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Operating System Principles AUA CIS331 Albert Minasyan Computer System Structures Handout Content: I/O structure o I/O hardware Isolated I/O ports Memory mapped I/O o I/O software Polling, Poll handshaking Interrupt o DMA o Blocking, Non-Blocking I/O o Buffering Textbook Silberschatz, Chapter 2 1 Operating System Principles AUA CIS331 Albert Minasyan I/O Structure A modern, general-purpose computer system consists of a CPU and a number of device controllers that are connected through a common bus that provides access to shared memory. Each device controller is in charge of a specific type of device (for example, disk drives, audio devices, and video displays). Depending on the controller, there may be more than one attached device. For instance, the small computer-systems interface (SCSI) controller can have seven or more devices attached to it. A device controller maintains some local buffer storage and a set of special-purpose registers. The device controller is responsible for moving the data between the peripheral devices that it controls and its local buffer storage. The CPU and the device controllers can execute concurrently, competing for memory cycles. To ensure orderly access to the shared memory, a memory controller is provided whose function is to synchronize access to the memory. Typically, operating systems have a device driver for each device controller. This device driver understands the device controller and presents a uniform interface to the device to the rest of the operating system. Memory Controller General purpose computer system • A modern, general-purpose computer system consists of • CPU • number of device controllers They are connected through a common bus BUS also provides access to shared memory • • • • • • The device controller is responsible for moving the data between the peripheral devices that it controls and the BUS. Each device controller is in charge of a specific type of device • Many same type of devices can be connected through the same controller (SCSI, USB) The CPU and the device controllers can communicate directly and exchange information • Between Device Controller and CPU registers. The CPU and the device controllers2.2.1. can execute concurrently, competing for memory I/O Hardware access (Direct Memory Access to Memory by Device Controller). • Memory controller’s function is to synchronize access to the memory 2 Operating System Principles AUA CIS331 Albert Minasyan Memory-mapped I/O and isolated I/O are two complementary methods of performing input/output between the CPU and peripheral devices in a computer. An alternative approach is using dedicated I/O processors — commonly known as channels on mainframe computers — that execute their own instructions. Despite the incredible variety of I/O devices we need only a few concepts to understand how the devices are attached and how the software can control the hardware. There are two ways of information exchange between the host and I/O devices: • Through the Isolated I/O ports • Through the Memory Mapped I/O Ports The information exchange could be done using one of below methods by the software: • Polling or programmed I/O • Interrupts How can the processor give commands and data to a controller to accomplish an I/O transfer? The short answer is that the controller has one or more registers for data and control signals. The processor communicates with the controller by reading and writing bit patterns in these registers. One way in which this communication can occur is through the use of special I/O instructions that specify the transfer of a byte or word to an I/O port address. The I/O instruction triggers bus lines to select the proper device and to move bits into or out of a device register. Information exchange through the I/O ports How can the processor give commands and data to a controller to accomplish an I/O transfer ? • • • The controller has one or more registers for data and control signals. These registers (we call them I/O ports) could be accessed by unique addresses – I/O port addresses. The processor communicates with the controller by reading and writing bit patterns in these registers CPU Bus I/O ports 378-37F I/O ports 320-32F HDD controller Printer controller I/O ports 060-064 Keyboard controller An I/O port typically consists of four registers, called the (1) status, (2) control, (3) data-in, and (4) data-out registers. The Data-Input Register is read by the host to get input. The Data-Output Register is written by the host to send output. 3 Operating System Principles AUA CIS331 Albert Minasyan The Status Register contains bits that can be read by the host. These bits indicate states, such as whether the current command has completed, whether a byte is available to be read from the data-in register, and whether a device error has occurred. The Control Register can be written by the host to start a command or to change the mode of a device. For instance, a certain bit in the control register of a serial port chooses between full-duplex and half-duplex communication, another bit enables parity checking, a third bit sets the word length to 7 or 8 bits, and other bits select one of the speeds supported by the serial port. The data registers are typically 1 to 4 bytes in size. Some controllers have FIFO chips that can hold several bytes of input or output data to expand the capacity of the controller beyond the size of the data register. A FIFO chip can hold a small burst of data until the device or host is able to receive those data. An I/O port typically consists of 4 registers, called the (1) status, (2) control, (3) data-in (4) data-out The Data-Input Register is read by the host to get the input data. f.e. the pressed key’s value from the keyboard The Data-Output Register is written by the host to send output data. (f.e. LED’s value of keyboard – NumLock, ScrollLock, CapsLock.) The Status Register contains bits that can be read by the host. These bits indicate states, such as whether the current command has completed, whether a byte is available to be read from the data-in register, and whether a device error has occurred. The Control Register can be written by the host to start a command or to change the mode of a device o one of the bits could define the full-duplex or half-duplex communication o another bit parity checking o a third bit sets the word length to 7 or 8 bits o other bits select one of the speeds supported by the serial port Status Register(s) Control Register(s) Data Input Register(s) Data Output Register(s) Some Logic Keyboard Controller 4 Operating System Principles AUA CIS331 Albert Minasyan Isolated I/O ports One way in which this communication can occur is through the use of special I/O instructions in al, 64h ;Read keyboard status register from 0x64 port. out 60h, al ;send the command to keyboard to port 0x60 In this case we use the Isolated I/O ports. The below Figure shows the general map of Isolated I/O port addresses for PCs. Alternatively, the device controller can support memory mapped I/O. In this case, the devicecontroller registers are mapped into the address space of the processor. The CPU executes I/O requests using the standard data-transfer instructions to read and write the device-controller registers. The graphics controller has I/O ports for basic control operations, but the controller has a large memory mapped region to hold screen contents. The process sends output to the screen by writing data into the memory-mapped region. The controller generates the screen image based on the contents of this memory. This technique is simple and convenient to use. Moreover, writing millions of bytes to the graphics memory is faster than issuing millions of I/O instructions. But the ease of writing to a memory-mapped I/O controller is offset by a disadvantage. Because a common type of software fault is a write through an incorrect pointer to an unintended region of memory, a memory-mapped device register is vulnerable to accidental modification. Of course, protected memory helps to reduce this risk. Some systems use both techniques. For instance, PCs use I/O instructions to control some devices and memory-mapped I/O to control others. 5 Operating System Principles AUA CIS331 Albert Minasyan Memory Mapped Input / Output The device-controller registers are mapped into the address space of the processor. The CPU executes I/O requests using the standard data-transfer (memory access) instructions to read and write the device-controller registers. The graphics controller has I/O ports for basic control operations, but the controller has a large memory mapped region to hold screen contents. The controller generates the screen image based on the contents of this memory. RAM FFFF FFFF E000 0000 Write patterns FF- white pixel 00 - black pixel CPU Video RAM buffer FF- white pixel, 00 - black pixel Video 0000 0000 Signals Continuously Retrieve Controller patterns and Transform to Signals Separate Video RAM. • • • • • The address space is the same (main RAM’s address space But physically the Video RAM is placed in the Video Controller Does not take any amount of memory from the existing RAM Not Separated VRAM – You have 2GB memory – 0.5 GB is lost for VRAM Separated VRAM – You have 2GB memory + 0.5 GB VRAM Most forms of video RAM are dual-ported, which means that • while the processor is writing a new image to video RAM • the display is reading from video RAM to refresh its current display content. 6 Operating System Principles AUA CIS331 Albert Minasyan Advantage: The Memory Mapped I/O technique is simple and convenient to use. Moreover, writing millions of bytes to the graphics memory is faster than issuing millions of I/O instructions. Disadvantage: Because a common type of software fault is a write through an incorrect pointer to an unintended region of memory, a memory-mapped device register is vulnerable to accidental modification. Of course, protected memory helps to reduce this risk. Video RAM (VRAM) means in general all forms of random access memory (RAM) used to store image data for a computer display. All types of video RAM are special arrangements of dynamic RAM (DRAM). Video RAM is really a buffer between the computer processor and the display and is often called the frame buffer. When images are to be sent to the display, they are first read by the processor as data from some form of main (non-video) RAM and then written to video RAM. From video RAM (the frame buffer), the data is converted by a RAM digital-to-analog converter (RAMDAC) into analog signals that are sent to the display presentation mechanism such as a cathode ray tube (CRT). Most forms of video RAM are dual-ported, which means that while the processor is writing a new image to video RAM the display is reading from video RAM to refresh its current display content. The dual-port design is the main difference between main RAM and video RAM. 7 Operating System Principles AUA CIS331 Albert Minasyan I/O handling by the software Polling or Programmed I/O The complete protocol for interaction between the host and a controller can be intricate, but the basic handshaking notion is simple. We explain handshaking with an example. Assume that 2 bits are used to coordinate the producer-consumer relationship between the controller and the host. The controller indicates its state through the busy bit in the status register. (Recall that to set a bit means to write a 1 into the bit and to clear a bit means to write a 0 into it.) The controller sets the busy bit when it is busy working and clears the busy bit when it is ready to accept the next command. The host signals its wishes via the command-ready bit in the command register. The host sets the command-ready bit when a command is available for the controller to execute. For this example, the host writes output through a port, coordinating with the controller by handshaking as follows. 