79 lines
4.4 KiB
Markdown
79 lines
4.4 KiB
Markdown
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---
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tags:
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- bus
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- computer-architecture
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---
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# Bus
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A bus is a communication system that transfers data between components inside a
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computer, or between computers.
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**A bus can be thought of as a kind of pipeline that allows different components
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of the computer to communicate with each other.**
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Note that the use of the word "bus" varies somewhat in points of emphasis. A bus
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can mean:
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- The physical wires that make up the bus
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- The communication channel that is established over those wires
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## Main buses
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| Bus type | Description |
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| ------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ |
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| System bus | The primary pathway between the CPU and [memory](Memory.md). It comprises the **data bus** that transfers data from the memory to the CPU and the **address bus** which transmits requests from the CPU to memory. |
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| Internal bus | Connects local devices for example the harddisk to the CPU. |
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| External bus | Connects peripheral devices such as disks and printers to the [motherboard](Motherboard.md) |
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| Expansion bus | Allows expansion boards to access the CPU and memory. |
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| Frontside bus | Main computer bus that determines data transfer rate speed and is the primary data transfer path between the CPU, RAM and other [motherboard](Motherboard.md) devices. |
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| Backside bus | Transfers secondary cache (L2 cache) data at faster speeds, allowing more efficient CPU operations |
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## Bus standards
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### Parallel and serial transmission types
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There is a distinction between buses that use **serial** and buses that use
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**parallel transmission**. They are distinguished by the way in which the bits
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are transferred from one point to another, along the bus.
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Serial Transmission is the type of transmission in which a single communication
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link is used to transfer the data from an end to another. On other hand Parallel
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Transmission is the transmission in which multiple parallel links are used that
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transmit each bit of data simultaneously.
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<img src="../img/serial-transmission.jpg" width="800px"/>
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<img src="../img/parallel-transmission.jpg" width="800px"/>
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- Serial buses are cheaper to implement than parallel buses
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- Serial buses operate at greater [latency](Bus.md#latency) than parallel buses
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## Latency
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Latency means _delay_: the delay from the time the data is requested until the
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time it arrives. In the context of a microprocesser and buses a key locus of
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latency would be between the time a request takes to travel accross the system
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bus from the CPU to the memory.
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### Bandwidth
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In general, bandwidth is a measure of the capacity for a communications channel
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to transmit data. A bus is a channel between two components thus we can talk
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about the bandwidth of buses. The speed and efficiency of a bus is a function of
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its bandwidth, although other factors such as its length and proximity to the
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source and receiver are also important.
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Bandwidth can be calculated in terms of how many bits can be transferred per
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second. The wider the bus, the more bits that can be transferred. If we have a
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bus that can run at 66MHz and transfer 64 bits, 64 bits is 8Mb thus the
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bandwidth is 8 x 66 = 528Mb.
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## Standards
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The mode of transmission is used to distinguish the different bus standards.
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SCSI (Small Computer System Interface) and ATA (Advanced Technology Attachment)
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are used for hard drive devices (i.e. internal buses). USB (Univeral Serial Bus)
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and SATA (Serial Advanced Technology Attachment) are used for peripheral
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devices, keyboards and modem devices (i.e external buses).
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