Basics of OS (I/O Structure)

Summary

This lecture delves into the input/output (I/O) structure of operating systems, emphasizing its significance for system reliability and performance. It explains the role of I/O devices, device controllers, and device drivers in managing data transfers. The process of initiating I/O operations, involving device driver interaction with device controllers and data transfer to local buffers, is outlined. The lecture also contrasts interrupt-driven I/O with Direct Memory Access (DMA), highlighting DMA's efficiency in handling bulk data movement by reducing CPU intervention and minimizing overhead.

Takeaways

• 📚 The lecture focuses on the input/output (I/O) structure, a fundamental aspect of the Operating System.
• 🖥️ Input/output devices are essential for providing inputs to and receiving outputs from a computer, including storage devices.
• 🔌 A significant portion of the operating system's code is dedicated to managing I/O due to its impact on system reliability and performance.
• 🛠️ Different I/O devices have different natures, requiring the operating system to manage a variety of device controllers.
• 🔄 A general-purpose computer system comprises CPUs, device controllers, and a common bus that provides access to shared memory.
• 🔑 Each device controller is responsible for a specific type of device and maintains local buffer storage and special-purpose registers.
• 🚀 The operating system includes a device driver for each device controller to provide a uniform interface for interaction.
• 🔧 The basic I/O operation involves the device driver loading registers in the device controller, which then determines the action to take based on the register contents.
• 🔄 After loading, the device controller transfers data from the device to its local buffer and then informs the device driver via an interrupt once the operation is complete.
• 🚨 Interrupt-driven I/O can be inefficient for bulk data movement due to high CPU overhead from frequent interrupts.
• 🚀 Direct Memory Access (DMA) is used to overcome this inefficiency by allowing data transfer between device buffers and memory without CPU intervention, reducing the need for interrupts.

Q & A

What is the primary focus of the lecture?

The lecture primarily focuses on the input/output (I/O) structure within the basics of the Operating System, discussing the importance of I/O devices, their management, and the role of device controllers and drivers.

Why is managing input/output devices considered important in an operating system?

Managing input/output devices is important because it greatly affects the reliability and performance of a system, and due to the varying nature of the devices, a significant portion of the operating system code is dedicated to managing I/O.

What is the role of a device controller in a computer system?

A device controller is responsible for a specific type of device, maintaining local buffer storage and a set of special-purpose registers, which are used for managing the data transfer and communication with the device.

What does a device driver do in the context of an operating system?

A device driver understands the device controller and presents a uniform interface to the device and the rest of the operating system, allowing for proper interaction despite the varying nature of different device controllers.

How does an I/O operation typically start in an operating system?

An I/O operation typically starts with the device driver loading the appropriate register within the device controller, which contains the information or data about the action or the exact I/O operation to be performed.

What is the purpose of the local buffer in a device controller?

The local buffer in a device controller temporarily stores data that is being transferred from the device during an I/O operation, before it is ultimately moved to or from the system memory.

How does a device controller inform the device driver that an I/O operation has been completed?

Once the data transfer and execution are complete, the device controller informs the device driver that it has finished the operation by generating an interrupt.

What is the main disadvantage of interrupt-driven I/O for bulk data movement?

Interrupt-driven I/O can produce high overhead when used for bulk data movement because it requires frequent CPU interruptions, which can consume significant CPU time and resources.

What is Direct Memory Access (DMA) and how does it solve the overhead problem of interrupt-driven I/O?

Direct Memory Access (DMA) allows a device controller to transfer an entire block of data directly to or from its own buffer storage to memory without CPU intervention, reducing the need for frequent interrupts and thus lowering the CPU overhead.

How does DMA differ from standard interrupt-driven I/O in terms of CPU usage?

In DMA, the CPU is not interrupted for each byte of data transfer, as it is in standard interrupt-driven I/O. Instead, only one interrupt is generated per block of data transfer, allowing the CPU to perform other tasks without being disturbed.

What is the significance of having a device driver for each device controller in terms of operating system design?

Having a device driver for each device controller allows the operating system to interact with a wide variety of devices in a standardized way, abstracting the complexity of different hardware and making the system more modular and extensible.

Outlines

00:00

💾 Basics of Input/Output and Storage Devices

This paragraph introduces the concept of input/output (I/O) structures within the operating system, emphasizing the importance of I/O devices in a computer system. It explains that storage devices are a subset of I/O devices and highlights the significance of managing I/O for system reliability and performance. The paragraph also describes the general purpose computer system's architecture, including CPUs, device controllers, and the common bus that connects them. It introduces the role of device controllers in managing local buffer storage and special-purpose registers, and the necessity of device drivers to provide a uniform interface for different device controllers to interact with the operating system.

