Single Bus Problems

   Lots of devices on one bus leads to :

• Physically long buses

           ~ Propagation delays –
               Long data paths mean that co-ordination of bus use can adversely effect performance​.    

           ~ Reflections / termination problems​.

• Aggregate data transfer approaches bus capacity​.

• Slower devices dictate the maximum bus speed.

 Traditional (ISA) Bus Architecture with cache

High Performance Bus Architecture

Bus Hierarchy

• The Processor Bus - This is the highest-level bus that the chipset uses to send information to and from the processor. 

• The Cache Bus - Higher-level architectures, such as those used by the Pentium Pro and Pentium II, employ a dedicated bus for accessing the system cache. This is sometimes called a backside bus. Conventional processors using fifth-generation motherboards and chipsets have the cache connected to the standard memory bus. 

• The Memory Bus - This is a second-level system bus that connects the memory subsystem to the     chipset and the processor. In some systems the processor and memory buses are basically the same thing. 

• The Local I/O Bus - This is a high-speed input/output bus used for connecting performance-critical       peripherals to the memory, chipset, and processor. For example, video cards, disk storage devices, high-speed networks interfaces generally use a bus of this. The two most common local I/O buses are the VESA Local Bus (VLB) and the Peripheral Component Interconnect Bus (PCI). 

• The Standard I/O Bus - Connecting to the above three buses is the "good old" standard I/O bus, used for slower peripherals (mice, modems, regular sound cards, low-speed networking) and also for compatibility with older devices. On almost all modern PCs this is the Industry Standard Architecture (ISA) bus.

How various bus connect to CPU

Characteristics of Bus

Address, Data, and Control Bus

    There are normally three type of bus in any processor system : 

• An address bus - this determines the location in memory that the processor will read data from                              or write data to.

• A data bus - this contain contents that have been read from the memory location or are to be                           written into the memory location.

• A control bus - this manages the information flow between components indicating whether the                           operation is a read or write and ensuring that the operation happens at the right                           time.

Bus Width

A bus is characterized by the amount of information that can be transmitted at once. It is call bus width. The amount expressed in bits, corresponds to the number of physical lines over which data is sent simultaneously. The term "width" is used to refer to the number of bits that a bus can transmit at once. Typical widths are 8, 16, 32, or 64 bits at a time.

In other words, the bus width means the size of the entity that the bus sends data in one cycle. For example 32 bits, this means that the bus has 32 wires that are used for the transmission. (This may also be different, depending on the transmission protocol).

Bus Speed

Bus speed means how many bits or bytes (1 byte = 8 bits) is the bus able to send in a unit of time (typically second). For example for a computer network it could be 100 Mbs means 100 Megabits (12, 5 Megabytes) in a second. The bus speed is also defined by its frequency. It expressed in Hertz. Frequency means the number of data packets sent or received per second. Each time that data is sent or received is called a cycle.

Besides, the bus speeds also commonly known as front side bus (FSB). It is shows how fast the components can communicate with each other. Faster bus speed means faster computer. You can easily see how a faster FSB helps your computer get more information from one place to the other. For example, if two computers are exactly the same in all ways except one has an FSB speed of 500 MHz and the other has an FSB of 1066 MHz, twice as much data will get through on the second computer compared to the first. This means your computer will get more work done in the same amount of time faster spreadsheet calculations, faster decoding of that movie, or smoother video games.

To increase the bus speed you can either increase the bus width or the frequency of the transmission on the bus.

Bus Bandwidth

Bandwidth, also called throughput, refers to the total amount of data that can theoretically be transferred on the bus in a given unit of time. Using the highway analogy, if the bus width is the number of lanes, and the bus speed is how fast the cars are driving, then the bandwidth is the product of these two and reflects the amount of traffic that the channel can convey per second.

The table below shows the theoretical bandwidth of most of the common I/O buses on PCs today. Note the italics on the word "theoretical"; most buses can't actually transmit anywhere near these maximum numbers because of command overhead and other factors. This is especially true of older buses. 

For example, the theoretical bandwidth of the 8-bit ISA bus might be about  MBytes/sec, but in reality there are wait states inserted during I/O that drop this figure down dramatically. Most of these buses can run at many different speeds; the speed listed is the one most commonly used for the bus type.