I/O and Device Drivers

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1 I/O and Device Drivers Minsoo Ryu Real-Time Computing and Communications Lab. Hanyang University

2 Topics Covered I/O Components I/O Interface I/O Operations Device Drivers 2

3 I/O Components

4 CPU Execution and I/O The two main jobs of a computer CPU execution Input/Output Usually, the main job is I/O The CPU execution is incidental 4

5 Typical I/O Data Rates 5

6 Variety of I/O Devices A computer is equipped with CPU and memory Many I/O devices VGA card, network card, disk controller, Network card CPU Main board Memory Hard disk 6

7 Types of I/O Devices Character devices (mouse, terminal, etc) Commands include get and put Block devices (disk drive, flash drive, etc) Commands include read, write, and seek Raw I/O or file-system access Memory-mapped file I/O access possible mmap( ) function: convenient programming interface mmap is faster than read, because of single copy operation Network devices NIC (network interface card) 7

8 Character Device: Mouse (1) 8

9 Character Device: Mouse (2) Brief history First mechanical mouse with a roller ball Bill English at Xerox PARC in the early 1970s Introduced by Apple Macintosh in 1984 They have helped to completely redefine the way we use computers since then Became the PC-human interface of choice quickly when Windows 3.1 made Graphical User Interface (GUI) a standard Optical Mouse Gary Gordon at Agilent Laboratories in

10 Character Device: Mouse (3) Optical mouse Tiny camera takes images per second Camera = laser + a CMOS sensor Images sent for analysis to a DSP operating typically at 18 MIPS DSP detects patterns in images and thus estimates motion Data ports are used for two-way communication Upon mouse movement, a 3/5-byte packet is sent to the port Typical description of the data (x s, y s ), (x d, y d ), mouse-up/down This data packet is decoded by the mouse driver and its internal co-ordinates are updated 10

11 Character Device: Mouse (4) PC mouse system (data transfer chain) Sensors (CMOS) Mouse Controller (DSP) Communication link (Cable/Wireless) Data interface (Serial, PS/2, USB) Device driver Application 11

12 Character Device: Mouse (5) How OS collaborate with mouse driver Applications wait for mouse movement or click When a mouse movement occurs, the mouse driver informs the event manager of OS about the event Event manager determines whether to queue the event or not Mouse driver automatically tracks the mouse and displays the cursor as the user moves the mouse When a mouse-up or mouse-down event occurs, the event manager records the action in the event queue and informs the active application about it The active program decides what action is to be taken Ex: Show the mouse cursor, hide the cursor and draw something onto screen, etc. 12

13 Character Device: Terminal (1) Example DEC VT100, Heathkit Z19 13

14 Character Device: Terminal (2) Terminal = keyboard + display Keyboard and display are handled independent in most systems (no automatic echo, full duplex serial link) I/O registers are connected to host via serial line Keyboard data/status registers Display data/status registers One interrupt per character One character (8-bit data or control function) is sent at a time ASCII encoding is used: A is 0x65 Slow speed characters per second Measure: Baud rate (bits per second) 14

15 Character Device: Terminal (3) Keyboard input When a character is typed: Its ASCII code is placed in bits [7:0] of keyboard data register The ready bit of keyboard status register is set to zero Keyboard is disabled Any typed characters will be ignored keyboard data KBDR ready bit KBSR When KBDR is read: The ready bit of KBSR is set to one Keyboard is enabled 15

16 Character Device: Terminal (4) Display output When monitor is ready to display another character The ready bit of display status register is set to one DDR display data ready bit DSR When data is written to DDR: The ready bit of DSR is set to zero Character in DDR is displayed Any other character data written to DDR is ignored 16

17 Character Device: Terminal (5) Keyboard echo Usually, input character is also printed to screen User gets feedback on character typed and knows it s ok to type the next character 17

18 Block Device: Disk Drive (1) 18

19 Block Device: Disk Drive (2) Disk characteristics (technology of 2012) 2-6 heads (platters x 2) Platter diameter between 0.8" and 8" 16,383 tracks (cylinders) per surface 63 sectors per track Sector size of 512 to 4096 bytes 4KB physical emulated at 512-byte sectors Capacity ranges up to 4 TB 19

20 Block Device: Disk Drive (3) Disk operation Select desired read/write head Move heads to the correct track ( seek ) Seek time Wait for disk to rotate desired sector into position Rotational delay Read and write sector while it spins by Transfer latency Disk performance Seek time: 0-50 ms (average ms) Rotational delay: 0-16 ms Typical drive spins at RPM 20

21 Network Interfaces Network interfaces UART and RS-232, USB, ethernet, etc 21

22 Layering (Protocol Stack) Port no. Data Message IP address TCP Data Segment/Datagram MAC address IP TCP Data Datagram Ethernet IP TCP Data Frame Bit stream Network Adapter (NIC) 22

