PCI Bus Architecture

PCI Bus Architecture

CS 350: Computer Organization

Sections 1 and 2

Spring 2004

Prepared by: Lauren Greenfield

Matthew Pozun

Lindsay Stenger

Olivia Ting
Table of Contents

Introduction………………………………………………………………………………1

History…………………………………………….………………………………………1

Performance…………………………...…………………………………………………2

§  How PCI Compares to Other Buses...... 3

§  Voltage Requirements...... 3

§  Features...... 4

Plug and Play………………………………………...…………………………………...4

§  Requirements for Full Implementation...... 4

§  Tasks it Automates...... 4

How PCI Works…………………………………..……………………………………...5

Other Types of PCI……………………………………..………………………………..6

The Future of PCI and PCI Express……………………………..…………………….7

Conclusion…………………………...…………………………………………………...9

Bibliography…………………………...………………………………………………..10

1

Introduction:

There are many different types of buses on a motherboard. What their purposes are, how they work, and how they originated are all details that will give a base of understanding and a more in depth knowledge of the PCI bus. A computer bus is used to transfer data from one location or device on the motherboard to the central processing unit where all calculations take place. A computer bus has two different parts: the address bus and the data bus. The data bus transfers the actual data while the address bus transfers information about where the data should go.

Some of the main buses inside a computer are the ISA bus, MCA bus, EISA bus, VLB bus, PCI bus, AGP bus, USB bus, mini PCI bus, and PCI-X bus. The PCI bus is what this paper will concentrate on, explaining the details of its history, its performance and how that compares to other buses. We will explain how Plug and Play plays a roll in the use of the PCI bus, how the PCI bus works, engage in a brief discussion of the other types of PCI, and the future of the PCI. PCI bus is based on the technology of the ISA (Industry Standard Architecture) bus and the VL Bus.

History:

The Peripheral Component Interconnect bus was introduced by Intel in 1992 and was revised in 1993 to version 2.0 and then again in 1995 to version 2.1. It started out as a 32-bit bus that later became the 64-bit bus that it is today. The one thing about this bus that made it even greater than other buses that were developed was that this bus was developed as an industry standard. It was not designed by one specific company that only wanted to use it on their computers, but rather it was made for all computer companies to use.

When installing an ISA device you had to set the jumpers and dip switches so that the device would work properly. The development of PCI made it much easier to install devices for people that did not understand a lot about computers. This is because the manufacturer could give you software for the device. The user would place the device in the PCI slot, start the computer, and then install the software the manufacturer provided.

The PCI bus is a combination of the old ISA bus developed by IBM and the VESA Local bus. ISA originated as an eight bit bus and then became sixteen bits and became known as ISA. ISA is capable of transmitting at sixteen MBps, which is capable of doing most applications even today.

VESA local bus (named after Video Electronics Standards Association) is 32 bits wide and can transmit at the speed of the local bus, which is typically the speed of the processor. The problem with the VL-bus is that if you connect too many devices into this bus it can start to introduce latency in the processor.

The PCI bus provides direct access to system memory for the devices that are connected to it, but all of these are connected through a bridge that then connects to the front side bus. Based on this configuration it can provide higher performance without slowing down the processor. The PCI bus originally worked at 33 MHz to 66 MHz. PCI uses a small number of pins because the hardware multiplexes by sending more than one signal over the same pin. PCI devices can be either 3.3 volts or 5 volts.

Figure 1 – Diagram of the Motherboard bus system (Tyson, 2001c)

PCI did not become famous until Microsoft came out with Windows 95 operating system and used Plug and Play technology. PCI did not take off then because it needed an operating system that could handle Plug and Play, and that really utilized the concept of the PCI bus.

Performance:

In any computer environment, performance of the system is crucial. Computers are used in military to medical environments, which at times require that the components of the system be as fast as possible. The PCI is one of the fastest buses available today at a reasonable cost with speeds ranging up to 4.3 GBps, in PCI-X 2.0, which is the most current revision (Quatech, 2004b). The following sections go over such topics as how the versions of PCI that we use compare to older popular buses, what the voltage requirements are for cards designed with PCI in mind, and features of PCI that help it operate as well as it does.

How PCI Compares to Other Buses:

Over the years, as technology has changed, different buses of varying speeds have been used to build computers. Two buses that are still prevalent today are the Industry Standard Architecture (ISA) bus and Peripheral Component Interconnect (PCI) bus, which has several revisions on the original PCI theme. A good overview of the aspects of a couple PCI variants, and ISA buses is featured in table 1 below.

Bus Type / Bus Width / Bus Speed / MB/sec / Advantages / Disadvantages
ISA / 16 bits / 8MHz / 16 MBps / low cost, compatibility, widely used / low speed, Jumpers & DIP switches. becoming obsolete
PCI / 64 bits / 133 MHz / 1 GBps / very high speed, Plug & Play, dominant board-level bus / incompatible with older systems,
can cost more
CompactPCI / 64 bits / 33MHz / 132 MBps / designed for industrial use, hot swapping/Plug & Play, ideal for embedded systems / lower speed than PCI, need adapter for PC use, incompatible with older systems

Table 1: How PCI compares to other buses (Tyson, 2004a; Quatech, 2004c)

As can be seen in the table above the PCI technology that is available today can reach speeds of 1 GBps which is high for today’s standards, but even this is changing. Soon, as stated above, PCI-X 2.0 will be showing up more and more, bringing with it speeds into the 4.3 GBps range. Overall, PCI has been the best bus for connecting devices such as Ethernet cards and USB cards, because it is very fast (Quatech, 2004b).

Voltage Requirements:

When PCI was originally developed all boards that plugged into the PCI slot had to be able to accept 5V power supplied by the motherboard. Over the years, only the power, which was provided by the motherboard, stayed the same (Quatech, 2004a).