05:03

🔄 The Working of Basic I/O Operations and Interrupts

This section delves into the process of initiating an I/O operation, where the device driver loads the necessary registers within the device controller. It explains how the device controller uses these registers to determine the required action and then transfers data from the device to its local buffer. The paragraph also discusses the notification mechanism via interrupts once the operation is complete, allowing the device driver to return control to the operating system. Additionally, it points out the inefficiency of interrupt-driven I/O for bulk data transfer due to high CPU overhead and introduces the concept of Direct Memory Access (DMA) as a solution.

10:07

🚀 Direct Memory Access (DMA) for Efficient Data Transfer

The final paragraph explains the advantages of Direct Memory Access (DMA) over traditional interrupt-driven I/O. It describes how, in DMA, the device controller can transfer an entire block of data directly to or from memory without CPU intervention, reducing the need for multiple interrupts. This approach allows the CPU to perform other tasks concurrently, thus enhancing system efficiency. The paragraph also contrasts DMA with the previous method, where an interrupt was generated per byte transferred, highlighting the significant reduction in CPU overhead and the improved performance for bulk data operations.

Keywords

💡Input/Output (I/O) Devices

Input/Output devices are hardware components that allow for the interaction between the user and the computer system, either by providing data to the system (input) or displaying data from the system (output). In the context of the video, storage devices are highlighted as a type of I/O device, emphasizing their importance in the computer system's operation. The script discusses how a significant portion of the operating system's code is dedicated to managing these devices due to their impact on system reliability and performance.

💡Operating System (OS)

The operating system is the foundational software that manages computer hardware and software resources. It acts as an interface between the user and the computer's hardware, handling tasks such as process management, memory allocation, and device management. The video script explains how the OS plays a crucial role in managing I/O devices, which is a fundamental aspect of its functionality.

💡Device Controller

A device controller is a specialized hardware component that manages a specific type of I/O device. It is responsible for the communication between the device and the rest of the computer system. The script mentions that each device is connected to a specific device controller, which in turn is connected to a common bus, highlighting the controller's role in facilitating data transfers to and from the device.

💡Local Buffer Storage

Local buffer storage refers to a temporary data storage area within a device controller. It is used to hold data during input/output operations, allowing for efficient data transfer between the device and the main memory. The script explains that the device controller maintains this buffer, which is crucial for managing the flow of data during I/O operations.

💡Special-Purpose Registers

Special-purpose registers are registers within a device controller that have specific functions related to the management of I/O operations. They store information necessary for the controller to perform its tasks, such as data transfer commands and status information. The script points out that the device controller maintains these registers to facilitate the execution of I/O operations.

💡Device Driver

A device driver is a software component within the operating system that provides a standardized interface for the interaction between the OS and a specific device controller. It understands the controller's functionality and presents a uniform interface to the rest of the operating system. The script explains the importance of device drivers in managing the varying nature of different device controllers.

💡I/O Operation

An I/O operation refers to the process of transferring data between an I/O device and the computer's memory. The script details the steps involved in starting an I/O operation, including loading registers within the device controller and the subsequent data transfer to the local buffer. This process is central to the video's discussion on how I/O devices are managed by the operating system.

💡Interrupt

An interrupt is a signal sent to the CPU to temporarily halt its current tasks and attend to a different task, such as responding to an I/O operation's completion. The script explains how the device controller uses interrupts to inform the device driver that an I/O operation has finished, which is a critical aspect of managing I/O operations efficiently.

💡Direct Memory Access (DMA)

Direct Memory Access is a method of allowing I/O devices to transfer data directly to and from memory without the intervention of the CPU, thus reducing the CPU's load during bulk data transfers. The script discusses DMA as a solution to the high overhead associated with interrupt-driven I/O for large data transfers, highlighting its efficiency and the CPU's availability for other tasks during DMA operations.

💡Bulk Data Movement

Bulk data movement refers to the process of transferring large amounts of data between storage devices and memory. The script points out that while interrupt-driven I/O is suitable for small data transfers, it can be inefficient for bulk data movement due to the high CPU overhead. DMA is introduced as a more efficient method for handling such transfers.

💡CPU

The CPU, or Central Processing Unit, is the primary component of a computer that performs most of the processing. It executes instructions and manages the computer's operation. In the context of the video, the script discusses the CPU's role in I/O operations and how DMA allows the CPU to be less involved in data transfers, thus freeing it to perform other tasks.

