Review of system architecture

1. USER Shell System and application programs systems calls | API | middleware user mode Review of system architecture Kernel access points file sys. memory process support mgt mgt kernel mode I/O subsystem Device drivers Hardware ports | buses | controllers DEVICES Chapter 11 I/O Systems

2. Review of File System File interface User friendly, abstractions, e.g., (user mode) GUI, folders, standard interface, portability/transparency/virtual services File implementation Resource manager, including (kernel mode) efficiency (e.g., clusters for CPU) (defragmentation, compression) security (e.g., encryption) availability reliability fault tolerance Chapter 11 I/O Systems

3. I/O management as compared to file management • File management is divided into file interface (services provided to application layer, user) and file implementation • File interface designed for user-friendliness, e.g.virtual file systems give consistent interface to users • File implementation handles implementation • I/O also divided into interface and implementation • I/O interfaces are standardized, simplified for services to other OS services, application layer • I/O implementation is becoming more and more complex • More devices • Compression and redundancy added to devices • Much of this is now in hardware (controllers) Chapter 11 I/O Systems

4. I/O Systems • Computing versus I/O • Early systems had minimal user interface • I/O was most tedious and repetitive part – beginning of OS • Early programming languages (FORTRAN, etc.) designed for computation • COBOL for business processing (1960s) • Lots of I/O • Systems integrated I/O and process-bound jobs • Current Applications (games, video, etc.) • “Challenge to incorporate them into our computers” Chapter 11 I/O Systems

5. I/O layers • I/O subsystem provides device independent interface to applications and OS • Device drivers – software that encapsulates devices for uniform device-access interface to I/O subsystem • Basic I/O hardware • Ports, buses, device controllers Chapter 11 I/O Systems

6. Question • Why are i/o functions divided into separate layers? a.To handle the increased complexity of devices b.To provide a uniform interface for application programs and OS using so many different types of devices ans: a and b Chapter 11 I/O Systems

7. Some Types of I/O Devices • Machine readable • Disk drives, sensors, controllers • Transmission • NICs, modems, FAX • Human-readable • Keyboard, mouse, screen, joystick, printer • Robotics Chapter 11 I/O Systems

8. Some differences between devices • Data rate • Keyboard 100bps; Gigabit ethernet Gbps • Application • Software and policies vary for applications • Priorities; access rights • Complexity • Unit of transfer • Data representation • Error conditions Chapter 11 I/O Systems

9. Device communication • Bus • Set of wires and circuitry, with functional information on each wire (data, address, control) and protocol for exchanging information (send, receive) on channel • Discuss PCI bus (next slide) • Discuss daisy chain • Port- Connection point • Serial (USB, firewire), parallel (IDE) • Timer (typically at port 040-043 hex) • Controller-firmware (microcode on chips) operates port, bus, or device • Host adapter (NIC) and SCSI controllers typically are on circuit boards Chapter 11 I/O Systems

10. PCI bus monitor Processor(s) SCSI controller to SCSI bus and multiple disks caches Graphics controller controller memory USB controller to USB bus ISA bridge mouse ISA bus keyboard Serial printers, cameras, etc. IDE controller to disks Parallel printers modem Chapter 11 I/O Systems

11. PCI bushttp://www.tech-pro.net/intro_pci.html • This is an edited version of an article that appeared a few years ago in PC Support Advisor. Although it provides a good general introduction to PCI bus concepts, further improvements have been made since the article was written.) • The acronym PCI stands for Peripheral Component Interconnect, which aptly describes what it does. PCI was designed to satisfy the requirement for a standard interface for connecting peripherals to a PC, capable of sustaining the high data transfer rates needed by modern graphics controllers, storage media, network interface cards and other devices. • Earlier bus designs were all lacking in one respect or another. The IBM PC-AT standard ISA (Industry Standard Architecture) bus, for example, can manage a data transfer rate of 8MB/sec at best. In practice, the throughput that can be sustained is much less than that. Other factors, like the 16-bit data bandwidth and 24 bit wide address bus - which restricts memory mapped peripherals to the first 16MB of memory address space - made the ISA bus seem increasingly outmoded . Chapter 11 I/O Systems

12. Personal Computer buses Bus PCI Clock 33MHz - 66MHz Number of bits 32 64 Data per Clock Cycle 1 1 Maximum Transfer Rate 133MB/s /266MB/s/533MHz Bus PCI -X Clock 66MHz - 133MHz Number of bits 64 Data per Clock Cycle 1, 2, 4 Maximum Transfer Rate 533MB/s to 4266MB/s Bus AGP (accelerated graphics port) Clock 66HMz Number of bits 32/64 Data per Clock Cycle 1-8 Maximum Transfer Rate 266MB/s- 2133MB/s and higher Chapter 11 I/O Systems