The amount of power supplied to the board has changed greatly. After the original 5V, an option for only 3.3V was developed and considered optional, and later, with PCI 2.3, 3.3V became required making 5V obsolete (Quatech, 2004a).

To make sure that the correct voltage travels to the card different arrangements of the gold fingers on the bottom of the card exist (these fingers are known as the key). Because of these different keys Universal PCI slots have been developed which support both 5V and 3.3V cards (Quatech, 2004a).

Features:

Due to features present in the PCI bus, PCI is the fastest general Input/Output bus used in Personal Computers today. Three specific features that make this possible are: (1) PCI has Burst Mode (allows multiple sets of data to be sent after an initial address has been received), (2) PCI is also capable of full Bus Mastering, which leads to improved performance by allowing devices on the PCI bus to take control and perform direct transfers (Kozierok, 2001c). (3) Lastly, PCI also has some High Bandwidth Options that have allowed the speed of PCI to be continually increased (Kozierok, 2001a).

Plug and Play:

Plug and Play is a feature that allows the user to add a new piece of hardware to the computer and have it recognized and configured to work on the system. Plug and Play is important in a PCI system because these systems were the first to make Plug and Play popular (Kozierok, 2001b). The Plug and Play system was developed by Intel with Windows 95 offering the first system-level support for Plug and Play (Tyson, 2004b).

Requirements for Full Implementation:

In order for your Personal Computer to fully implement Plug and Play it needs to have three things:

·  Plug and Play BIOS – these BIOS do two things. First, they allow Plug and Play to operate on the computer and they retrieve configuration information about Plug and Play devices that are already installed on the system.

·  Extended System Configuration Data (ESCD) – A file on the system that contains information about Plug and Play devices that are already installed on the system.

·  Plug and Play operating system – This requirement can be fulfilled by any operating system that supports Plug and Play. Various operating systems that fulfill this requirement would be Windows 95/98/ME/2000/XP. The operating system is needed because it has Plug and Play handlers that will complete the configuration process that was started in the Plug and Play BIOS.

(Tyson, 2004b)

Tasks It Automates:

The Plug and Play feature automates four tasks that previously had to be done by hand or with an installation utility that had to be provided by the hardware manufacturer. These four tasks include:

·  Interrupt Requests (IRQ) – An IRQ is a request from the hardware device to the Central Processing Unit (CPU) that indicates that the hardware has information that needs to be done. Originally, each device connected to the computer needed its own IRQ to operate, but PCI changed that. PCI connects at the bus bridge, which allows multiple PCI devices to use a single IRQ line.

·  Direct Memory Access (DMA) – PCI devices are set up so that they can directly access the system memory without needing to ask the CPU first.

·  Memory Addresses – Plug and Play automatically assigns memory addresses to different hardware devices to ensure that the devices will have the necessary system resources to allow it to operate properly.

·  Input/Output (I/O) Configuration – The last task that Plug and Play automates is determining what ports the devices will send and receive information on.

By providing these automatic capabilities, Plug and Play was able to make PCI the bus of choice (Tyson, 2004b).

How PCI Works:

All buses hold specific standards for their bus architecture. The standards are based on MHz, Mbps, Bits, and Voltage. MHz (or Megahertz) is the speed of the data transfer. MBps (Mega Bytes processed per second) is the transfer rate, measuring how much data can be transferred within a period of one second. A higher Mbps will indicate a faster device. The Bits specification is similar to the number of lanes on a highway that can accommodate traffic. 32 bits will represent 32 lanes on the highway, which can handle a lot of cars. If there are more bits, then the more data it can accommodate. Finally, the Voltage standard is the amount of electrical energy it takes to do its work. The lower the voltage, the better, as it would satisfy the new “Green PC” energy requirements. The PCI is one of the best buses out there, comparable to ISA, EISA, and VESA, as shown on table 2 (Advanced Horizons, 2003):

ISA / EISA / VESA / PCI

MHz

/ 8.3 / 8.3 / 33 / 33
MBps / 8.3 / 33 / 160 / 132 or 264
Bits / 16 / 32 / 32 / 32 or 64
Voltage / 5 / 5 / 5 / 3.3 or 5

Table 2: PCI bus standards compared to other bus standards

Some computer motherboards can have anywhere from four to six PCI slots. Because the PCI is usually controlled by Windows software, it has the capability to be automatically configured once a new device card has been inserted into a PCI slot. Before being able to understand how PCI works, it must first be properly installed into the computer. The following steps explain how to properly install a new external device into the computer.

1. With the computer turned off, take off the computer case, and then insert the card into an empty PCI slot on the motherboard.
2. Put the case back on, and with all the cables and cords properly plugged in, turn on the computer.
3. The computer system Basic Input/Output System (BIOS) will initiate the Plug and Play (PnP) BIOS.
4. The PnP BIOS will scan the PCI bus for the hardware by sending out a signal to any device connected to the bus, asking for identification.
5. The card will respond with its identification and its device ID will be sent back to the BIOS through the bus.

6.  PnP checks the Extended System Configuration Data (ESCD) to make sure the configuration data already exists for the card. If the card is new, then there will be no data for it.

1. PnP will assign and Interrupt Request Line, Direct Memory Access, memory address and Input/Output settings to the card, then stores the information in the ESCD.
2. When the Windows software loads, it will check the PCI bus and the ESCD to see if there is new hardware. If there is new hardware, Windows will alert the user that new hardware has been found and will identify the hardware.
3. Windows will determine the type of device and attempt to install its driver. The operating system may ask the user to insert a disk containing the driver or direct it to where the driver is located. In the event that Windows is unable to determine what the device is, it will provide a dialog window so the user can identify the hardware and load its driver.

(Tyson, 2001c)