Highlights

Lecture focuses on the input/output structure in the context of Operating System basics.

Storage is identified as a type of input/output device within a computer.

Importance of managing I/O devices for system reliability and performance emphasized.

Explanation of the varying nature of different input/output devices.

Operating system code is largely dedicated to I/O management due to its critical role.

General purpose computer systems consist of CPUs and device controllers connected through a common bus.

Each device is connected to a specific device controller responsible for its operation.

Device controllers maintain local buffer storage and special-purpose registers.

Device drivers provide a uniform interface between device controllers and the operating system.

I/O operation begins with the device driver loading appropriate registers within the device controller.

Device controller examines register contents to determine the required action for I/O operation.

Data transfer from the device to the device controller's local buffer is initiated.

Device controller informs the device driver of operation completion via an interrupt.

Diagrammatic representation of the I/O operation cycle involving CPU, device, and memory.

Interrupt-driven I/O can produce high overhead for bulk data movement.

Introduction of Direct Memory Access (DMA) to solve inefficiencies in bulk data transfer.

DMA allows device controllers to transfer data directly to memory without CPU intervention.

Only one interrupt is generated per block in DMA, reducing CPU overhead.

CPU remains available for other tasks during DMA operations, enhancing efficiency.

Summary of I/O structure and DMA as fundamental aspects of operating system basics.

Transcripts

In the last lecture, we have studied the storage structure

under the basics of the Operating System.

And in this lecture we will study input/output structure,

which also comes under the basics of the Operating system.

So, as we studied about storage structure in the previous lecture,

we see here that storage is only one of the many types of

input/output devices within a computer.

So, what are the input/output devices?

Input/output devices are the devices that are used for either

giving inputs or getting outputs from your computer.

And storage is also just one of the many input/output devices that we have.

So, we see that input/output devices are something that is very important.

So, a large portion of operating system code is dedicated to managing I/O,

both because of its importance to the reliability and performance of a system

and because of the varying nature of the devices.

So, as I told you managing input/output devices is very important

So, when you code the operating system

a very large portion of that code

is dedicated for managing the input/output.

And why is that?

It is because of its reliability and performance of the system.

The reliability and performance of a system

greatly depend upon how its inputs and outputs are managed.

And also because of the varying nature of the devices.

We know that we have different input/output devices

and all these input/output devices are different in nature.

So, since they are very different and varying in nature

and because it is very important for the reliability and performance of a system

a large portion of operating system code are dedicated for managing this input/output.

So, we see that a general purpose computer system

consists of CPU, and multiple device controllers,

that are connected through a common bus.

Now this was something that we have studied

when I started teaching about Operating System.

Just go back and look into it once more.

So, here when we studied about computer system operation.

We said that a general purpose computer contains one or more CPUs,

and a number of device controllers, connected through a common bus.

that provides access to the shared memory.

So, here we see that we have different devices

and each device is connected to a particular device controller like this.

Here we see that the disk is connected to a disk controller

and then the USB devices like your mouse, keyboard, and printer

were connected to the USB controllers

and monitor is connected to the video adapter.

So, each device is connected to a device controller

which is connected by a common bus.

This line here represents the common bus.

And they provide access to the shared memory

So, that is what we have said over here.

And now each device controller is in charge of a specific type of device.

We know that there are different types of device

and each device controller is in charge of a specific type of device.

And this device controller maintains two things.

What are they?

One is local buffer storage.

And the other one is the set of special-purpose registers.

So, we will see why it maintains this

and what is the use of this as we proceed further.

But remember that a device controller maintains a local buffer storage,

and a set of special purpose registers.

Typically operating system has a device driver for each device controller.

We have talked about device controller,

Now, we are talking about an another term called device driver.

Now, operating system has a device driver for each device controller.

Lets see why do we have that!

This device driver understands the device controller

and presents a uniform interface to the device

and the rest of the operating system.

So, as I told you each device is very different

They have varying nature, as we have said.

And then each device controller is different for different devices.

How will these device controllers interact with the operating system?

We need a proper interface,

that allows the device controller to interact with the operating system.

So, the device drivers understand the nature of the device controller

and provide a uniform interface to the device and the rest of the operating system.

So, that is the function of your device driver.

Now, let's see how does a basic I/O operation works?

So here we will see the working of an I/O operation.

So, here we have a diagram and its explanation.

So, let's see!

First of all, to start an I/O Operation,

the device driver loads the appropriate register within the device controller.

So, when an I/O operation has to be performed by any device,

What it does?

the respective device driver of the device

loads the appropriate register within the device controller.

