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November 10, 2007 ©2003 Craig Zilles (derived fromslides by Howard Huang)1Introduction to I/O Where does the data for our CPU and memory come from or go to? Computers communicate with the outside world via I/O devices.— Input devices supply computers with data to operate on.— Results of computations can be sent to output devices. Today we’ll talk a bit about I/O system issues.— I/O performance affects the overall system speed.— We’ll look at some common devices and estimate their performance.— We’ll look at how I/O devices are connected (by buses).November 10, 2007 Introduction to I/O 2I/O is important! Many tasks involve reading and processing enormous quantities of data.— CCSO has two machines and 144GB of storage for a local web cache.— Institutions like banks and airlines have huge databases that must beconstantly accessed and updated.— Celera Genomics is a company that sequences genomes, with the helpof computers and 100 trillion bytes of storage! I/O is important for us small people too!— People use home computers to edit movies and music.— Large software packages may come on multiple compact discs.— Everybody and their grandparents surf the web!November 10, 2007 Introduction to I/O 3I/O is slow! How fast can a typical I/O device supply data to a computer?— A fast typist can enter 9-10 characters a second on a keyboard.— Common local-area network (LAN) speeds go up to 100 Mbit/s, whichis about 12.5MB/s.— Today’s hard disks provide a lot of storage and transfer speeds around40-60MB per second. Unfortunately, this is excruciatingly slow compared to modern processorsand memory systems:— Modern CPUs can execute more than a billion instructions per second.— Modern memory systems can provide 2-4 GB/s bandwidth. I/O performance has not increased as quickly as CPU performance,partially due to neglect and partially to physical limitations.— This is changing, with faster networks, better I/O buses, RAID drivearrays, and other new technologies.November 10, 2007 Introduction to I/O 4I/O speeds often limit system performance Many computing tasks are I/O-bound, and the speed of the input andoutput devices limits the overall system performance. This is another instance of Amdahl’s Law. Improved CPU performancealone has a limited effect on overall system speed.Amount of improvementTime unaffectedby improvement+Time affected by improvement=Executiontime afterimprovementNovember 10, 2007 Introduction to I/O 5Common I/O devices Hard drives are almost a necessity thesedays, so their speed has a big impact onsystem performance.— They store all the programs, moviesand assignments you crave.— Virtual memory systems let a harddisk act as a large (but slow) part ofmain memory. Networks are also ubiquitous nowadays.— They give you access to data fromaround the world.— Hard disks can act as a cache fornetwork data. For example, webbrowsers often store local copies ofrecently viewed web pages.April 30, 2003 Introduction to I/O 6I/O Performance Question How long does it take to “rip” a song from a CD? Why?November 10, 2007 PC I/O 7The Hardware (the motherboard)CPU socketMemory slotsSerial,parallel,and USBports(Back) (Front)IDE driveconnectorsPCI slotsAGP slotNovember 10, 2007 PC I/O 8What is all that stuff? Different motherboards support different CPUs, types of memories, andexpansion options. The picture is an Asus A7V.— The CPU socket supports AMD Duron and Athlon processors.— There are three DIMM slots for standard PC100 memory. Using 512MBDIMMs, you can get up to 1.5GB of main memory.— The AGP slot is for video cards, which generate and send images fromthe PC to a monitor.— IDE ports connect internal storage devices like hard drives, CD-ROMs,and Zip drives.— PCI slots hold other internal devices such as network and sound cardsand modems.— Serial, parallel and USB ports are used to attach external devices suchas scanners and printers.November 10, 2007 PC I/O 9How is it all connected?North BridgechipSouth BridgechipModem Sound cardHard disks CD-ROMVideocardMemoryCPUAGPportPCI busPCI slotsIDE controllerSerial, paralleland USB portsNovember 10, 2007 PC I/O 10Frequencies CPUs actually operate at two frequencies.— The internal frequency is the clock rate inside the CPU, which is whatwe’ve been talking about so far.— The external frequency is the speed of the processor bus, which limitshow fast the CPU can transfer data. The internal frequency is usually a multiple of the external bus speed.— A 2.167 GHz Athlon XP sits on a 166 MHz bus (166 x 13).— A 2.66 GHz Pentium 4 might use a 133 MHz bus (133 x 20).• You may have seen the Pentium 4’s bus speed quoted at 533MHz. This isbecause the Pentium 4’s bus is “quad-pumped”, so that it transfers 4 dataitems every clock cycle. Processor and Memory data rates far exceed PCI’s capabilities:— With an 8-byte wide “533 MHz” bus, the Pentium 4 achieves 4.3GB/s— A bank of 166MHz Double Data Rate (DDR-333) Memory achieves2.7GB/sNovember 10, 2007 PC I/O 11The North Bridge To achieve the necessary bandwidths, a “frontside bus” is oftendedicated to the CPU and main memory.— “bus” is actually a bit of a misnomer as, in most systems, theinterconnect consists of point-to-point links.— The video card, which also need significant bandwidth, is also given adirect link to memory via the Accelerated Graphics Port (AGP). All this CPU-memory traffic goes through the “north bridge” controller,which can get very hot (hence the little green heatsink).North BridgechipVideocardMemoryCPUAGPport1.1GB/s @ 133MHz x 2322.7GB/s @ 133MHz64AGP 4xNovember 10, 2007 PC I/O 12PCI Peripheral Component Interconnect is a synchronous 32-bit bus running at33MHz, although it can be extended to 64 bits and 66MHz. The maximum bandwidth is about 132 MB/s.33 million transfers/second x 4 bytes/transfer = 132MB/s Cards in the motherboard PCI slots plug directly into the PCI bus. Devices made for the older and slower ISA bus standard are connected viaa “south bridge” controller chip, in a hierarchical manner.North BridgechipSouth Bridgechip33 MHz PCI busPCI slotsNovember 10, 2007 PC I/O 13External buses External buses are provided to support the frequent plugging and un-plugging of devices— As a result their designs significantly differ from internal buses Two modern external buses, Universal Serial Bus (USB) and


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U of I CS 232 - Introduction to I/O

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