Encyclopedia of Computer Science and Technology

multiprocessing 323music and video players, digital; smartphones; and digitalconvergence). Further, equipped with digital camerasand camcorders (even cell-phone cameras), togetherwith easy-to-use editing software, more and more peopleare becoming not just consumers of multimedia, but creatorsas well (see user-created content and YouTube).Further ReadingCoorough, Calleen, and James E. Shuman. Multimedia for the Web:Creating Digital Excitement. Boston: Thomson Course Technology,2005.Lauer, David, and Stephen Pentak. Design Basics: Multimedia Edition.6th ed. Belmont, Calif.: Wadsworth Publishing, 2006.MIT Media Lab. Available online. URL: http://www.media.mit.edu.Accessed August 15, 2007.“Multimedia and Authoring Resources on the Internet.” NorthwesternUniversity Library. Available online. URL: http://www.library.northwestern.edu/dms/multimedia.html.Accessed August 15, 2007.Packer, Randall, and Ken Jordan. Multimedia: From Wagner to VirtualReality. Expanded ed. New York: Norton, 2002.Vaughan, Tay. Multimedia: Making It Work. 7th ed. New York:McGraw-Hill/Osborne Media, 2006.multiprocessingOne way to increase the power of a computer is to use morethan one processing unit. In early computers (see mainframe)a single processor handled both program executionand input/output (I/O) operations. In the late 1950s,however, machines such as the IBM 709 introduced theconcept of channels, or separate processing units for I/Ooperations. In such systems the central processor sends aset of I/O commands (such as to read a file into memory) tothe channel, which has its own processor for carrying outthe operation.True multiprocessing, however, involves the use of morethan one central processing unit (CPU). One successfuldesign, Control Data Corporation’s CDC 6600 (1964), containedboth multiple arithmetic/logic units (the part of theCPU that does calculations) and multiple controllers for I/Oand memory access control. IBM soon added multiprocessingcapability to its 360 line of mainframes.Multiprocessing can be either asymmetric or symmetric.Asymmetric multiprocessing essentially maintains a singlemain flow of execution with certain tasks being “handedover” by the CPU to auxiliary processors. (For example,the Intel 80386 processor could be purchased with an additionalfloating-point processor, allowing such calculationsto be performed using more efficient hardware. When thePentium line was developed, floating-point was integratedinto the main CPU).Symmetric multiprocessing (SMP) has multiple, fullfledgedCPUs, each capable of the full range of operations.The processors share the same memory space, whichrequires that each processor that accesses a given memorylocation be able to retrieve the same value. This coherenceof memory is threatened if one processor is in the midstof a memory access while another is trying to write datato that same memory location. This is usually handled bya “locking” mechanism (see concurrent programming)that prevents two processors from simultaneously accessingthe same location.A subtler problem occurs with the use by processors ofseparate internal memory for storing data that is likely tobe needed (see cache). Suppose CPU “A” reads some datafrom memory and stores it in its cache. A moment later,CPU “B” writes to that memory location, changing the data.At this point the data in “A’s” cache no longer matches thatin the actual memory. One way to deal with this problemis called bus snooping. Each CPU includes a controller thatmonitors the data line (see bus) for memory locations beingused by other CPUs. When it sees an address that refers toan area of memory currently being stored in the cache, thecontroller updates the memory from the cache. This writeoperation sends a signal that lets other CPUs know that anycached data they have for that location is no longer valid.This means the other CPUs will go back to memory andreread the current data.Alternatively, all CPUs can be given a single sharedcache. While less complicated, this approach limits thenumber of CPUs to the maximum data-handling capacityof the bus.Larger-scale multiprocessing systems consist of latticelikearrays of hundreds or even thousands of CPUs,which are referred to as nodes. Indeed, small clusters ofCPUs using the architecture given above can be connectedtogether to form larger arrays. Each cluster can have itsown shared memory cache. Because accessing memory ata remote node takes considerably longer than accessingThis example shows what can happen if processes do not properlymanage a shared memory resource. At (1) processor A retrieves3 from the memory location. At (2) processor B copies 7 from itscache to that same memory location. Finally, at (3) processor Aadds the 3 it had retrieved to a 2 in its register, storing 5 back in alocation where processor B probably expects there to still be a 7.