1. 2. 3. 4. 5. 6. 7. The host repeatedly reads the busy bit until that bit becomes clear. The host sets the write bit in the command register and writes a byte into the data-out register. The host sets the command-ready bit. When the controller notices that the command-ready bit is set, it sets the busy bit. The controller reads the command register and sees the write command. It reads the data-out register to get the byte and does the I/O to the device. The controller clears the command-ready bit, clears the error bit in the status register to indicate that the device I/O succeeded, and clears the busy bit to indicate that it is finished. This loop is repeated for each byte. Status Control Busy bit Write bit, Command ready bit Data Intput Register(s) Data Output Byte Some Logic Keyboard Controller Polling or Programmed I/O Advantage: This technique is straightforward and simple and could be used when there is no performance requirements or I/O devices are very fast and not too many. Disadvantage: CPU waits the I/O device in a loop and polling becomes inefficient when it is attempted repeatedly yet rarely finds a device to be ready for service, while other useful CPU processing remains undone. Alternative: In such instances, it may be more efficient to arrange for the hardware controller to notify the CPU when the device becomes ready for service, rather than to require the CPU to poll repeatedly for an I/O completion. The hardware mechanism that enables a device to notify the CPU is called an interrupt. 8 Operating System Principles AUA CIS331 Albert Minasyan In step 1, the host is busy-waiting or polling: it is in a loop, reading the status register over and over until the busy bit becomes clear. If the controller and device are fast, this method is a reasonable one. But if the wait may be long, the host should probably switch to another task. How, then, does the host know when the controller has become idle? For some devices, the host must service the device quickly, or data will be lost. For instance, when data are streaming in on a serial port or from a keyboard, the small buffer on the controller will overflow and data will be lost if the host waits too long before returning to read the bytes. In many computer architectures, three CPU-instruction cycles are sufficient to poll a device: • read a device register, • logical-and to extract a status bit, • and branch if not zero. Clearly, the basic polling operation is efficient. But polling becomes inefficient when it is attempted repeatedly yet rarely finds a device to be ready for service, while other useful CPU processing remains undone. In such instances, it may be more efficient to arrange for the hardware controller to notify the CPU when the device becomes ready for service, rather than to require the CPU to poll repeatedly for an I/O completion. The hardware mechanism that enables a device to notify the CPU is called an interrupt. Poll Handshaking 9 Operating System Principles AUA CIS331 Albert Minasyan I/O Interrupts If the CPU uses polling to watch the control bit, constantly looping to see whether the device is ready, this method of operation is called programmed I/O (PIO). If the CPU does not poll the control bit, but instead receives an interrupt when the device is ready for the next byte, the data transfer is said to be interrupt driven. The basic interrupt mechanism works as follows The CPU hardware has a wire called the Interrupt-Request Line CPU senses that line after executing every instruction. The device controller raises an interrupt by asserting a signal on the interrupt request line The CPU catches the interrupt and dispatches it to the interrupt handler And the handler clears the interrupt by servicing the device. CPU Bus I/O ports 378-37F I/O ports 320-32F Interrupt request HDD controller Printer controller I/O ports 060-064 Keyboard controller When the CPU detects that a controller has asserted a signal on the line, the CPU performs a state save and jumps to the Interrupt-Handler Routine at a fixed address in memory. The interrupt handler determines the cause of the interrupt, performs the necessary processing, performs a state restore, and executes a return from interrupt instruction to return the CPU to the execution state prior to the interrupt. We say that the device controller raises an interrupt by asserting a signal on the interrupt request line, the CPU catches the interrupt and dispatches it to the interrupt handler, and the handler clears the interrupt by servicing the device. 10 Operating System Principles AUA CIS331 Albert Minasyan The Figure below summarizes the interrupt-driven I/O cycle. 1. User process in CPU requests I/O operation for I/O device 2. I/O device starts I/O operation 4. CPU interrupts and processes the I/O result. 3. I/O device finished I/O operation. Interrupt request is sent to CPU Interrupt time line for a single process doing output. 