1 COS 318: Operating Systems I/O Device and Drivers 2 Input and Output  A computer’s job is to process data  Computation (CPU, cache, and memory)  Move data into and out of a system (between I/O devices and memory)  Challenges with I/O devices  Different categories: storage, networking, displays, etc.  Large number of device drivers to support  Device drivers run in kernel mode and can crash systems  Goals of the OS  Provide a generic, consistent, convenient and reliable way to access I/O devices  As device-independent as possible  Don’t hurt the performance capability of the I/O system too much 3 Revisit Hardware  Compute hardware  CPU and caches  Chipset  Memory  I/O Hardware  I/O bus or interconnect  I/O controller or adaptor  I/O device  Two types of I/O  Programmed I/O (PIO) • CPU does the work of moving data  Direct Memory Access (DMA) • CPU offloads the work of moving data to DMA controller CPU Memory I/O bus CPU CPU CPU Network 4 Definitions and General Method  Overhead  Time that the CPU is tied up initiating/ending an operation  Latency  Time to transfer one bit (typ. byte)  Overhead + 1 bit reaches destination  Bandwidth  Rate of I/O transfer, once initiated  Mbytes/sec  General method  Higher level abstractions of byte transfers  Batch transfers into block I/O for efficiency to amortize overhead and latency over a large unit Initiate Data transfer2 5 Programmed Input Device  Device controller  Status register ready: tells if the host is done busy: tells if the controller is done int: interrupt …  Data registers  A simple mouse design  Put (X, Y) in data registers on a move  Interrupt  Input on an interrupt  Read values in X, Y registers  Set ready bit  Wake up a process/thread or execute a piece of code 6 Programmed Output Device  Device  Status registers (ready, busy, … )  Data registers  Example  A serial output device  Perform an output  Wait until ready bit is clear  Poll the busy bit  Writes the data to register(s)  Set ready bit  Controller sets busy bit and transfers data  Controller clears the ready bit and busy bit 7 Direct Memory Access (DMA)  DMA controller or adaptor  Status register (ready, busy, interrupt, …)  DMA command register  DMA register (address, size)  DMA buffer  Host CPU initiates DMA  Device driver call (kernel mode)  Wait until DMA device is free  Initiate a DMA transaction (command, memory address, size)  Block  Controller performs DMA  DMA data to device (size--; address++)  Interrupt on completion (size == 0)  Interrupt handler (on completion)  Wakeup the blocked process 8 I/O Software Stack User-Level I/O Software Device-Independent OS software Device Drivers Interrupt handlers Hardware3 9 Recall Interrupt Handling  Save context (registers that hw hasn’t saved, PSW etc)  Mask interrupts if needed  Set up a context for interrupt service  Set up a stack for interrupt service  Acknowledge interrupt controller, perhaps enable it  Save entire context to PCB  Run the interrupt service  Unmask interrupts if needed  Possibly change the priority of the process  Run the scheduler  Then OS will set up context for next process, load registers and PSW, start running process … 10 I/O System Device Drivers Rest of the operating system Device driver Device driver . . . Device driver Device controller Device controller . . . Device controller Device Device Device Device Interrupt Handling  Manage the complexity and differences among specific types of devices (disk vs. mouse, different types of disks …)  Each handles one type of device or small class of them (eg SCSI) 11 Typical Device Driver Design  Operating system and driver communication  Commands and data between OS and device drivers  Driver and hardware communication  Commands and data between driver and hardware  Driver responsibilities  Initialize devices  Interpreting commands from OS  Schedule multiple outstanding requests  Manage data transfers  Accept and process interrupts  Maintain the integrity of driver and kernel data structures Simplified Device Driver Behavior  Check input parameters for validity, and translate them to device-specific language  Check if device is free (wait or block if not)  Issue commands to control device  Write them into device controller’s registers  Check after each if device is ready for next (wait or block if not)  Block or wait for controller to finish work  Check for errors, and pass data to device-indept software  Return status information  Process next queued request, or block waitng for next  Challenges:  Must be reentrant (can be called by an interrupt while running)  Handle hot-pluggable devices and device removal while running  Complex and many of them; bugs in them can crash system 124 Types of I/O Devices  Block devices  Organize data in fixed-size blocks  Transfers are in units of blocks  Blocks have addresses and data are therefore addressable  E.g. hard disks, USB disks, CD-ROMs  Character devices  Delivers or accepts a stream of characters, no block structure  Not addressable, no seeks  Can read from stream or write to stream  Printers, network interfaces, terminals  Like everything, not a perfect classification  E.g. tape drives have blocks but not randomly accessed  Clocks are I/O devices that just generate interrupts 13 Typical Device Speeds  Keyboard  Mouse  Compact Flash card  USB 2.0  52x CD-ROM  Scanner  56K modem  802.11g wireless net  Gigabit Ethernet  FireWire-1  FireWire 800  SCSI Ultra-2 disk  SATA disk  PCI bus  Ultrium tape 14 10 B/s 100 B/s 40 MB/s 60 MB/s 7.8 MB/s 400 KB/s 7 KB/s 6.75 MB/s 320 MB/s 50 MB/s 100 MB/s 80 MB/s 300 MB/s 528 MB/s 320 MB/s 15 Device Driver Interface 