So, when we were talking about the device controller, I told you that

Device controllers maintain a set of registers.

So, from these registers, the appropriate registers that are required for

performing a particular input/output operation

will be loaded by the device driver.

And then the second point is

The device controller , in turns, And then the second point is

The device controller , in turns,

examine the contains of these registers to determine what action to take.

now why does it loads those appropriate registers?

Those registers are loaded because in that registers,

we have the information or the data about,

what is the action?

or what is the exact input/output operation that has to be performed?

So, after the device driver loads the those

appropriate register within the device controller,

The device controller will examine the contains of those registers,

and determine what action to take.

So, the device controller will examine that

and let you know what is the action that has to be taken. So, the device controller will examine that

and let you know what is the action that has to be taken.

And then, the controller starts to transfer of data

from the device to its local buffer.

So, whatever has to be performed, the data for that

will be transferred to the local buffer of the device controller.

So, when I talked about the device controller, I told you,

It has a local buffer and a set of registers.

So, we saw what the registers are used for.

And now, comes the local buffer.

So, the data from the device

is transferred to the local buffer of the device controller.

And then, once the transfer of data is complete

the device controller informs the device driver

via an interrupt that it has finished the operation.

so, we saw that the data was transferred to the local buffer,

and then once the data transfer and execution is complete,

the device controller will inform the device driver that it has finished its operation.

So, once operation is finished the device controller will inform the device driver.

And how will it inform?

It will inform via an interrupt.

I have already explained it in one of my previous lectures on interrupt.

So, via interrupt, it will inform the device driver by the device controller that

it has finished its operation when it is done.

And then what happens?

The device driver then returns control to the operating system.

After it has been done, It continues the other task that it has to do.

Now, this is the diagram that represents whatever I was explaining over here.

Here you have the CPU and a device, and this is the memory.

And then when the device has an input/output request

after loading the registers and everything,

the device controller loads the data to the local buffer,

and then it goes via the CPU to the memory.

So, here we have the instruction execution cycle and data movement

between the CPU and the memory.

And whatever task has to be done will be done.

And then the data that has to be transferred back to the device will be transferred back.

And once everything is done.

Via an interrupt, it tells that I am done with my operation.

This is the diagrammatic representation of whatever is written here.

now we see that there is an disadvantage of this kind of operation.

Let see what it is.

This kind of interrupt-driven I/O is fine for moving small amount of data

but can produce high overhead when used for bulk data movement.

So, we see that when this kind of system is used

It is okay, when it is used for moving a small amount of data.

But If you want to move

or if you want to do an input/output operation that involves

a large or bulk amount of data movement,

then this is not a very efficient system.

Why?

Because it is going to take up so much of your CPU and time.

because the CPU is always being interrupted over here.

In order to solve that, what do we do?

To solve this problem direct memory access also known as DMA is used.

So, in order to solve this problem,

we have something called direct memory access or DMA.

So, that is what represented over here.

And let's see what happens in DMA?

After setting up buffers, pointers, and counters for the I/O devices,

the device controller transfer an entire block of data

directly to or from its own buffer storage to memory,

with no intervention by the CPU.

So, even in this direct memory access,

the first three points remain the same.

they will have to load the register, the buffer, and everything.

And after doing that, instead of going via CPU in this way.

the device controller transfers the entire block of data

directly to the memory from its buffer storage without intervention of the CPU.

So, in DMA we see that the CPU is not affected.

It just goes directly to the memory.

So, whatever has to be sent to the memory can be directly sent

and whatever has to be taken back from the memory

can directly be done without the intervention of the CPU.

So, this is direct memory access.

So, in this direct memory access, only one interrupt is generated per block

to tell the device driver that the operation has completed.

So, here only one interrupt per block is required.

But in case of the other system that we have previously explained,

there will be interrupt per byte of data that is being transferred.

So, the CPU is interrupted so many times making it an inefficient system

But here only one interrupt is generated per block.

and when it is generated?

It is generated to tell the device driver that the operation has completed.

So, while the device controller is performing this operation

the CPU is available to accomplish other works.

So, this is one of the greatest advantages of this.

As we said!

In DMA, the CPU is not intervened.

Even when this I/O operation is occurring,

the CPU is free to perform its other task and it will not be disturbed.

So, this is the advantage of direct memory access.

So, that is how input/output operation works.

and how DMA works.

So, I hope this was clear to you.

This was about the input/output structure,

which is also the basics of your operating system.

So, I hope this was clear to you.

Thank you for watching and see you in the next one.