Bus overview
Bus ： A group of common information transmission lines that can share time for multiple components . Sharing means that multiple components can be attached to the bus , The information exchanged between the various components can be transmitted through this group of common lines ; Time sharing means that only one component can transmit information on the bus at the same time .
Classification of bus ：
Classification by function
1> On chip bus ： Bus in chip , It is CPU Between registers in chip , Register and ALU Public connection between Line . The on-chip bus is inside the chip , It's usually invisible .
2> system bus ： Functional components in computer system (CPU, Main memory ,I/O Interface ) The buses are connected to each other , The system bus is also known as the internal bus
Line , Is the main component of the computer . System bus can be divided into data bus according to different information , Address bus and control bus .
3> Communication bus ( External bus )： Used between computer systems or between computer systems and other systems ( Telecommunication equipment , Test equipment ) Information between Transmitted bus .
Sort by number of data lines
1> Parallel bus ： A bus with multiple bidirectional data lines , It can realize multi bit simultaneous transmission of a data . Parallel bus has data transmission
Advantages of high rate , However, the transmission characteristics of each data line cannot be completely consistent , When the data line is long , The delay of data bits arriving at the receiving end may be inconsistent , Will cause transmission error .
2> Serial bus ： A bus with only one bidirectional or two unidirectional data lines , Serial bus can realize a data bit according to one
Fixed speed and sequence transmission . Because the serial transmission of data by bit does not require high transmission characteristics of data line , In the case of long-distance connection, data can still be effectively transmitted , So the advantage of serial bus is long distance communication .
Bus structure ：
Single bus structure ： There is only one system bus , All components are connected through the system bus .
Dual bus structure ： On the basis of single bus structure, a dedicated CPU The data transfer path between and main memory .
Three bus structure ： Add another one on the basis of dual bus I/O Bus .
Performance index of bus ：
1> Bus width ： Refers to the number of lines in the bus , It determines the physical space and cost of the bus . The most direct impact on bus width is the address line and data
Number of lines , The width of the address line indicates the address space range of the memory that the bus can directly access , The width of the data indicates the number of bits of data that can be exchanged when accessing a memory or peripheral device .
2> Bus bandwidth ： Maximum transfer rate of the bus , This is the number of bytes transferred per second .
3> Bus load ： Refers to the maximum number of devices connected to the bus . The load capacity of most buses is limited .
4> Bus multiplexing ： The same signal line on the bus is used to transmit different signals in different time periods , For example, the address bus and the data bus share a set of signal lines . The purpose of this approach is to reduce the number of buses , Improve bus utilization .
5> Bus burst transmission ： In a bus cycle, multiple data with consecutive storage addresses can be transmitted .
Bus arbitration ：
The bus control mechanism is equipped with bus optimization and arbitration control logic , That is, according to a certain priority to determine which component first uses the bus , Only components that have access to the bus , To start data transfer .
Chain query mode ：
I/O Interface through BR Request to bus , Only in BS Before the signal is established ,BR Can be responded by the bus controller , And send it out BG Answer the signal .BG The signal passes through each component in series , If a part itself has no bus request , The signal is passed on to the next component , Until the corresponding part is found . According to the priority of query mode , In the query chain, the device closer to the bus controller has the highest priority .
Counter timing query mode ：
The bus is receiving BR After request , stay BS by 0 In the case of , Let the counter start counting , Periodically query each component to determine who made the request . When the count value on the query line is consistent with the part number of the request , This component makes BS Set as 1, Access to the bus . This counting can be done from 0 start , You can also start at the midpoint . The initial value of the counter can be set by the program , Increase the flexibility of the system .
Independent request mode ：
Each component sharing the bus has a pair of control wires . When a part requests to use the bus , It will be sent out BR, There is a queuing circuit in the bus controller , According to a certain priority, it determines which part to respond to the request first .
Bus timing control ：
Synchronous timing mode ： The system uses a unified clock signal to coordinate the transmission timing relationship between the sender and the receiver . Clocks produce equal time intervals , Each time interval constitutes a bus cycle . In a bus cycle , The sender and receiver can transmit data once . Because the operation is carried out within the specified time period , therefore , The sender does not have to wait for the receiver to respond , When this period is over , Automatically proceed to the next operation . The clock frequency in the synchronous mode must be able to meet the needs of the longest delay and slowest interface on the bus , So the efficiency is low ; It is also impossible to know whether the visited peripheral is responding , So the reliability is low .
Asynchronous timing mode ： There is no fixed time interval , Depending on the transmission, both sides restrict each other " handshake " Signal to achieve timing control . The main equipment proposes to exchange information " request " signal , Through the interface to the slave device , Then it happens from the slave device to the master device " answer " signal ." request " reach " answer " Is determined by the actual time of the operating system , It has strong flexibility . Asynchronous control can ensure the reliable exchange of information between two parts or equipment with great difference in working speed , Automatic completion time coordination , But the control is more complex than the synchronous mode , The cost will also be higher .
Asynchronous according to " request " and " answer " Whether interlocking is divided into 3 Situation in China ：
1. Non interlock ：" request " and " answer " All signals have a certain time width ," request " The end of the signal and " answer " The end of the signal is not interlocked .
2. Semi interlock ：" request " The cancellation of the signal depends on the response received " signal ", and " answer " The cancellation of the signal depends on the device .
3." request " The cancellation of the signal depends on " answer " The arrival of the signal , and " request " The cancellation of the signal leads to an answer " signal " Revocation of .
External bus ： A bus connecting computer systems , Standard interface plug is usually used , Its structure and communication protocol are also standard .
1. Serial port and parallel port
1> Serial port ： Also known as communication port and COM mouth , It is mainly used for equipment that needs two-way communication with the system . The traditional serial port is asynchronous transmission , And oriented characters . Each character sent over a serial connection is formed by a standard start stop signal , Each character is preceded by a separate binary "0"( Start bit ), Subsequent 8 Binary numbers will make up a byte of data , There are 1 Or 2 Binary "1"( Stop bit ). At the receiving end of communication , Character recognition is based on the start stop signal .
2> Parallel port ： Yes 8 Data lines , You can transfer data through this 8 Data lines occur at the same time, containing all units of one byte of data .
2.USB Interface ： A peripheral bus standard , With plug and play function .USB A dedicated port is no longer needed , It's also down I/O Use of cards , It greatly reduces important system resources .
reference material ：<< Principles of Computer Organization >> Jiang Benshan
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