Memory Unit (I/O)

This article will cover various I/O devices:

Compiled by “Md. Farrukh Asif”

Computer Memory:

The computer memory holds the data and instructions needed to process raw data and produce output. The computer memory is divided into a large number of small parts known as cells. Each cell has a unique address which varies from 0 to memory size minus one.

Computer memory is of two types: Volatile (RAM) and Non-volatile (ROM). The secondary memory (hard disk) is referred to as storage, not memory.

But, if we categorize memory on behalf of space or location, it is of four types:v

v Cache memory

v Primary memory

v Secondary memory

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Cache Memory:

Cache memory is a high-speed memory, which is small in size but faster than the main memory (RAM). The CPU can access it more quickly than the primary memory. So, it is used to synchronize with high-speed CPU and to improve its performance.

Cache memory can only be accessed by the CPU. It can be a reserved part of the main memory or a storage device outside the CPU. It holds the data and programs which are frequently used by the CPU. So, it makes sure that the data is instantly available for the CPU whenever the CPU needs this data. In other words, if the CPU finds the required data or instructions in the cache memory, it doesn't need to access the primary memory (RAM). Thus, by acting as a buffer between RAM and CPU, it speeds up the system performance.

Types of Cache Memory:

L1: It is the first level of cache memory, which is called Level 1 cache or L1 cache. In this type of cache memory, a small amount of memory is present inside the CPU itself. If a CPU has four cores (quad-core CPU), then each core will have its own level 1 cache. As this memory is present in the CPU, it can work at the same speed as the CPU. The size of this memory ranges from 2KB to 64 KB. The L1 cache further has two types of caches: The instruction cache, which stores instructions required by the CPU, and the data cache which stores the data required by the CPU.

L2: This cache is known as Level 2 cache or L2 cache. This level 2 cache may be inside the CPU or outside the CPU. All the cores of a CPU can have their own separate level 2 cache, or they can share one L2 cache among themselves. In case it is outside the CPU, it is connected to the CPU with a very high-speed bus. The memory size of this cache is in the range of 256 KB to 512 KB. In terms of speed, they are slower than the L1 cache.

L3: It is known as Level 3 cache or L3 cache. This cache is not present in all the processors; some high-end processors may have this type of cache. This cache is used to enhance the performance of Level 1 and Level 2 cache. It is located outside the CPU and is shared by all the cores of a CPU. Its memory size ranges from 1 MB to 8 MB. Although it is slower than the L1 and L2 cache, it is faster than Random Access Memory (RAM).

How does cache memory work with the CPU?

When the CPU needs the data, first of all, it looks inside the L1 cache. If it does not find anything in L1, it looks inside the L2 cache. If again, it does not find the data in the L2 cache, it looks into the L3 cache. If data is found in the cache memory, then it is known as a cache hit. On the contrary, if data is not found inside the cache, it is called a cache miss.

If data is not available in any of the cache memories, it looks inside the Random Access Memory (RAM). If RAM also does not have the data, then it will get that data from the Hard Disk Drive.

So, when a computer is started for the first time, or an application is opened for the first time, data is not available in cache memory or in RAM. In this case, the CPU gets the data directly from the hard disk drive. Thereafter, when you start your computer or open an application, the CPU can get that data from cache memory or RAM.

Primary Memory:

Primary Memory is of two types: RAM and ROM.

RAM (Volatile Memory)

It is a volatile memory. It means it does not store data or instructions permanently. When you switch on the computer the data and instructions from the hard disk are stored in RAM.

CPU utilizes this data to perform the required tasks. As soon as you shut down the computer the RAM loses all the data.

ROM (Non-volatile Memory)

It is a non-volatile memory. It means it does not lose its data or programs that are written on it at the time of manufacture. So it is a permanent memory that contains all important data and instructions needed to perform important tasks like the boot process.

Secondary Memory:

The secondary storage devices that are built into the computer or connected to the computer are known as the secondary memory of the computer. It is also known as external memory or auxiliary storage.

The secondary memory is accessed indirectly via input/output operations. It is non-volatile, so permanently stores the data even when the computer is turned off or until this data is overwritten or deleted. The CPU can't directly access the secondary memory. First, the secondary memory data is transferred to primary memory then the CPU can access it.

