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Expansion Slots An expansion slot is used to add an adapter to the motherboard, and it has rules that control how many bits can be transferred at a time to the adapter. Expansion slots used in PCs are usually some form: PCI-E PCI (peripheral component interconnect) AGP (accelerated graphics port) Older PC Expansion Slots: ISA(industry standard architecture) EISA (extended industry standard architecture) MCA (micro channel architecture) ISA (Industry Standard Architecture) ISA is the oldest expansion slot. ISA allows 16-bit transfers to adapters installed in ISA slots. The number of expansion slots available depends on the motherboard manufacturer. ISA is also referred to as the AT bus. Because computer manufacturers want customers to be able to use their old adapters in an upgraded motherboard or a new computer, ISA is still available on the market. ISA operates at 8MHz, although some vendors reliably achieve 10MHz throughput. Some vendors have achieved 12MHz, but the industry pronounced 10MHz the maximum speed for ISA. With today’s microprocessor speeds, it’s easy to see how the ISA architecture can be a detriment. Adapters that require high-speed transfers, such as network memory and video, are hampered by the slowness of the ISA standard. ISA was designed to be backward compatible with IBM’s first two computer models, the PC and the XT, which had an 8-bit external data bus. The only adapters that worked in the PC and the XT computers were 8-bit adapters. The ISA architecture allows an 8-bit adapter to fit and operate in the 16-bit ISA slot. Today, motherboards normally do not come with ISA slots.

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Page 1: Expansion Slots

Expansion Slots

An expansion slot is used to add an adapter to the motherboard, and it has rules that control how many bits can be transferred at a time to the adapter.

Expansion slots used in PCs are usually some form:PCI-EPCI (peripheral component interconnect) AGP (accelerated graphics port)

Older PC Expansion Slots: ISA(industry standard architecture)EISA (extended industry standard architecture) MCA (micro channel architecture)

ISA (Industry Standard Architecture)

ISA is the oldest expansion slot. ISA allows 16-bit transfers to adapters installed in ISA slots. The number of expansion slots available depends on the motherboard manufacturer. ISA is also referred to as the AT bus. Because computer manufacturers want customers to be able to use their old adapters in an upgraded motherboard or a new computer, ISA is still available on the market.

ISA operates at 8MHz, although some vendors reliably achieve 10MHz throughput. Some vendors have achieved 12MHz, but the industry pronounced 10MHz the maximum speed for ISA.

With today’s microprocessor speeds, it’s easy to see how the ISA architecture can be a detriment. Adapters that require high-speed transfers, such as network memory and video, are hampered by the slowness of the ISA standard.

ISA was designed to be backward compatible with IBM’s first two computer models, the PC and the XT, which had an 8-bit external data bus. The only adapters that worked in the PC and the XT computers were 8-bit adapters. The ISA architecture allows an 8-bit adapter to fit and operate in the 16-bit ISA slot. Today, motherboards normally do not come with ISA slots.

PCI (Peripheral Component Interconnect)

A previously popular expansion slot is PCI(peripheral component interconnect) bus. PCI comes in four varieties: 32-bit 33MHz, 32-bit 66MHz, 64-bit 33MHz, and 64-bit 66MHz. Figure 2.18 shows the most common type of PCI expansion slot.

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An upgrade to the PCI bus is called PCI-X. PCI-X can operate at 66, 133, 266, 533, and 1066MHz. The PCI-X bus is backward compatible with the previous versions of the bus, but it allows faster speeds. A chip called the PCI bridge controls the PCI devices and PCI bus. With the PCI-X bus, a separate bridge controller chip is added. Figure 2.20 shows how the PCI-X bus integrates into the system board.

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PCI-X adapters are more often found in network servers or high-end gaming workstations to control video, network adapters (such as gigabit Ethernet), and SCSI adapters (that control hard drives, tape drives, CD/DVD drives, scanners, and other internal and

Tech Tips

PCI cards in PCI-X slots Remember that older PCI cards can fit in a PCI-X expansion slot, but a PCI-X adapter requires a PCI-X expansion slot. Today’s motherboards have a limited number of PCI or PCI-X expansion slots because of a newer standard called PCI Express or PCIe.

