System Bus Types, Functions, and Features
| The heart of any motherboard is the various signal pathways or buses that carry signals between the components . A bus is a common pathway across which data can travel within a computer. This pathway is used for communication and can be established between two or more computer elements. The PC has a hierarchy of different buses. Most modern PCs have at least three main buses; some have four or more. They are hierarchical because each slower bus is connected to the faster one above it. Each device in the system is connected to one of the buses, and some devices (primarily the chipset) act as bridges between the various buses. The main buses in a modern system are as follows :
Some newer laptop motherboards feature a special MDC (mobile daughter card) connector, which provides an interface for modem, audio, and networking solutions based upon the AC'97 (audio coder /decoder) interface. Some laptop manufacturers refer to MDC cards and sockets as communications daughter card (CDC) cards and sockets, although they are the same thing. The MDC connector provides a low-cost, small form factor module suitable for mobile systems. This is a dedicated connector for cards that are specific to the system or motherboard design to offer communications and networking options. They are not designed to be general-purpose bus interfaces, and cards for these connectors are not sold on the open market. Usually, they're offered only as a preinstalled option with a given system. They are designed so that a system manufacturer can easily offer configurations with and without various communications options, without having to reserve space on the motherboard for optional chips. For example, I recently purchased a laptop in which I wanted to integrate every type of wireless networking possible, including tri-mode 802.11a/b/g as well as Bluetooth. Unfortunately, the manufacturer did not offer a system that incorporated all of these devices, although various models included one or the other. Rather than add external CardBus devices, which would be unwieldy and use up all of the available slots, I purchased an internal 802.11a/b/g Mini PCI card and a combination Modem/Bluetooth Mobile Daughter Card (MDC) and installed them myself. Because such cards are not normally sold retail, I merely ordered them as if they were repair parts for the various models that incorporated them and installed them myself . The particular laptop in question (an IBM R40) already had integrated WiFi antennas, so all I needed for the ultimate WiFi solution was the $99 Mini PCI 802.11a/b/g card. My particular system did not include an internal Bluetooth antenna, although one could be ordered and installed as a repair part. So, I ordered a Modem/Bluetooth MDC module for $37 and an internal Bluetooth antenna for $8, and installed them in a matter of minutes. Because of the standardization available with Mini PCI and MDC, it's possible to upgrade many other laptops in a similar manner. Now I have all of these communications devices internally mounted, and the CardBus socket remains free for other uses. For the most part, modem, sound, and Bluetooth cards are implemented via MDC modules, while higher-speed WiFi wireless networking options are normally installed via Mini PCI cards. Mini PCI cards are available that support the 802.11a, 802.11b, or 802.11g standards individually, or in combo cards supporting 802.11a/b/g all in one. The system chipset is the conductor that controls the orchestra of system components, enabling each to have its turn on its respective buses. Table 5.1 shows the widths, speeds, data cycles, and overall bandwidth of virtually all PC buses. Table 5.1. Bandwidth (in MBps) and Detail Comparison of Most PC Buses and Interfaces
Note that many of the buses use multiple data cycles (transfers) per clock cycle to achieve greater performance. Therefore, the data-transfer rate is higher than it would seem for a given clock rate, which provides an easy way to make an existing bus go faster in a backward-compatible way. The following sections discuss the processor and other subset buses in the system and the main I/O buses mentioned in the previous table. The Processor Bus (Front-Side Bus)The processor bus (also called the front-side bus, or FSB) is the communication pathway between the CPU and motherboard chipset ”more specifically, the North Bridge or memory controller hub. This bus runs at the full motherboard speed ”typically between 66MHz and 533MHz in modern laptops, depending on the particular board and chipset design. This same bus also transfers data between the CPU and an external (L2) memory cache on older 386, 486, and Pentium systems. Intel 800-series chipsets use what Intel calls hub architecture instead of the older North/South Bridge design. This moves the main connection between the chipset components to a separate 266MBps hub interface (which has twice the throughput of PCI) and enables PCI devices to use the full bandwidth of PCI without fighting for bandwidth with a South Bridge. Also, the flash ROM BIOS