Some of the secondary memory or storage devices are described below:

1) Hard Disk:

It is a rigid magnetic disc that is used to store data. It permanently stores data and is located within a drive unit.

The hard disk is also known as a hard drive. It is a rigid magnetic disc that stores data permanently, as it is a non-volatile storage device. The hard disk is located within a drive unit on the computer's motherboard and comprises one or more platters packed in an air-sealed casing. The data is written on the platters by moving a magnetic head over the platters as they spin. The data stored on a computer's hard drive generally includes the operating system, installed software, and the user's files and programs, including pictures, music, videos, text documents, etc.

Main Components of a Hard Drive

A hard disk drive (HDD) is a storage device that uses magnetic storage to retain and retrieve data. It consists of several key components working in unison:

1. Platters

The core of the HDD, platters are circular disks made of aluminum, glass, or ceramic.
Coated with a thin layer of magnetic material, they are where data is actually stored.
Multiple platters are stacked on top of each other in a hard drive, increasing storage capacity.

2. Spindle

A motor-driven shaft that holds the platters in place.
Rotates the platters at high speeds (typically 5400, 7200, or 15000 RPM) to enable rapid data access.

3. Actuator Arm

A mechanical arm containing the read/write heads.
Moves across the surface of the platters to locate and access data.
Driven by a small motor, it positions the read/write heads accurately.

4. Read/Write Heads

Tiny electromagnetic coils located at the end of the actuator arm.
Convert magnetic patterns on the platters into electrical signals (reading) and vice versa (writing).
Hover incredibly close to the platter surface without touching it.

5. Controller

The "brain" of the hard drive, responsible for managing data transfer and disk operations.
Interprets commands from the computer and translates them into actions for the other components.
Handles error correction, data formatting, and other essential tasks.

6. Enclosure

The protective casing that houses all the components.
Made of metal or plastic to shield the delicate internal parts from damage.
Includes connectors for power and data transfer.

How it Works

The computer sends a request to access data.
The controller interprets the request and determines the platter, track, and sector where the data is located.
The actuator arm moves the read/write heads to the correct position.
The spindle rotates the platters until the desired data is under the read/write head.
The read/write head converts the magnetic data into electrical signals, which are sent to the computer.
For writing data, the process is reversed, with the read/write head creating magnetic patterns on the platter.

2) Solid-state Drive:

SSD (Solid State Drive) is also a non-volatile storage medium that is used to hold and access data. Unlike a hard drive, it does not have moving components, so it offers many advantages over SSD, such as faster access time, noiseless operation, less power consumption, and more.

As the cost of SSD has come down, it has become an ideal replacement for a standard hard drive in desktop and laptop computers. It is also suitable for notebooks, and tablets that don't require lots of storage.

3) SATA: Serial Advanced Technology Attachment

What is SATA?

SATA is a computer bus interface used to connect host bus adapters to mass storage devices like hard disk drives (HDDs), solid-state drives (SSDs), and optical drives.It replaced the older Parallel ATA (PATA) standard and became the predominant interface for storage devices.

How does SATA work?

SATA uses a serial communication method, meaning data is transferred one bit at a time over a single data line.This is different from PATA, which used multiple lines to send data simultaneously.This serial approach offers several advantages:

· Higher data transfer rates: SATA can achieve significantly faster speeds than PATA.

· Thinner cables: Serial cables are smaller and easier to manage than the bulky PATA cables.

· Hot swapping: Some SATA devices support hot swapping, allowing you to connect and disconnect them without turning off the computer.

SATA Components

SATA cable: Connects the storage device to the motherboard.
SATA connector: The physical interface on both the device and the motherboard.
SATA power connector: Provides power to the storage device.

Advantages of SATA

· Faster data transfer speeds compared to PATA.

· Thinner and more flexible cables

· Improved error correction.

· Hot plugging support (optional).

· Lower power consumption.

In essence, SATA is a crucial technology that has significantly improved the performance and efficiency of storage devices in computers.

