Section Objectives
After you complete this section you will:
Understand the components on the system board.
Understand the different expansion bus types.
Understand the interrupts and DMA in relation to expansion busses.
System Board Overview
The system board, also referred to as the “main board” or “motherboard” contains all of the elements that are central to the operation of the computer including the CPU, expansion slots, memory, and components which connect to other devices.
System Board Components
The CPU (Central Processing Unit) is the central chip to the operation of the computer. The CPU processes the instructions from the software. A CPU can be from several different manufacturers, examples include a Pentium II, an AMD Athlon, or a PowerPC. CPU speeds are measured in Megahertz. The higher the Megahertz, the faster the chip (generally speaking, CPUs are also measured in how many instructions per second it can perform and so a chip with a lower MHz could work much faster than one with a higher MHz, though on the Intel-compatible side of computing, the higher the MHz number, the faster the chip). For example, a Pentium III running at 800 MHz is faster than a Pentium III running at 700 MHz.
Newer chips are measured in GHz (Gigahertz), which is simply 1000 MHz. A 1.2 GHz chip is 1,200 MHz.
The expansion bus allows the computer to be upgraded using different modules. Expansion buses are made up with copper slots where a circuit board can plug into it. The motherboard contains a bus clock which controls how fast information flows to the expansion boards and back.
Memory, or more specifically RAM (Random Access Memory), is the place for storages of software, computer instructions, and operations while the computer is operating. RAM is a very fast storage location; though it loses all of its information with a loss of power.
The System Board
The System Board, or Motherboard, is discussed in great detail in the System Architecture course. For the exam, you need to know the general layout of a motherboard. Become familiar with the general layout of the system board. Here is an example of a motherboard with common components referenced:
Expansion Bus
Expansion Bus Speed
The different types of expansion busses operate at different speeds. The speed is controlled by a bus clock. The speed is controlled so that the motherboard/CPU only has to receive and transfer the amount of data it can handle or process at a given time. For example, a slow processor may not be able to handle the amount of data a very fast bus could transfer, so the data transfer speed is regulated to ensure the data does not overload the processor.
Interrupts
Interrupts are commands from a device to signify to the computer to stop. Basically, the operating system will continue running software code until the code is complete or until it receives an interrupt request. In hardware, the interrupt request (e.g. IRQ) has a value associated with it for each particular device.
DMA Channels
DMA Channels, or Direct Memory Access Channels, were designed to allow certain devices direct access to the memory. In the original PC, the system was designed so the CPU would control all access and act as the “traffic cop.” DMA was designed to allow certain expansion cards or devices to bypass the “middle man” and access memory directly. DMA was only used in ISA and its cousins EISA and VLB. PCI expansion cards do not use DMA.
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8-bit Bus |
8-bit
The 8-bit expansion bus operates at a maximum of 4.77 MHz (approx. 5 MHz), has eight interrupts, four DMA Channels, and one large 62 slot card.
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ISA Bus |
ISA Bus
The ISA (Industry Standard Architecture) bus is a 16-bit bus with a card slightly larger than the 8-bit. The 16-bit cards have an extra piece extending beyond the 8-bit card length. This bus has 16 interrupts and 8 DMA channels. ISA also runs at 8 MHz. ISA buses are backwards compatible with the 8-bit cards.
MCA Bus
The MCA (Micro Channel Architecture) bus was a proprietary bus designed by IBM. It is a 16-bit or 32-bit bus and it’s clock speed is 10 MHz. It also offered software configuration instead of dip switches and jumper settings.
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EISA Bus |
EISA
EISA (Extended ISA) bus borrowed a lot of features from the MCA bus and expanded on them. It has a 32-bit bus and has more I/O addresses (Input/Output addresses or memory addresses). It also still uses the 8 MHz of the ISA bus to allow for backwards compatibility.
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VLBus |
Vesa Local Bus
The VESA Local Bus (VLB) is a local bus type, meaning it is a bus which runs at the same speed as t
he processor. Typically, it is used for video to gain the advantage of the high speed of transfer of the processor. It is backwards compatible with the ISA, but has an extra slot to make it 32-bits.
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PCI Bus |
PCI Bus
PCI (Peripheral Component Interconnect) Bus is the newest and most advantageous type of all of the buses. It supports both 32-bit and 64-bit data paths to be compatible with 486 and Pentium class processors. PCI is also processor independent, allowing it to operate in Macintosh, PC, and RISC computers. PCI runs at 33 MHz and has a maximum throughput of 256 megabytes per second.
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AGP Bus |
AGP Bus
AGP, or Accelerated Graphics Port, is a bus technology developed by Intel to support high speed video cards. The AGP interface uses the RAM in your computer to generate 3-D images and video very quickly.
PCMCIA Bus
PCMCIA (Personal Computer Memory Card Association) is the last type of bus we will discuss. It is mainly used in laptops or other small computers and in some digital cameras. PCMCIA comes in three types, cleverly called Type I, Type II, and Type III.
Type I cards are 3.3 mm thick. Type II cards are the most common type and are 5 mm thick. Type III cards are mainly used for hard drives and are 10.5 mm thick.
Summary of Expansion Busses
| Bus Type | Bus Size | Speed (MHz) |
| 8-bit | 8-bit | 4.77 |
| ISA | 16-bit | 8 (10 in turbo mode) |
| MCA | 16-bit or 32-bit | 10 |
| EISA | 32-bit | 8 |
| VESA Local | 32-bit | Processor Speed |
| PCI | 64-bit | Processor Speed |
| PCMCIA Card | 16-bit | 33 MHz |
After studying this section you should:
Understand the components on the system board.
Understand what the CPU is, what an expansion bus is, and what memory is.
Understand the different expansion bus types.
Understand the differences between 8-bit, ISA, MCA, EISA, VLBus, PCI, AGP and PCMCIA.
Understand the interrupts and DMA in relation to expansion busses.
Understand what an interrupt is and what DMA allows a device to do.