AGP (Accelerated Graphics Port)

AGP(accelerated graphics port) is a bus interface for graphics adapters developed from the PCI bus. Intel does the majority of the development for AGP and the specification was originally designed around the Pentium II processor. AGP speeds up 3-D graphics, 3-D acceleration, and full-motion playback.

With AGP, the processor on the video adapter can directly access RAM on the motherboard when needed. This helps with video-intensive applications. 3-D graphics, for example, are resource-intensive and use a lot of memory. Software developers can produce better and faster 3-D graphics using AGP technology. The best performance is achieved when applications use the RAM on the AGP adapter. However, because more memory than the amount on the adapter is needed, motherboard RAM is the next best option. Previous video adapters have been limited by the bottleneck caused by going through an adapter and a bus shared with other devices. With AGP, the video subsystem is isolated from the rest of the computer. The different versions of AGP are known as 1X, 2X, 4X, and 8X. All versions transfer 32-bits at a time. Table 2.9 summarizes the differences between the AGP versions.

Figure 2.21 shows an illustration of an AGP slot compared with PCI and ISA expansion slots. All of the expansion slots previously covered are being replaced by PCIe (covered next).

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Figure 2.22 shows various expansion slots.

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PCIe (Peripheral Component Interconnect Express)

The PCI bus has almost reached its limit in terms of speed. PCI, PCI-X, and AGP are being replaced with PCIe(PCI Express), which is also seen as PCI-E. PCIe is better than the other types of PCI expansion slots. PCIe 3.0 allows transfers up to 1GBps per lane in one direction with a maximum of 32 lanes. PCIe 2.0 has a transfer rate from 2.5GT/s (gigatransfers per second) to 5.0 GT/s, and version 3.0 increases it to GT/s. This allows a 16-lane PCIe link to transfer data at a rate up to 32GBps. Competing tech-nologies to PCIe include Rapid IO, HyperTransport, InfiniBand, and StarFabric. These are great types of technologies to research if you are interested in hard-ware development. The older PCI standard is half-duplex bidirectional, which means that data is sent to and from the PCI or PCI-X card, but in only one direction at a time. PCIe sends data full-duplex bidirectionally; in other words, it can send and receive at the same time. Figure 2.23 shows this concept.

The older PCI standard including PCI-X uses a parallel bus where data is sent with multiple 1s and 0s simultaneously. PCIe is a serial bus and data is sent one bit at a time.

Another difference is that PCIe slots come in different versions depending on the maximum number of lanes that can be assigned to the card inserted into the slot.

For example, an x1 slot can have only one transfer lane used by the x1 card inserted into the slot—x4, x8, and x16 slots are also available. An x16 slot accepts up to 16 lanes, but fewer lanes can be assigned. An x16 slot accepts x1, x4, x8, and x16 PCIe adapters. Figure 2.24 shows the concepts of PCIe lanes. Figure 2.25 (on page 50) shows some sample PCIe slots.

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AMD has a different method of interfacing with PCI and PCIe interfaces than the traditional FSB. AMD uses a technology called HyperTransport, which is a high-speed bus used to connect multiple CPUs, interface an AMD CPU with input/output devices, interface the CPU with PCI, PCI-X, and PCIe slots, and interface the CPU with RAM. Figure 2.26 shows how the HyperTransport bus connects various technologies.

HyperTransport is a feature of AMD’s Direct Connect architecture. With Direct Connect, there are no front side buses. Instead, the CPUs, memory controller, and input/output functions directly connect to the CPU at CPU speed.

PCI, PCI-X, AGP, and PCIe are important for connectivity in both workstation and portable computers. Traditional PCI connectivity will need to be supported for several more years in new machines for backward compatibility and in computers already in use. PCIe is the bus of the future for internal and external device connectivity. Figure 2.32 shows how AMD and Intel connect PCI and PCIe cards to other motherboard components.

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