4) Pen drive:

A pen drive is a compact secondary storage device. It is also known as a USB flash drive, thumb drive, or jump drive. It connects to a computer via a USB port. It is commonly used to store and transfer data between computers. For example, you can write a report using a computer and then copy or transfer it to a pen drive. Later, you can connect this pen drive to a computer to see or edit your report. You can also store your important documents and pictures, music, and videos in the pen drive and keep them in a safe place.

Pen drive does not have movable parts; it comprises an integrated circuit memory chip that stores the data. This chip is housed inside a plastic or aluminum casing. The data storage capacity of the pen drive generally ranges from 2 GB to 128 GB. Furthermore, it is a plug-and-play device as you don't need additional drives, software, or hardware to use it.

5) SD Card:

SD Card stands for Secure Digital Card. It is most often used in portable and mobile devices such as smartphones and digital cameras. You can remove it from your device and see the things stored in it using a computer with a card reader.

There are many memory chips inside the SD card that store the data; it does not have moving parts. SD cards are not created equal, so they may differ from each other in terms of speed, physical size, and capacity. For example, standard SD cards, mini SD cards, and micro SD cards.

6) Compact Disk (CD):

Compact Disk is a portable secondary storage device in the shape of a round medium disk. It is made of polycarbonate plastic. The concept of CD was co-developed by Philips and Sony in 1982. The first CD was created on 17 August 1982 at the workshop of Philips in Germany.

In the beginning, it was used for storing and playing sound recordings, later it was used for various purposes such as for storing documents, audio files, videos, and other data like software programs in a CD.

Physical characteristics of a CD/ Structure of CD:

A standard CD is around 5 inches in diameter and 0.05 inches in thickness. It is made of a clear polycarbonate plastic substrate, a reflective metallic layer, and a clear coating of acrylic plastic. These thin circular layers are attached one on top of another as described below:

A polycarbonate disc layer at the bottom has the data encoded by creating lands and pits.

The polycarbonate disc layer is coated with a thin aluminum layer that reflects the laser.

The reflective aluminum layer is coated with a lacquer layer to prevent oxidation to protect the below layers. It is generally spin-coated directly on the top of the reflective layer.

The label print is applied on the lacquer layer, or artwork is screen printed on the top of the disc on the lacquer layer by offset printing or screen printing.

How Does a CD Work?

The data or information is stored recorded or encoded on CD digitally using a laser beam that etches tiny indentations or bumps on its surface. The bump is called a pit, which represents the number 0. Space, where the bump is not created, is called land, and it represents the number 1. Thus, the data is encoded into a compact disc by creating pits (0) and lands (1). The CD players use laser technology to read the optically recorded data.

7) DVD:

DVD is short for digital versatile disc or digital video disc. It is a type of optical media used for storing optical data. Although it is the same size as a CD, its storage capacity is much more than a CD. So, it is widely used for storing and viewing movies and to distribute software programs as they are too large to fit on a CD. DVD was co-developed by Sony, Panasonic, Philips, and Toshiba in 1995.

Types of DVDs:

DVDs can be divided into three main categories which are as follows:

DVD-ROM (Read-Only):

These types of DVDs come with media already recorded on them, such as movie DVDs. As the name suggests, data on these discs cannot be erased or added, so these discs are known as read-only or non-writable DVDs.

DVD-R (Writable):

It allows you to record or write information on the DVD. However, you can write information only once as it becomes a read-only DVD once it is full.

DVD-RW (Rewritable or Erasable):

This type of disc can be erased, written, or recorded multiple times.

Memory Units:

Memory units are used to measure and represent data. Some of the commonly used memory units are:

1) Bit: The computer memory units start from bit. A bit is the smallest memory unit to measure data stored in main memory and storage devices. A bit can have only one binary value out of 0 and 1.

2) Byte: It is the fundamental unit to measure data. It contains 8 bits or is equal to 8 bits. Thus a byte can represent 2^8 or 256 values.

3) Kilobyte: A kilobyte contains 1024 bytes.

4) Megabyte: A megabyte contains 1024 kilobytes.

5) Gigabyte: A gigabyte contains 1024 megabytes.

