and Secondary Storage ### RAM (Random-Access Memory) - **Volatile**: loses contents when power is off. - Used as **working memory** for programs and data being actively processed. - Fast compared to secondary storage, but more expensive per bit and lower capacity. - Often used as a **cache** and **write buffer** to improve performance. - Operating systems may borrow RAM for caching; if needed, contents can be transferred to storage via **virtual memory**. - Types: DRAM (primary storage) and SRAM (CPU cache). ### ROM (Read-Only Memory) - **Non-volatile**: retains data without power. - Used for **start-up instructions** (e.g., BIOS/UEFI). - Typically read-only, but variants like PROM, EPROM, EEPROM exist. ### Secondary Storage - **Magnetic** (e.g., hard disk drives): high capacity, lower cost per bit, slower than RAM. - **Optical** (e.g., CDs, DVDs): portable, removable, slower. - **Solid-state** (e.g., flash memory, SSDs): fast, durable, no moving parts, but more expensive per bit than magnetic. - Used for long-term storage of data and programs. ### Why a Computer Needs All Three - RAM for speed and active work, ROM for booting, secondary storage for permanent storage. - Each balances speed, cost, and capacity. ### Key Concepts - **Memory cell**: stores one bit (0 or 1); flash can store multiple bits per cell. - **Word**: group of bits (e.g., 1, 2, 4, 8, 16, 32, 64, 128 bits) accessed by a binary address. - With N-bit address, can store 2^N words. ### Historical Context - Early memory: vacuum tubes (ENIAC), delay lines, Williams tubes. - Magnetic-core memory: non-volatile, dominant until 1960s. - MOS memory (semiconductor): cheaper, faster, led to modern RAM.

RAM: Random-Access Memory; volatile memory used as working memory for programs and data being actively processed.
ROM: Read-Only Memory; non-volatile memory that retains data without power, used for start-up instructions.
Volatile memory: Memory that loses its contents when power is turned off; e.g., RAM.
Non-volatile memory: Memory that retains data without power; e.g., ROM, flash memory.
Secondary storage: Non-volatile storage for long-term data and programs, such as magnetic, optical, and solid-state drives.
Magnetic storage: Storage using magnetic media, e.g., hard disk drives; high capacity, lower cost per bit.
Optical storage: Storage using lasers to read/write data on discs like CDs and DVDs; portable and removable.
Solid-state storage: Storage with no moving parts, using flash memory; fast and durable, e.g., SSDs.
Memory cell: A component that stores one bit (0 or 1); flash can store multiple bits per cell.
Word: A group of bits (e.g., 1, 2, 4, 8, 16, 32, 64, 128) accessed together by a binary address.
Virtual memory: A memory management technique that transfers contents of RAM to storage when needed.
DRAM: Dynamic Random-Access Memory; a type of volatile memory used for primary storage.
SRAM: Static Random-Access Memory; a type of volatile memory used mainly for CPU cache.

闪卡

What does RAM stand for and what is its main characteristic?

RAM stands for Random-Access Memory. It is volatile memory, meaning it loses its contents when the power is turned off.

What is the role of RAM in a computer system?

RAM is the main working memory where the computer stores data and programs that are currently in use. It allows the CPU to access this information quickly.

What does ROM stand for and what is its main characteristic?

ROM stands for Read-Only Memory. It is non-volatile memory, meaning it retains its contents even when the power is off.

What is the primary purpose of ROM in a computer?

ROM is used to store the firmware, including the boot-up instructions (like the BIOS) that the computer needs to start up. It is not typically modified during normal operation.

What is secondary storage?

Secondary storage is non-volatile storage used to store data and programs permanently, even when the computer is turned off. Examples include hard drives, SSDs, and optical discs.

Name three types of secondary storage.

Magnetic (e.g., hard disk drives), optical (e.g., CDs, DVDs), and solid-state (e.g., USB flash drives, SSDs).

What is the difference between volatile and non-volatile memory?

Volatile memory requires power to retain data (e.g., RAM), while non-volatile memory retains data without power (e.g., ROM, secondary storage).

Why does a computer need both RAM and secondary storage?

RAM provides fast, temporary storage for running programs and data, but it is volatile and limited in capacity. Secondary storage provides large, permanent storage for files and programs, but it is slower. Together, they balance speed and capacity.

What is the main advantage of solid-state storage over magnetic storage?

Solid-state storage has no moving parts, making it faster, more durable, and more energy-efficient than magnetic storage, though it is often more expensive per gigabyte.

What is a common example of magnetic storage?

A hard disk drive (HDD) is a common example of magnetic storage. It uses spinning platters and magnetic heads to read/write data.

What is a common example of optical storage?

CDs, DVDs, and Blu-ray discs are common examples of optical storage. They use lasers to read and write data on the disc surface.

What is a common example of solid-state storage?

USB flash drives and solid-state drives (SSDs) are common examples of solid-state storage. They use flash memory to store data.

How does the speed of RAM compare to secondary storage?

RAM is much faster than secondary storage. RAM can be accessed in nanoseconds, while secondary storage (like HDDs or SSDs) takes milliseconds or microseconds.

How does the cost per gigabyte compare between RAM and secondary storage?

RAM is more expensive per gigabyte than secondary storage. Secondary storage is cheaper per gigabyte, allowing for larger capacities at lower cost.

What is the typical capacity of RAM compared to secondary storage?

RAM typically has a smaller capacity (e.g., 8-32 GB) compared to secondary storage, which can be terabytes (e.g., 1-4 TB).

What is portability in the context of storage devices?

Portability refers to how easily a storage device can be moved and used with different computers. For example, USB flash drives are highly portable, while internal hard drives are not.

Why is RAM considered 'working memory'?

RAM is where the computer holds data and instructions that are actively being used by the CPU. It acts as the 'workspace' for the computer.

What happens to the contents of RAM when the computer is turned off?

The contents of RAM are lost because RAM is volatile memory. It requires power to maintain the stored data.

What is the boot-up process and how does it relate to ROM?

When a computer starts, it executes instructions stored in ROM (like the BIOS) to initialize hardware and load the operating system from secondary storage into RAM.

What is the difference between primary and secondary storage?

Primary storage (like RAM) is directly accessible by the CPU, fast, and volatile. Secondary storage (like HDDs, SSDs) is not directly accessible, slower, non-volatile, and used for long-term storage.

What is a memory cell?

A memory cell is the basic unit of semiconductor memory that stores one bit of data (0 or 1).

What is the role of the CPU cache in relation to memory?

The CPU cache is a small, very fast type of volatile memory (SRAM) that stores frequently used data and instructions to speed up processing, acting as a buffer between the CPU and main memory (RAM).

What is virtual memory?

Virtual memory is a technique where the operating system uses a portion of secondary storage as an extension of RAM, allowing the computer to run programs that require more memory than physically available.

What is the difference between DRAM and SRAM?

DRAM (Dynamic RAM) is used for main memory (RAM) and needs constant refreshing, while SRAM (Static RAM) is used for CPU cache and is faster but more expensive; SRAM does not need refreshing.

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