11 Operating System Principles AUA CIS331 Albert Minasyan The Efficiency of an Interrupt Driven System Interrupts introduce a considerable amount of complexity to a software system. One might ask if using interrupts is really worth the trouble. The answer of course, is yes. Why else would people use interrupts if they were proven not to be worthwhile? However, interrupts are like many other nifty things in computer science – they have their place; if you attempt to use interrupts in an inappropriate fashion they will only make things worse for you. The following sections explore the efficiency aspects of using interrupts. As you will soon discover, an interrupt driven system is usually superior despite the complexity. However, this is not always the case. For many systems, alternative methods provide better performance. Interrupt Driven I/O vs. Polling The whole purpose of an interrupt driven system is to allow the CPU to continue processing instructions while some I/O activity occurs. This is in direct contrast to a polling system where the CPU continually tests an I/O device to see if the I/O operation is complete. In an interrupt driven system, the CPU goes about its business and the I/O device interrupts it when it needs servicing. This is generally much more efficient than wasting CPU cycles polling a device while it is not ready. The serial port is a perfect example of a device that works extremely well with interrupt driven I/O. You can start a communication program that begins downloading a file over a modem. Each time a character arrives, it generates an interrupt and the communication program starts up, buffers the character, and then returns from the interrupt. In the meantime, another program (like a word processor) can be running with almost no performance degradation since it takes so little time to process the serial port interrupts. Contrast the above scenario with one where the serial communication program continually polls the serial communication chip to see if a character has arrived. In this case the CPU spends all of its time looking for an input character even though one rarely (in CPU terms) arrives. Therefore, no CPU cycles are left over to do other processing like running your word processor. Suppose interrupts were not available and you wanted to allow background downloads while using your word processing program. Your word processing program would have to test the input data on the serial port once every few milliseconds to keep from losing any data. Can you imagine how difficult such a word processor would be to write? An interrupt system is the clear choice in this case. If downloading data while word processing seems far fetched, consider a more simple case – the PC’s keyboard. Whenever a keypress interrupt occurs, the keyboard ISR reads the key pressed and saves it in the system type ahead buffer for the moment when the application wants to read the keyboard data. Can you imagine how difficult it would be to write applications if you had to constantly poll the keyboard port yourself to keep from losing characters? Even in the middle of a long calculation? Once again, interrupts provide an easy solution 12 Operating System Principles AUA CIS331 Albert Minasyan Direct Memory Access (DMA) Above forms of Programmed I/O (PIO) or Interrupt-driven I/O are fine for moving small amounts of data but can produce high overhead when used for bulk data movement such as disk I/O. Many computers avoid burdening the main CPU with PIO by offloading some of this work to a special-purpose processor called a Direct-Memory-Access (DMA) Controller. The system bus consists of 3 main groups of lines: • Address Bus: Transfers the memory address or the device I/O port address • Data Bus: Transfers the data between CPU registers, Memory and Device Controllers’ Registers. The data is accompanied with the Address. • Control Bus: Signals on these lines show the availability or the direction of sending the Data or the Address. Many lines show the Availability or Absent of some events in the system. To initiate a DMA transfer, the host writes a DMA command block into memory. This block contains a pointer to the source of a transfer, a pointer to the destination of the transfer, and a count of the number of bytes to be transferred. The CPU writes the address of this command block to the DMA controller, then goes on with other work. The DMA controller proceeds to operate the memory bus directly, placing addresses on the bus to perform transfers without the help of the main CPU. When the entire transfer is finished, the DMA controller interrupts the CPU. When the DMA controller seizes the memory bus, the CPU is momentarily prevented from accessing main memory although it can still access data items in its primary and secondary caches. Although this Cycle-Stealing can slow down the CPU computation, offloading the data-transfer work to a DMA controller generally improves the total system performance. Some computer architectures use physical memory addresses for DMA, but others perform Direct Virtual Memory Access (DVMA) using virtual addresses that undergo translation to physical addresses. DVMA can perform a transfer between two memory-mapped devices without the intervention of the CPU or the use of main memory. DMA operations can be performed in either burst or single-cycle mode. Some DMA controllers support both. In burst mode, the DMA controller keeps control of the bus until all the data buffered by the requesting device has been transferred to memory (or when the output device buffer is full, if writing to a peripheral). In single-cycle mode, the DMA controller gives up the bus after each transfer. This minimizes the amount of time that the DMA controller keeps the processor off of the memory bus, but it requires that 13 Operating System