6) Terabyte: A terabyte contains 1024 gigabytes.

FAQ on Cache Memory, Primary Memory, and Mass Memory

Cache Memory

Q1: What is cache memory?
Answer: Cache memory is a small, high-speed memory located inside or very close to the CPU. It stores copies of frequently accessed data and instructions to speed up processing by reducing the time needed to access data from the main memory (RAM).

Q2: What are the different levels of cache memory?
Answer: Cache memory is typically divided into three levels:

L1 Cache: The smallest and fastest cache, located within the CPU core.
L2 Cache: Larger than L1, located close to the CPU core, and slower but still much faster than RAM.
L3 Cache: Shared among multiple cores within a CPU, larger and slower than L2 but still faster than RAM.

Q3: How does cache memory improve CPU performance?
Answer: Cache memory reduces the time the CPU spends waiting for data from the slower main memory. By storing frequently used data and instructions closer to the CPU, cache memory minimizes latency, leading to faster execution of programs.

Q4: What is cache hit and cache miss?
Answer:

Cache Hit: Occurs when the CPU finds the data it needs in the cache memory, leading to faster data retrieval.
Cache Miss: Occurs when the data is not found in the cache, forcing the CPU to fetch it from the slower main memory, which takes more time.

Q5: Can cache memory be upgraded?
Answer: No, cache memory is integrated into the CPU and cannot be upgraded or expanded separately.

Primary Memory (Main Memory or RAM)

Q1: What is primary memory?
Answer: Primary memory, or main memory, refers to the Random Access Memory (RAM) in a computer. It temporarily stores data and instructions that the CPU needs while performing tasks, allowing for quick access and execution.

Q2: What are the types of primary memory?
Answer: Primary memory mainly includes:

DRAM (Dynamic RAM): Most common type of RAM used in general computing.
SRAM (Static RAM): Faster and more expensive than DRAM, often used for cache memory.
ROM (Read-Only Memory): Non-volatile memory that contains essential instructions for booting up the system.

Q3: How does primary memory differ from secondary memory?
Answer: Primary memory is volatile, meaning it loses its data when the power is turned off, whereas secondary memory (e.g., hard drives, SSDs) is non-volatile and retains data permanently. Primary memory is faster but has a smaller capacity compared to secondary memory.

Q4: What role does primary memory play in computer performance?
Answer: Primary memory directly affects the performance of a computer by determining how much data and how many programs can be processed simultaneously. More RAM generally allows for better multitasking and faster data access.

Q5: Can primary memory be upgraded?
A5: Yes, most computers allow for RAM upgrades, which can improve overall system performance.

Mass Memory (Secondary Memory or Storage)

Q1: What is mass memory?
Answer: Mass memory, also known as secondary memory or storage, refers to devices that store large amounts of data permanently. This includes hard drives (HDDs), solid-state drives (SSDs), optical drives (CD/DVD/Blu-ray), and external storage devices.

Q2: What are the types of mass memory?
Answer: Common types of mass memory include:

Hard Disk Drives (HDDs): Magnetic storage devices with moving parts, typically offering large storage capacities at lower cost.
Solid-State Drives (SSDs): Faster, more reliable storage devices with no moving parts, offering quicker data access and better performance.
Optical Drives: Use lasers to read and write data on discs like CDs, DVDs, and Blu-rays.
Flash Drives: Portable storage devices using flash memory, commonly used for data transfer.

Q3: How does mass memory differ from primary memory?
Answer: Mass memory is non-volatile and retains data even when the power is off, whereas primary memory (RAM) is volatile and only stores data temporarily while the computer is running. Mass memory also has a much larger capacity but is slower than primary memory.

Q4: Can mass memory be upgraded?
Answer: Yes, mass memory can be upgraded or expanded by adding or replacing hard drives, SSDs, or other storage devices, depending on the system’s compatibility.

Q5: What is the difference between HDD and SSD?
Answer:

HDDs use spinning disks to read/write data, making them slower and more prone to mechanical failure but generally cheaper with higher capacities.
SSDs use flash memory, providing faster data access, no moving parts (which makes them more durable), but they are typically more expensive per gigabyte than HDDs.

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