13. Device Controllers • Device (mechanical component) separated from Controller (electronic component) • Controller is sometimes a chip on the motherboard or else a printed circuit card to be inserted into a PCI expansion slot • Controller connected to device(s) by connecting cables • Interface between device and controller is standardized for IDE, SCSI, USB, FireWire, etc. Chapter 11 I/O Systems

14. Example: device controller for disk • Serial bit stream feeds between disk to controller • Controller organizes it into preamble (cylinder & sector #, sector size, bit and sector synchronization data) ; perhaps 4096 bit data sector; checksum (or ECC) • ECC verified • If everything checks, data is assembled as block and forwarded to memory Chapter 11 I/O Systems

15. Controller / CPU Communication • Registers on controller written to/ read from • Contain commands, device status, etc. • May include data buffers • Bus contains address, control, data lines • If CPU wants to read/ write data • Places address on bus address line • For a write, it places data on data lines. • Asserts read/write signal on bus control line • Device responds to signal Chapter 11 I/O Systems

16. Memory-Mapped I/O or Port I/O or hybrids • Port I/O uses dedicated instructions, dedicated addresses • IN Reg1, Port • CPU to get value in controller register(@ Port) • and store in CPU register Reg1 • OUT Port, Reg1 • CPU sends value in Reg1 to Port for controller • Memory-mapped I/O instruction might be • MOV Reg1, 4 • data in memory 4 sent to Reg1 • Same instructions as for memory; same address notation Chapter 11 I/O Systems

17. Memory-Mapped I/O • Each control register is assigned a unique memory address (devices and memory share memory address space); uses same instructions as CPU programs • Programmer can use C code (Port I/O requires some assembly code to distinguish between instructions) • Each method has some disadvantages Chapter 11 I/O Systems

18. Controller/host handshake with interface registers • Status register - set by controller • Completed/failure • Busy bit for availability • Control register – set by host • Control information for speed, mode (read/write), parity checking on/off, word length, full/half duplex • Command ready bit • Data-in register – set by controller; read by host • Data-out register – written by host; read by controller for output to device • Possibly input/output is cached in multiple registers Chapter 11 I/O Systems

19. Sample Polling Protocol • Host polls for busy bit to be off (busy waiting) • If busy bit it off (device free), host can send signals (on bus lines) to • set mode bit in command register to write • place byte(s) in data-out register (other info possible) • set command-ready bit • Controller sets busy bit • fetches data-out from register • outputs data-out to device • sets status bit for success/ failure ` ` • clears command-ready bit, busy bit Chapter 11 I/O Systems

20. Interrupts for device I/O • In hardware • Device sends signal (IRQ) on interrupt-request line • Interrupt controller asserts signal; identifies interrupt # • Signal causes CPU to interrupt what it is doing • Interrupt # identifies position in interrupt vector that holds address of ISR • CPU saves value of current PC (possibly other values) • Transfers to interrupt vector, indirectly • Location contains (changeable) address of interrupt handler (ISR) • In software • ISR may disable interrupts, save state of registers it needs, enable interrupts • ISR determines nature of interrupt and handles interrupt • ISR may restore registers of interrupted process Chapter 11 I/O Systems

21. Interrupt-controller • Prevents interruption during critical processing • Prioritizes interrupts • Determines which device raised interrupt • Programmable or Advanced Programmable Interrupt-controllers • Enables different prioritization methods (hard, rotating, cascading), for example Chapter 11 I/O Systems

22. Question • Why are interrupts prioritized? • It is faster • It is fairer • It is important not to lose data that may be overwritten by fast devices • Prioritization prevents deadlock Answer: c Chapter 11 I/O Systems

23. Masking and disabling interrupts • Maskable or nonmaskable • Nonmaskable interrupts (power or memory failure) • Maskable interrupts – devices, etc. • PCI bus has 2 interrupt request lines • Maskable interrupt line can be temporarily turned off during execution of critical instructions • Interrupt chaining • Each address in interrupt vector may point to a chain of ISRs • Nested prioritized interrupts • Interrupts stored and handled in priority order Chapter 11 I/O Systems

24. Precise and Imprecise Interrupts • Pipelining of instructions - multiple instructions are in a state of execution • When an interrupt occurs, multiple instructions may be partially executed • This is called an imprecise interrupt • System must ensure that the instructions previous to the instruction whose PC has been saved are completed • System can roll back those instructions after the one whose PC has been saved (possibly including PC instruction) • Adds complexity to current systems Chapter 11 I/O Systems

25. Exceptions or traps • Interrupt mechanism is used to handle software traps • Examples: memory access violations, division by zero • Interrupt mechanism may be used for page or segment faults • Interrupt mechanism may be used for calls to the kernel • Threads may communicate thru interrupts • Place common data on system stack Chapter 11 I/O Systems