Principles AUA CIS331 Albert Minasyan the bus request/acknowledge sequence be performed for every transfer. This overhead can result in a drop in overall system throughput if a lot of data needs to be transferred. Programmed I/O (PIO) or Interrupt-driven I/O are fine for moving small amounts of data but can produce high overhead when used for bulk data movement such as disk I/O. Many computers avoid burdening the main CPU with PIO by offloading some of this work to a special-purpose processor called a Direct-Memory-Access (DMA) Controller. To initiate a DMA transfer, the host writes a DMA command block into memory. This block contains: o a pointer to the source of a transfer o a pointer to the destination of the transfer o a count of the number of bytes to be transferred. The CPU writes the address of this command block to the DMA controller, then goes on with other work. The DMA controller proceeds to operate the memory bus directly to perform transfers without the help of the main CPU. o When the DMA controller seizes the memory bus, the CPU is momentarily prevented from accessing main memory although it can still access data items in its primary and secondary caches. When the entire transfer is finished, the DMA controller interrupts the CPU. DMA Advantage: CPU does not take care of data transfer. DMA Problems: Hardware implementation of DMA is needed. DMA modes In burst mode, the DMA controller keeps control of the bus until all the data buffered by the requesting device has been transferred to memory (or when the output device buffer is full, if writing to a peripheral). In single-cycle mode, the DMA controller gives up the bus after each transfer. Advantages, Disadvantages of Burst and Single-Cycle modes Burst mode: CPU is isolated from the bus longer time but large data could be transferred fast. Single-Cycle mode: CPU is not isolated too much from the bus but large data transfer causes the overhead of bus requests. 14 Operating System Principles AUA CIS331 Albert Minasyan Blocking and Nonblocking, Synchronous and Asynchronous I/O To start an I/O operation, the CPU loads the appropriate registers within the device controller. The device controller, in turn, examines the contents of these registers to determine what action to take. For example, if it finds a read request, the controller will start the transfer of data from the device to its local buffer. Once the transfer of data is complete, the device controller informs the CPU that it has finished its operation. It accomplishes this communication by triggering an interrupt. This situation will occur, in general, as the result of a user process requesting I/O. Once the I/O is started, several courses of action are possible. Blocking or Synchronous I/O When an application issues a Blocking system call, the execution of the application is suspended. It resumes execution when receives the values returned by the system call. Disadvantage: As the physical actions performed by I/O devices are generally asynchronous-the I/O takes unpredictable amount of time. Advantage: Nevertheless, most operating systems use blocking system calls for the application interface, because blocking application code is easier to understand than nonblocking application code. Non-Blocking I/O, Asynchronous I/O A nonblocking call does not halt the execution of the application for an extended time. Instead, it returns quickly, with a return value that indicates how many bytes were transferred. An asynchronous call returns immediately, without waiting for the I/O to complete. The application continues to execute its code. The completion of the I/O at some future time is communicated to the application. The difference between nonblocking and asynchronous system calls is that a nonblocking read() returns immediately with whatever data are available-the full number of bytes requested, fewer, or none at all. An asynchronous read () call requests a transfer that will be performed in its entirety but will complete at some future time. Disadvantage: Complexity of implementation. Advantage: The program initiated I/O runs in parallel with the I/O process. 15 Operating System Principles AUA CIS331 Albert Minasyan When an application issues a Blocking system call, the execution of the application is suspended. The application is moved from the operating system's run queue to a wait queue. After the system call completes, the application is moved back to the run queue, where it is eligible to resume execution. When it resumes execution, it will receive the values returned by the system call. The physical actions performed by I/O devices are generally asynchronous-they take a varying or unpredictable amount of time. Nevertheless, most operating systems use blocking system calls for the application interface, because blocking application code is easier to understand than nonblocking application code. Some user-level processes need NonBlocking I/O. One example is a user interface that receives keyboard and mouse input while processing and displaying data on the screen. Another example is a video application that reads frames from a file on disk while simultaneously decompressing and displaying the output on the display. One way an application writer can overlap execution with I/O is to write a multithreaded application. Some threads can perform blocking system calls, while others continue executing. Some operating systems provide nonblocking I/O system calls. A nonblocking call does not halt the execution of the application for an extended time. Instead, it returns quickly, with a return value that indicates how many bytes were transferred. An alternative to a nonblocking system