chip is now referred to as a firmware hub and is connected to the system via the LPC bus instead of via the Super I/O chip, as in older North/South Bridge designs. Because the ISA bus is no longer used in most of these systems, the Super I/O is connected via the LPC bus instead of ISA. The Super I/O chip also can easily be eliminated in these designs. This is commonly referred to as a legacy-free system because the ports supplied by the Super I/O chip are now known as legacy ports. Devices that would have used legacy ports must then be connected to the system via USB instead, and such systems would feature two USB controllers, with up to four total ports (more can be added by attaching USB hubs). AMD processor systems adopted a Socket A design, which is similar to Socket 370, except that it uses faster processor and memory buses. Although early versions retained the older North/South Bridge design, more recent versions use a design similar to Intel's hub architecture. Note that the high-speed CPU bus runs up to 333MHz (2664MBps throughput), and DDR SDRAM SO-DIMM modules are used that support a matching bandwidth of 2664MBps. It is always best for performance for the bandwidth of memory to match that of the processor. Finally, note that most of the South Bridge components include functions that are otherwise found in Super I/O chips; when these functions are included, the chip is called a Super South Bridge. The Mobile Pentium 4 and Pentium 4-M use a Socket 478 design with the hub architecture. This design is most notable for including a 400 533, or 800MHz CPU bus with a bandwidth of 32004266, or 6400MBps. The 533MHz and 800MHz models are currently faster than anything else on the market. Many newer laptops use PC2100 (DDR266) or PC2700 (DDR333) DDR SDRAM. Although many desktop systems use dual-channel memory, which doubles the throughput of the modules by accessing two modules simultaneously , that is not yet common on laptop systems. Because the purpose of the processor bus is to get information to and from the CPU at the fastest possible speed, this bus typically operates at a rate faster than any other bus in the system. The bus consists of electrical circuits for data, addresses (the address bus, which is discussed in the following section), and control purposes. Most processors since the original Pentium have a 64-bit data bus, so they transfer 64 bits (8 bytes) at a time over the CPU bus. The processor bus operates at the same base clock rate as the CPU does externally. This can be misleading because most CPUs these days run at a higher clock rate internally than they do externally. For example, an AMD Athlon XP-M 2800+ system has a processor running at 2.13GHz internally but only 266MHz externally, whereas a Mobile Pentium 4 3.2GHz runs at 3.2GHz internally but only 533 or 800MHz externally. In these newer systems, the actual processor speed is some multiple (2x, 2.5x, 3x, and higher) of the processor bus. The processor bus is tied to the external processor pin connections and can transfer 1 bit of data per data line every cycle. Most modern processors transfer 64 bits (8 bytes) of data at a time. To determine the transfer rate for the processor bus, you multiply the data width (64 bits or 8 bytes for a Celeron/Pentium III/4/M or Athlon/Duron/Athlon XP) by the clock speed of the bus (the same as the base or unmultiplied clock speed of the CPU). For example, if you are using a Mobile Pentium 4 3.2GHz processor that runs on a 533MHz processor bus, you have a maximum instantaneous transfer rate of roughly 4,266MBps. You get this result by using the following formula:
With slower versions of the Mobile Pentium 4-M, you get this:
With Socket A (Athlon XP-M), you get:
or:
With Socket 370 (Pentium III), you get:
or:
This transfer rate, often called the bandwidth of the processor bus, represents the maximum speed at which data can move. Refer to Table 5.1 for a more complete list of various processor bus bandwidths. The Memory BusThe memory bus is used to transfer information between the CPU and main memory ”the RAM in your system. This bus is connected to the motherboard chipset North Bridge or memory controller hub chip. Depending on the type of memory that your chipset (and, therefore, the motherboard) is designed to handle, the North Bridge runs the memory bus at various speeds. The best solution is for the memory bus to run at the same speed as the processor bus. Systems that use PC133 SDRAM have a memory bandwidth of 1066MBps, which is the same as the 133MHz CPU bus. In another example, Athlon systems running a 266MHz processor bus also run PC2100 DDR-SDRAM, which has a bandwidth of 2133MBps ”exactly the same as the processor bus in those systems. Note Notice that the main memory bus must normally transfer data in the same width as the processor bus. This defines the size of what is called a bank of memory. For most Pentium and newer mobile systems, a single small outline DIMM (SO-DIMM) is equal to a bank. | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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