Understanding Your Motherboard

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Understanding your motherboard's bus system These days, pick up any electronic device and you'll find some acronyms on the box describing the various busses it supports. A computer will have a list of busses as long as your arm. In this Daily Drill Down, I'll discuss the different busses currently available and in use to help you understand exactly what your equipment is capable of. What's a bus? A bus is the path through which a device sends its data so that it can communicate with the CPU and
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    Understanding your motherboard's bus system These days, pick up any electronic device and you'll find some acronyms on thebox describing the various busses it supports. A computer will have a list ofbusses as long as your arm. In this Daily Drill Down, I'll discuss the differentbusses currently available and in use to help you understand exactly what yourequipment is capable of.What's a bus?A bus  is the path through which a device sends its data so that it cancommunicate with the CPU and/or other devices. For example, a PCI device,such as an audio card, will send its data through the PCI bus. Each device willhave an access point to the bus using a particular kind of interface. The word interface  refers not only to the physical port the devices plug into, but to theelectrical operating parameters and the communication format as well. Typically,each bus has a uniquely shaped interface to prevent you from damaging yourdevices by plugging them into the wrong ports. PCs have three or more busses.The motherboard's bus system has been compared to a mass transit system thatcarries data over many routes through the city (your motherboard) and usesdifferent types of vehicles (fast and slow, and small and large) to carry it.The differences between computer busses break down into these categories: ã   Data width ã   Cycle rate ã   Device management ã   TypeThe data width and cycle rate are used to determine the bandwidth, or the totalamount of data that the bus can transmit. An 8-bit bus (1-byte data width) thatoperates at a cycle rate of 1,000 MHz (1,000,000 times per second) can transfer8 Mbps (1 MBps).The device-management specification indicates the maximum number ofsupported devices and the difficulty of configuring them. There are two types ofbus communications, serial and parallel. On a parallel bus, all devices have theirown interface to the bus, which is the norm. Serial devices are tied together in,well, a series; the last one has to talk through the first one. This can causeobvious performance problems. These busses typically are used in conditionswhere data throughput isn't critical.Front Side Bus (FSB)The Front Side Bus is the interface between the CPU and the motherboard,specifically the North Bridge/Memory Controller Hub. See below for details on the   FSBs in use by Intel and AMD. For more information on this subject, see myDaily Drill Down, Motherboard chipsets—the good, the bad, and the ugly. Intel GTL+ Front Side BusAt the simplest level, Intel's GTL+ FSB provides a single connection to the NorthBridge shared between all CPUs. In a dual-CPU system, this halves the availablebandwidth and quarters it for a quad CPU board. Someone is sure to point outthe fact that CPUs almost never need the full bandwidth of the bus. Quite true.Unfortunately, since the bus is in an all-or-nothing situation, the CPUs must taketurns. (Hello Mr. Latency, care to have a seat and chat while we're waiting for thebus to arrive?) The problem is even worse in today's world of 800-MHz CPUsrunning on 133-MHz memory where even a single CPU has to wait up to sixprocessor cycles for a data request. Imagine a quad CPU server being used forsomething other than show; if the application isn't smart enough to fill that L1cache, or—heaven forbid—your L1 cache is too small to last the duration, you'llhave processors sitting idle. So you might as well use fewer or slower CPUs andsave some cash. This is why only Intel's Xeon processors, coming with up toeight times as much L1 cache as a Pentium III, are usable in systems with morethan two processors.AMD EV-6 Front Side BusThe EV-6 FSB is more like a network switch than a bus, since each processorhas a full connection to the North Bridge running at an effective 200 MHz; 50percent faster than Intel's 133-MHz FSB. Between per-processor links to theNorth Bridge and that high-speed bus, the EV-6 is an excellent multiprocessorbus. Naturally, the total effective CPU bandwidth can't exceed the bandwidthavailable from the other interfaces combined, but unlike the GTL+ bus, one CPUcould access peripherals on the PCI bus while another accesses memory.Of course, the EV-6 bus is nothing new to the computing world since it was oneof Alpha's weapons in the server wars. This is also academic until AMD releasesthe multiprocessor version of the 760 chipset in late December 2000.Memory bussesThe memory bus is the interface between the RAM and the motherboard.Because each variant requires a different type of controller, few motherboardssupport more than one type of memory. There have been many forms of memorythat are now considered obsolete. The current types are discussed below.DDR-SDRAM (Double Data Rate Synchronous Dynamic Random AccessMemory)This upcoming replacement for SDRAM