23 TCP Socket Programming Flow Client socket() well-known port socket() bind() listen() Server Implicit bind connect() write() Connection establishment TCP three-way handshake Data (request) accept() Block until connection from client read() Process requests read() close() Data (reply) End-of-file notification write() read() close() 23

24 Clocks and Timers Provide three basic functions Give the current time Give the elapsed time Set a timer to trigger operation X at time Y Programmable interval timer The hardware for last two functions Wait and generate an interrupt CPU scheduler uses this Unfortunately, the system calls for timer functions are not standardized across operating systems alarm() in UNIX timer_create() and timer_settime() in POSIX 24

25 Unix Time Values Two different time values Calendar time Counts the number of seconds since the Epoch (Jan 1, 1970) UTC (coordinated Universal Time, Greenwich Mean Time) Process time Measures the CPU time used by a process Process time is measured in clock ticks Usually, 50, 60, or 100 ticks per second CPU time = User CPU time + System CPU time Lack of timer support on Unix We can get around by using alarm( ) system call The unit is second ioctl on UNIX covers odd aspects of I/O such as clocks and timers Catchall for I/O operations 25

26 Linux The timer interrupt is set to a default frequency linux/param.h 100 Hz for most hardware platforms Jiffies The number of clock ticks since the operating system was booted When the timer interrupt occurs, the jiffies value is incremented 26

27 I/O Interface and Operations

28 I/O Bus and Connection Devices are attached to I/O bus ISA, PCI, EISA, SCSI, CPU VGA Ethernet Sound I/O BUS 28

29 I/O Buses and Connections 29

30 Generic Model of I/O Interface I/O Interface 30

31 I/O Registers An I/O port typically consists of four registers Status register: states read by the host Whether the current command has completed Whether a byte is available to be read from data-in register Whether there is a device error Control register: command written by the host Start a command and change the mode of a device Data-in register: input read by the host Data-out register: output written by the host 31

32 I/O Register Access Memory-mapped IO Data Bus, Address Bus, Control Bus are shared e.g., Motorolla Memory and I/O access use the same instructions Port-mapped IO Data Bus and Address Bus are shared, but Control Bus not shared e.g., Intel x86 and Z80 Memory and I/O access use different instructions Data Channel Separate Data Bus, Address Bus, and Control Bus Dedicated I/O processors are used 32

33 Memory-Mapped I/O Memory mapped I/O uses special memory locations in the normal address space Possible to design address decoder to divert read or write to a device instead of memory Use move instead of in and out PCs use both port-mapped mapped I/O and memory-mapped I/O Graphics controller has I/O ports for basic control operations But it also has a large memory-mapped region to hold screen contents The controller generates the screen image based on the contents of this memory Memory-mapped I/O is vulnerable to accidental modification by using pointers, but protected memory reduces this risk 33

34 Port-mapped I/O and Address Space Port-mapped IO structure provides two address spaces Memory address space and I/O (or port) address space The I/O address space uses the same address pins on the processor as the memory address space RD and WR lines determine whether data being read or written When M/IO line is high, memory is selected When M/IO line is low, I/O is selected 34

35 Instructions for Port-Mapped I/O Port-mapped input/output uses special instructions Register I/O instructions e.g., in and out (80386) Move data between I/O ports and general registers Data may be a byte, word, or doubleword Block I/O instructions e.g., ins and outs (80386) Move blocks of data between I/O ports and memory space 35

36 Device I/O Port Locations on PCs (partial) 36

37 Basic I/O Operations 1. CPU checks I/O module status 2. I/O module return status 3. If ready, CPU requests data transfer 4. I/O module gets data from device 5. I/O module transfers data to CPU Variations for output such as DMA 37

38 Two Types of I/O Polling (Programmed I/O) The system repeatedly checks the status of the I/O module if data can be read or written Usually used for character devices Interrupts (Interrupt-Driven I/O) The system sleeps the process until data can be read or written and wakes it up upon receiving the awaited interrupt Usually used for character and block devices 38

39 DMA (Direct Memory Access) Interrupt driven and programmed I/O require active CPU intervention Transfer rate is limited CPU is tied up DMA (Direct Memory Access) DMA controller is required Bypasses CPU to transfer data directly between I/O device and memory 39

DMA Controllers To do DMA, I/O device attached to DMA controller Multiple devices can be connected to one controller Controller itself seen as a memory mapped I/O device Processor

40 DMA Controllers To do DMA, I/O device attached to DMA controller Multiple devices can be connected to one controller Controller itself seen as a memory mapped I/O device Processor initializes start memory address, transfer size, etc. DMA controller takes care of bus arbitration and transfer details That s why buses support arbitration and multiple masters 40

