I/O Systems Sunu Wibirama If you are obviously IT student Then - PowerPoint PPT Presentation

I/O Systems Sunu Wibirama

If you are obviously IT student Then you should know these pictures...

Introduction  Main job of computer : I/O and processing (the latter is rarely happened)  Browsing: read and enter information, not compute an answer  OS: manage, control I/O devices and operations  What we will explore today:  I/O Hardware  Application I/O Interface  Kernel I/O Subsystem  What should you read by yourself:  Transforming I/O Requests to Hardware Operations  STREAMS  Performance 13.

KERNEL Kernel is the core of operating system Kernel’s main task: resource management, communication between hardware & software

I/O Hardware  Major concern of OS designer is: I/O support. Why?  Incredible variety of I/O devices: mouse, hard disk, cd-rom, usb, etc .  Common concepts of I/O hardware elements:  Port / connection point (ex.: serial port, parallel port)  Bus: wires with spesific protocol  Controller ( host adapter ) : collection of electronics that can operate a port, a bus, or a device. Serial-port controller v.s. SCSI controller  Details are encapsulated in device-driver module in OS kernel  Device drivers: uniform I/O instructions to control I/O devices  Controller has one or more registers for data and control signals  How processor give commands / data to a controller for I/O transfer?  Direct I/O instructions using bus: send special instructions to an I/O port address. Must select proper device and send data to device register  Memory-mapped I/O: device registers are mapped into the address space of the processor 13.

A Typical PC Bus Structure PCI bus - connected to fast devices Expansion bus - connected to slow devices

Device I/O Port Locations on PCs (partial) Graphic controller : memory-mapped I/O to hold screen contents, because writing data into memory-mapped region is faster than issuing millions of instructions 13.

I/O Port’s Register  Data-in register: read by the host to get input  Data-out register: written by the host to send output  Status register: contain status bits that can be read by the host  Control register: can be written by the host to start command or to change the mode of the device. Example:  Full-duplex or half-duplex mode; Duplex: communicate (send and receive message). Half-duplex: only one activity at one time. Full-duplex: can either send or receive information at the same time. 13.

I/O Transfer Control  Determine when the next data transfer occurs:  Polling  Interrupts  Direct Memory Access (DMA) 13.

Polling  CPU (Host) keeps checking I/O Controller status register until detect new input data arrives OR output devices ready for next data.  Going on a trip an ask: “Are we there yet?” “Are we there yet?” “Are we there yet?”  Controller:  Status register: 1 - busy and 0 - clear  Host:  Command register: command-ready bit  Handshaking step (repeated for each byte):  Host repeatedly read busy bit until that bit becomes clear (busy waiting or polling)  Host sets the write bit in the command register and writes a byte into the data-out register  Host sets command-ready bit  Controller notices that command-ready bit is set, it sets the busy bit  Controller reads the command register, sees the write command. It reads data-out register to get the byte and does the I/O to the device  Controller clears command-ready bit, clears error bit in the status register to indicate I/O succeeded, clears the busy bit to indicate that I/O operation is finished 13.

Interrupt  Polling is inefficient if rarely find idle devices, but large tasks remain undone. How to solve?  How about: device sends special signal to CPU when new data arrives OR device ready for next data  Going on a trip an ask: “Wake me when we get there!”  Interrupt : Notify CPU if device is ready, instead of polling repeatedly 13.

Interrupt-Driven I/O Cycle 13.

Direct Memory Access (DMA)  Large data transfer: disk drive  CPU is kept from exhausted work: avoid programmed I/O for large data movement ( programmed I/O = watch status bits and to feed data into a controller register one byte at a time)  How? Bypasses CPU to transfer data directly between I/O device and memory. Delegating CPU tasks to DMA controller.  Initiate DMA transfer:  CPU writes a DMA command block into memory  This command block contains: pointer to the source of transfer, pointer to the destination of transfer, and a count of the number of bytes to be transferred  CPU writes the address of command block to DMA controller and let the DMA controller do the rest 13.

Six Step Process to Perform DMA Transfer 13.

Application I/O Interface  Explain interface and techniques of OS to treat I/O devices in standard way (such as open a file on a disk without knowing what kind of disk it is).  I/O system calls encapsulate device behaviors in generic classes  Device-driver layer hides differences among I/O controllers from kernel  Devices vary in many dimensions  Character-stream or block  Sequential or random-access  Sharable or dedicated  Speed of operation  read-write, read only, or write only 13.

Characteristics of I/O Devices 13.

A Kernel I/O Structure uniformity think the structure as organization structure: head of organization doesn’t need to know the detail 13.

Kernel I/O Subsystem  Kernel provides many services to I/O, such as:  scheduling  buffering  caching  spooling  device reservation  error handling  These are provided by I/O subsystem, build on hardware and device-driver infrastructure 13.

Kernel I/O Subsystem  Scheduling  Meaning: to determine a good order to execute I/O requests  Can improve overall system performance  Some I/O request ordering via per-device queue  Some OSs try fairness  Buffering - store data in memory while transferring between devices  To deal with device speed mismatch ( modem v.s. storage )  To deal with device transfer size mismatch ( large data re-assembling in computer networking )  To maintain “copy semantics” (data reliability when write() system call is executed. Copying an opened large Ms. Word file to USB flash drive ) 13.

Sun Enterprise 6000 Device-Transfer Rates Logarithmic scale 13.

Kernel I/O Subsystem  Caching - fast memory holding copy of data (do you remember the function of proxy server in networking? - you need to press CTRL+F5 to renew the cache )  Always just a copy (not the only existing item - as buffer does)  Key to performance  Spooling - hold output for a device  If device can serve only one request at a time  i.e., Printing - no interleave allowed  Device reservation - provides exclusive access to a device  System calls for allocation and deallocation  Watch out for deadlock 13.

Error Handling  OS can recover from disk read, device unavailable or defective, transient write failures  Most return an error number or code when I/O request fails  Unix : errno 2 - no such file or directory  System error logs hold problem reports 13.

I/O Protection  User process may accidentally or purposefully attempt to disrupt normal operation via illegal I/O instructions  All I/O instructions defined to be privileged: I/O must be performed via OS ʼ system calls  Memory-mapped and I/O port memory locations must be protected too (ex.: graphic games and video editing should access the memory directly. OS should protects a block of memory for graphic processing at one time) Memory layout 13.

Kernel Data Structures  Kernel keeps state info for I/O components, including open file tables, network connections, character device state  Many, many complex data structures to track buffers, memory allocation, “dirty” blocks  Unix uses object-oriented methods to encapsulate “differences” (for example: read() operation, the semantics is different for each I/O devices)  Windows uses message passing to implement I/O  I/O request is converted into a message through the kernel to the I/O manager and then to device driver, each of which may change the message contents.  For output, the message contains data to be written. For input, the message contains buffer to receive data. 13.

Life cycle of I/O Request (using DMA)

Thank You