is basically the same product, but itoperates twice per clock cycle. Two grades are expected to be introducedinitially: the 2x 100-MHz PC1600 (1.6 GBps) and the 2x 133-MHz PC2100 (2.1GBps). DDR-SDRAM is only about 10 to 20 percent more expensive thantraditional SDRAM and provides better performance than single-channelRDRAM. A 2x 200-MHz PC3200 (3.2 GBps) is currently under development that   will provide the same performance level of dual-channel RDRAM while using onlya single memory module.RDRAM (Rambus Dynamic Random Access Memory)Rambus is a proprietary memory architecture being touted by Intel. It has a serialmemory format with a very narrow 16-bit interface but operates very fast at 800MHz on a DDR-type 400-MHz bus, resulting in 1.6 GBps of bandwidth. A dual-channel RDRAM system is used on a rare few workstation systems: It has twoRDRAM controllers for 3.2 GBps of bandwidth but requires the RDRAM to beinstalled in pairs.RDRAM is several times more expensive than SDRAM and provides increasedlatency. It will shortly be challenged by DDR-SDRAM in the marketplace on bothprice and performance.SDRAM (Synchronous Dynamic Random Access Memory)SDRAM is the standard memory format for the majority of computers on themarket. This 64-bit memory comes in three grades: PC66 (66 MHz or 528MBps), PC100 (100 MHz or 800 MBps), and PC133 (133 MHz or 1.06 GBps).PC66 was used on early Intel Pentium IIs and all Intel Celeron PCs. PC100 is inuse on the vast majority of Intel Pentium II and Pentium III processors. PC133 isthe preferred memory of all AMD Athlon and Duron processors and the latestPentium III systems.VCM (Virtual Channel Memory)This subset of SDRAM is a low-latency variant that provides increasedperformance. It runs at 133 MHz and has the same 1 GB of bandwidth as PC133SDRAM but shaves off about 10 nanoseconds of latency from SDRAM's normal40-nanosecond latency. It does this by using special fast registers that keeptrack of memory pages. These registers provide a fast link, or channel, to thememory used by an application. VCM actually works better for complexapplications like games and databases that have memory spanning multiplememory banks.VCM is supported on a number of Pentium II, Pentium III, and Athlonmotherboards but is very difficult to acquire. The low production volumes havekept prices out of proportion to the performance compared to standard PC133.Motherboards that support VCM can use it or standard SDRAM.High-speed I/O bussesNow let's take a look at the high-speed I/O busses.AGP/Pro (Advanced Graphics Port)This interface is a 32-bit system based on the PCI standard, revision 2.1. Theinitial version, 1x, operated at 66 MHz for 266 MBps with direct memory-accessabilities that PCI didn't have. The 2x variant is a double-data-rate system thattransmits data twice per clock cycle, for a functional frequency of 133 MHz (532   MBps).The 4x again doubles the bandwidth up to 1,066 MBps and has additionalmemory-access features. The bandwidth of AGP 4x is greater than SDRAM'sabilities, making most of the improvements of only limited use on systems notcurrently using Rambus RDRAM (1.6 GBps) or the upcoming double-data-rateDDR memory (2.1 GBps).The Pro is a 4x variant that includes additional power leads to run today's high-transistor-count video cards. Standard AGP slots provide up to 25 watts ofpower, far less than AGP Pro's maximum available 110 watts.The AGP port is typically a dark-colored port that resembles a PCI slot. It's setfarther back from the edge of the motherboard than the PCI slots and is locatednear the power supply and processor on most boards.EIDE (Enhanced Integrated Drive Electronics)IDE hard drives include a device controller mounted on the drive, the result of a1986 collaboration between Compaq and Western Digital to develop a cheapdrive with good performance. They decided to limit the number of pins and thecable length as it was intended for lower-end systems that would not need alarge number of internal devices.Because each device has its own controller, only two devices can be on eachchain to prevent excess interference. Modern IDE host adapters can operate twochains, each with a master and a slave. The master can interrupt the slavedevice at any time, making it inappropriate for the primary system drive orsensitive devices like CD-R, CD-RW, and tape drives to be slaves. Themaximum of four devices per controller (two chains with two devices each) limitsthe number of devices an IDE system can handle.The current generation of integrated IDE host adapters requires somemanagement by the computer's processor, creating a load on the system. Thevarious implementations of direct memory access (DMA) protocols helpedtransfer data to the computer's RAM from devices with less management by theprocessor. Transfer rates increased under the ATA-2 specification from 11.1MBps to 16.66 MBps—a significant increase but still a bottleneck for PCs.Beginning with ATA-2 (also known as Fast ATA), the improved interface becameknown as EIDE, rather than IDE.In 1996, the ATA/33 specification utilized newer DMA techniques to reach 33-MBps transfer rates. Also known as Ultra DMA or UDMA/33, it was completelybackward-compatible with previous devices and became the standard for PChard drives.The year 1999 saw the introduction of the improved ATA/66 format. The 66-
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