Cycle Stealing When the DMA controller seizes the memory bus, The CPU is momentarily prevented from accessing main memory But, it can still access data items in its primary and

41 Cycle Stealing When the DMA controller seizes the memory bus, The CPU is momentarily prevented from accessing main memory But, it can still access data items in its primary and secondary cache The cycle stealing can slow down the CPU computation However, offloading the data-transfer work to a DMA controller generally improves the total system performance 41

42 Device Drivers

43 Types of Linux Devices Character Devices Transfer unit: character (byte) Can be accessed like a file (open, close, read, write, ) (Ex) console, keyboard, mouse, (/dev/tty1, /dev/lp1, ) Block Devices Transfer unit: block (usually some kilobytes) Can be accessed like a file (open, close, read, write, ) (Ex) hard disk drive, CD-ROM drive, (/dev/hda1, ) Network Interfaces Not mapped to neither character or block devices (Ex) eth0 43

44 Device Files Represent an I/O device /dev/tty0 = first serial port Attributes Type: block or character Major number: specifies device driver Minor number: argument to device driver, kernel don t care Name Type Major Minor Desc /dev/fd0 Block 2 0 Floppy Disk /dev/had Block 3 0 First IDE disk /dev/hda2 Block 3 2 Second partition of first IDE disk /dev/hdb Block 3 64 Second IDE disk /dev/console Char 5 1 Console /dev/null char 1 3 Null device 44

45 Structure of Character Device Drivers Character Device Table 0 Index = major number 1 2 Name File operations Character Device Driver Device Name Character Device Registered when device being installed Open Read Write Close Ioctl Open code Read code write code 45

46 Character Device Drivers To applications Device file = Regular file To file system Regular file: read from or write to disk drive Device file: invoke device driver operations open: initialize device read: device data to user buffer write: user buffer to device close: called when removed from device table ioctl: device specific control (Ex) baud rate change, access permission set 46

47 Structure of Block Device Drivers Block Device Table Block Device Driver Open Read Write Close Ioctl Open code Handling request code (strategy routine) Block Device Buffer Cache Manager Request queue block Block Read Block Write char Buffer Buffer Block Buffer Buffer (Buffer Cache) 47

48 Interrupt Handling in Device Drivers Register interrupt handling routine Kernel invokes registered handler when interrupt occurs Linux interrupt handling (two-level processing) Interrupt handler (top half) High priority function (such as acknowledging to PIC) Run with interrupt disabled Invoked every time interrupt occurred Marks a bottom half as active Bottom half Low priority function (such as transferring data from device) Run with interrupt enabled Execution may be deferred 48

49 Bottom Half Proc In this example, the bottom half is activated twice but executed once Proc Intr_1 Ack Intr_1, activate bottom_half_1 Int_handler_1 Interrupt disabled Intr_2 Ack Intr_2, activate bottom_half_1 Int_handler_2 Interrupt disabled Bottom_half_1 Interrupt enabled 49

50 General Device Driver Routines (1) Open Reads minor number Initializes the appropriate device Initializes the device driver internal data structures If needed, changes the file operation table (according to the minor number) Increases the usage counter If needed, registers the interrupt handler and enable IRQ Close Frees dynamically allocated data structures Decreases the usage counter Un-registers the interrupt handler 50

51 General Device Driver Routines (2) Read/Write Polling vs. interrupt driven Polling Checks the device if data can be read or written Usually used for character devices Interrupt Driven Sleeps the process until data can be read or written Usually used for character and block devices Blocking vs. non-blocking Blocking If data cannot be read or written, waits until ready Non-blocking If data cannot be read or written, immediately returns 51

52 General Device Driver Routines (3) Character Device: Polling and Blocking I/O Read Character Device: Non-Blocking I/O Read Check Device Check Device Device ready? No Device ready? No Yes Yes Read Data (device user) Read Data (device user) Return to User Return to User 52

53 General Device Driver Routines (4) Character Device: Interrupt Driven and Blocking I/O Read Check Device Read ready interrupt Device Ready? No Wakeup process Yes Read Data (device user) Sleep process Return to User 53

54 General Device Driver Routines (5) Block Device: Interrupt Driven and Blocking I/O Write Write complete interrupt Check write buffer Wakeup process Buffer full? No Yes Sleep process Wakeup by another process Issue Write Command Sleep process Return to User 54

55 55

Input/Output Problems. External Devices. Input/Output Module. I/O Steps. I/O Module Function Computer Architecture

Input/Output Problems. External Devices. Input/Output Module. I/O Steps. I/O Module Function Computer Architecture 168 420 Computer Architecture Chapter 6 Input/Output Input/Output Problems Wide variety of peripherals Delivering different amounts of data At different speeds In different formats All slower than CPU