The primary pathway between the CPU and [memory](Memory/Basics.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.
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.
Transfers secondary cache (L2 cache) data at faster speeds, allowing more efficient CPU operations
## Bus standards
### Parallel and serial transmission types
There is a distinction between buses that use serial and buses that use parallel transmission. They are distinguished by the way in which data in the form of bits is transferred from one point to another, along the bus.
Serial Transmission is the type of transmission in which a single communication link is used to transfer the data from an end to another. On other hand Parallel Transmission is the transmission in which multiple parallel links are used that transmit each bit of data simultaneously.
* Serial buses operate at greater [latency](/Hardware/Bus.md#latency) than parallel buses
## Latency
Latency means _delay_: the delay from the time the data is requested until the time it arrives. In the context of a microprocesser and buses a key locus of latency would be between the time a request takes to travel accross the system bus from the CPU to the memory.
In general, bandwidth is a measure of the capacity for a communications channel to transmit data. A bus is a channel between two components thus we can talk about the bandwidth of buses. The speed and efficiency of a bus is a function of its bandwidth, although other factors such as its length and proximity to the source and receiver are also important.
Bandwidth can be calculated in terms of how many bits can be transferred per second. The wider the bus, the more bits that can be transferred. If we have a bus that can run at 66MHz and transfer 64 bits, 64 bits is 8Mb thus the bandwidth is 8 x 66 = 528Mb.
SCSI (Small Computer System Interface) and ATA (Advanced Technology Attachment) are used for hard drive devices (i.e. internal buses). USB (Univeral Serial Bus) and SATA (Serial Advanced Technology Attachment) are used for peripheral devices, keyboards and modem devices (i.e external buses).