26. DMA (Direct Memory Access) • Fast I/O processor • Kernel process writes command block to DMA • Address in memory, address on device, r/w, # of blocks • DMA transfers block to/from memory to device; interrupt upon completion • Memory cycle stealing • DMA “handshake” with device controller (perhaps IDE) • DVMA can transfer block between two devices Chapter 11 I/O Systems

27. Question • DMAs use cycle stealing. What does that mean? a. CPU cycles are stolen by the DMA controller b. Memory cycles are stolen from other devices by the DMA controller c. Memory cycles are stolen from the CPU by the DMA controller • Ans: c (i.e., CPU is prevented from accessing memory while DMA is transferring data in or out) Chapter 11 I/O Systems

28. Kernel I/O structure • Kernel requests uniform set of services from subsystem • (through kernel-subsystem interface) • Typically- read(), write(), seek() • Subsystem requests services of device drivers, which encapsulate peculiarities of devices • (through subsystem- driver interface) • Example: PCI device driver • Drivers requests services (data, error flags, etc.) of controllers • (driver- controller interface) • Example of controller: PCI bus device controller • Controllers handle I/O devices - ever more different and complex • Speed, sharing, unit of transfer, access method, in sync or not • They all transfer bits • Example of device: PCI bus Chapter 11 I/O Systems

29. Back door to device driver • Bypasses subsystem • In UNIX, for example, the command ioctl () • Returns File descriptor • In UNIX, devices are handled as files • Determines command to device driver • Defined integer value • Provides pointer to data structure with control information, data for output if necessary Chapter 11 I/O Systems

30. Clocks and Timers • Either hardware clock or very fast (high frequency) counter is used for timing services • Generates periodic interrupts (clock ticks) • Programmable clocks control frequency of interrupt • Clock Driver • Maintains time of day • May limit CPU time allowed to process • Handles the alarm system call • Provides software timers for the system • Statistics, monitoring, etc. Chapter 11 I/O Systems

31. Software Timers • Software interrupts are attached to clock ticks • Time slice interrupt • Preemptive priority scheduling • Periodic write-out of cache to disk • Network time-out and resend, break connections, etc. • Can provide services to user programs – • /usr/bin/time (Cshell) • sleep () in programs Chapter 11 I/O Systems

32. Kernel I/O Subsystem services • Management of name space for files and devices • Access control authentication for files and devices • Operation control (modem cannot do a seek) • File-system space allocation • Device allocation • Buffer, i/o caches, spooling management • I/O scheduling • Device monitoring • Error handling, failure recovery • Device-driver configuration and initialization • Scheduling of disk operations • Table Maintenance Chapter 11 I/O Systems

33. Performance Issues -Buffers • Buffering ameliorates speed mismatch between producer and consumer • Ex: modem and hard disk • Typically, double buffering is used • Once buffer is filled, disk write is requested • Modem can continue; writes to other buffer until output is completed • Ameliorates differences in data-transfer size • Perhaps holds data until it can be defragmented (reassembled) Chapter 11 I/O Systems

34. Performance Issues - Caching • Cache is high speed storage that holds, by definition, copy of data from slower storage • Used for efficiency purposes • Cache on CPU is completely hardware controlled • Only issue for OS would be size, replacement • Caches in memory, on devices Chapter 11 I/O Systems

35. Performance issues - Spooling • Disk buffers holding data for output (or input) • Control is handled by system daemon process or kernel thread • Spooling list is available • Job can be removed • Output is delayed until complete (usually) • Can cause deadlock over printer (or other hardware) if a process holds (locks) printer while waiting for other resource • Output process can continue executing if printer is busy • Consistent output as processes complete one at a time Chapter 11 I/O Systems

36. I/O protection • All I/O operations are privileged • User program traps to kernel, kernel suspends process, verifies user rights, completes I/O, signals user • Memory-mapped and i/o port locations are denied to users. Users must trap to kernel • But graphic games need direct access to memory-mapped graphics controller memory • Performance issue – cost of trap to kernel • OS can provide a section of graphics memory to a single user at a time using dedicated bus Chapter 11 I/O Systems

37. Magnetic Disks • “Platters” coated with magnetic material • logically divided into circular tracks • Cylinders are concentric tracks • tracks divided into sectors • sector is smallest unit of access • Access time (about 10ms) • Read-Write head on disk arm positioned over track (seek time – random access) • Rotational latency – disk rotated to proper sector (direct access) • Transfer rate – copying data to/from disk (33MB/s) • Transfer by Disk Controller – one on each disk • Sometimes controller is shared by disks (e.g., SCSI) • Head crash – head damages magnetic surface Chapter 11 I/O Systems