call is an asynchronous system call. An asynchronous call returns immediately, without waiting for the I/O to complete. The application continues to execute its code. The completion of the I/O at some future time is communicated to the application, either through the setting of some variable in the address space of the application or through the triggering of a signal or software interrupt or a call-back routine that is executed outside the linear control flow of the application. The difference between nonblocking and asynchronous system calls is that a nonblocking read() returns immediately with whatever data are available-the full number of bytes requested, fewer, or none at all. An asynchronous read () call requests a transfer that will be performed in its entirety but will complete at some future time. Interrupt before or after the Data Transfer. Interrupt before Data Transfer is started • The key is pressed on keyboard • Interrupt is requested by keyboard controller to notify the OS that the key value is ready to be taken from the keyboard. • Interrupt is serviced by OS (application) • Key value is taken from the Keyboard Controller Port (Data Transfer) Interrupt after Data Transfer is finished • The application requests data to read (write) to the HDD • I/O is started and the information flow to the destination is completed • Interrupt request is sent from HDD controller to notify about the completion the CPU. • The Interrupt is serviced by OS (Application). • Application knows that the data is transferred (arrived or is sent) and could take the appropriate action. 16 Operating System Principles AUA CIS331 Albert Minasyan 17 Operating System Principles AUA CIS331 Albert Minasyan Buffering One way in which the I/O system improves the efficiency of the computer is by using storage space in main memory or on disk via techniques called buffering. A Buffer is a memory area that stores data being transferred between two devices or between a device and an application. Buffering is done for three reasons. One reason is to cope with a speed mismatch between the producer and consumer of a data stream. Suppose, for example, that a file is being received via modem for storage on the hard disk. The modem is about a thousand times slower than the hard disk. So a buffer is created in main memory to accumulate the bytes received from the modem. When an entire buffer of data has arrived, the buffer can be written to disk in a single operation. Since the disk write is not instantaneous and the modem still needs a place to store additional incoming data, two buffers are used. After the modem fills the first buffer, the disk write is requested. The modem then starts to fill the second buffer while the first buffer is written to disk By the time the modem has filled the second buffer, the disk write from the first one should have completed, so the modem can switch back to the first buffer while the disk writes the second one. This Double Buffering decouples the producer of data from the consumer, thus relaxing timing requirements between them. A second use of buffering is to provide adaptations for devices that have different data-transfer sizes. Such disparities are especially common in computer networking, where buffers are used widely for fragmentation and reassembly of messages. At the sending side, a large message is fragmented into small network packets. The packets are sent over the network, and the receiving side places them in a reassembly buffer to form an image of the source data. A third use of buffering is to support copy semantics for application I/O. An example will clarify the meaning of "copy semantics." Suppose that an application has a buffer of data that it wishes to write to disk. It calls the write() system call providing a pointer to the buffer and an integer specifying the number of bytes to write. After the system call returns, what happens if the application changes the contents of the buffer? With the Copy Semantics the version of the data written to disk is guaranteed to be the version at the time of the application system call, independent of any subsequent changes in the application's buffer. A simple way in which the operating system can guarantee copy semantics is for the write () system call to copy the application data into a kernel buffer before returning control to the application. The disk write is performed from the kernel buffer, so that subsequent changes to the application buffer have no effect. One way in which the I/O system improves the efficiency of the computer is by using storage space in main memory or on disk via techniques called buffering. A Buffer is a memory area that stores data being transferred between two devices or between a device and an application. Buffering is done for three reasons: To cope with a speed mismatch between the producer and consumer of a data stream. To provide adaptations for devices that have different data-transfer sizes. To support copy semantics for application I/O. 18 Operating System Principles AUA CIS331 Albert Minasyan Speed Mismatch Slow transfer Modem Fast transfer Buffer Hard Disk Buffer 2 For Double Buffering Different data-transfer sizes Large file to transfer Buffer Network Card Large file after transfer Small packets Network Card Buffer Support Copy Semantics Do not write the data on disk from the App. Buffer Application Buffer But make a Copy of the data before the writing on disk. Kernel Buffer Hard Disk 19 Operating System Principles AUA CIS331 Albert Minasyan Using the tools. Windows: Device Manager - to look at I/O devices, I/O ports and Interruption Number Memory mapped I/O ports Interrupt number I/O ports 20