38. Magnetic Tape • Audio and video recording; archival data storage (disks have taken many of these functions) • Sequential access • “direct” access is achieved by fast wind, rewind • Capacity • 2010 - the highest capacity Hitachi tape cartridges stored up to 50 TB of data without compression Chapter 11 I/O Systems

39. Optical Disks • Cheaper • R or R/W • Capacity – 30GB and more • Slower than hard disks (at least currently) • Have replaced floppy disks • Capacity is greater • Cost per byte is less • Reliability is probably better as well Chapter 11 I/O Systems

40. Disk Structure • Sectors, blocks, clusters • A sector is the minimal unit of allocation (hardware determination) • A block is a minimal unit of transfer • OS determines the number of sectors to a block • Blocks (logical) are mapped onto sectors • Mapping difficulties • Number of sectors may decrease for inner cylinders • Bad sectors are mapped onto other positions holding “spare” sectors • In Windows, a cluster is the minimal unit of file allocation • OS determines the number of blocks in cluster to increase contiguity in file storage Chapter 11 I/O Systems

41. Device Attachment • Host-attached storage • SCSI – up to 16 devices on bus • Controller card on host; up to 15 storage targets • Polling is used to give each device a turn • Handshakes for connection • IDE-ATA(Advanced Technology Attachment) – parallel interface standards • SATA, Firewire, USB are all serial bus interface standards • Fiber channel (perhaps over Ethernet) • Basis for SANs; switched “fabric” with large address space to connect large number of devices • Alternatively, arbitrated loop (FDDI) is used to connect up to 126 devices Chapter 11 I/O Systems

42. Device attachment • Firewire/ USB • High-speed serial data bus standards to connect a number of different peripheral devices • Have been implemented across different platforms • Firewire developed by Apple; {USB by Intel} • Firewire (speed of version S3200 up to 3.2 Gbps); SCSI (160Mbps, defined for both ATA and SATA); and super speed USB(up to 4.8Gbps); Firewire can daisy-chain 63+ devices Chapter 11 I/O Systems

43. Device Attachment • USB and firewire standards define the cables and connectors between devices and computers (e.g., keyboards, cameras, printers) as well as the communication protocol used for connection. • Ports, interfaces, power supply, cable connectors are all included in the standard Chapter 11 I/O Systems

44. Disk Attachment (cont.) • Network-Attached Storage • Network cables rather than SCSI (or other) cables • Location transparency • host does not know whether I/O devices are directly attached or distant • (except for the increased latency (delay) and for outputting a hard copy) • RPC over TCP/IP is commonly used to request service from storage devices Chapter 11 I/O Systems

45. Disk scheduling • Assume that we have a large number of requests for disk i/o • Scheduling these requests in specific orders can increase throughput and decrease average waiting time • Issues and algorithms are similar to process scheduling although here we try to minimize hardware issues such as seek time (time for disk arm to move the head to the required cylinder). Chapter 11 I/O Systems

46. Disk Management • Disk formatting • Disk must be formatted before it is used • Low level formatting – usually when manufactured • Divides magnetic recording units into sectors • Each sector contains a header, a trailer (control info for controller) and a data area (usually 512-1024 bytes) • Sector number, error-correcting code • Soft error if ECC can recover from mismatch • Bad sectors are isolated • Spare sectors are set aside for later needs Chapter 11 I/O Systems

47. OS disk formatting • High Level formatting • OS installs multi-level data structures on a disk • Partitions disk into sets of cylinders • Logical disks are mapped onto the physical disk • Can provide a separate logical disk for OS code with limited access rights for users • Separate user data to make backups easier • Logical formatting • Create disk’s file system • Maps of free and allocated space, initial directory of files on disk Chapter 11 I/O Systems

48. Raw Disks • OS activity is limited for raw disks (except for combining sectors into blocks) • Swap disks • Process data may be stored contiguously • Eliminate cost of i-node directories, linking blocks, etc. • Database systems • Provide their own high level formatting • These services bypass buffer cache, file locking, prefetching, directories, file naming Chapter 11 I/O Systems

49. Boot block • Bootstrap program on system disk in static location • Minimal boot program in ROM executed when machine is powered on • Brings rest of bootstrap program from disk into memory and transfers to it • Bootstrap program stored in “boot blocks” at fixed location on (boot or system) disks • Program does system and register initialization • Copies OS kernel from disk into memory • Transfers control to (jumps to) some address in memory to begin executing kernel Chapter 11 I/O Systems

50. Swap Space Management • Swap space is an extension of memory • Disk access is much slower than memory access • Access to swap space should be minimized for required virtual memory performance • Swap space was typically much larger than main memory until the large increases of main memory today • Swap space may be used for entire process or only for swapping out pages when necessary Chapter 11 I/O Systems