Bus (distributed connection): An information transmission line that connects multiple components, and is a transmission medium shared by all components.
Scattered connections: Use separate wires between components
2. Information transmission on the bus
Transmission of information on the bus: serial and parallel
serial
parallel
3. Scattered Connection Graph
Distributed connection 1
Distributed connection 2
4. Note
(1) When two or more components send information to the bus at the same time, it will cause signal conflict and the transmission will be invalid. At a time, only one component is allowed to send information to the bus.
(2) Multiple components can receive the same information from the bus at the same time.
(3) The bus is actually composed of many transmission lines or paths.
According to data transmission mode: parallel transmission bus, serial transmission bus
According to the scope of use computer bus, measurement and control bus, network communication bus
Classified by connecting parts: on-chip bus, system bus, communication bus
1. On-chip bus
The on-chip bus refers to the bus inside the chip . For example, in the CPU chip, registers and registers, registers and ALU are all connected by the on-chip bus.
2. System bus ( board-level bus or inter-board bus )
According to the different information transmitted by the system bus, it can be divided into three categories: data bus, address bus and control bus
2.1 Data bus
The data bus is used to transmit data information between functional components. It is a two-way transmission bus . The number of bits is related to the length of the machine word and the length of the storage word . Generally, it is 16 bits or 32 bits. The number of bits of the data bus is called the data bus width , which is an important parameter to measure the performance of the system.
If the width of the data bus is 8 bits and the instruction word length is 16 bits, then the CPU must access the main memory twice during the instruction fetch phase.
2.2 Address bus
The address bus is mainly used to indicate the address of the source data or destination data on the data bus in the main memory unit or the address of the I/O device.
The code on the address bus is used to indicate the address of the storage unit or 1/0 port that the CPU wants to access, and is output by the CPU for one-way transmission.
Related to storage address, I/O address
The number of bits of the address line is related to the number of storage units, if there are 20 address lines, the corresponding number of storage units is 2^20
2.3 Control bus
Since the data bus and address bus are shared by all the components on the bus, how to enable each component to occupy the right to use the bus at different times depends on the control bus, so the control bus is used to send various control signals transmission line.
Control signals are both output and input .
Output: memory read, memory write bus enable, interrupt acknowledge
Input: interrupt request, bus request
Common control signals are as follows:
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clock
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Reset
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Bus request
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The bus allows
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Interrupt request
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Interrupt response
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Memory write
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Memory read
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I/O copy
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I/O notes
3. Communication bus
This type of bus is used for communication between computer systems or between computer systems and other systems (such as control instruments, mobile communications, etc.) .
According to the transmission mode, it can be divided into two types: serial communication and parallel communication.
4. Bus characteristics and performance indicators
1. Physical implementation of the bus
2. Bus characteristics
2.1 Mechanical properties
Mechanical characteristics refer to some performances of the bus in terms of mechanical connection, such as the standards used by plugs and sockets, their geometric dimensions , shapes, number of pins and the order of arrangement, reliable contact at the joints , etc.
2.2 Electrical characteristics
Electrical characteristics refer to the transmission direction and effective level range of signals on each transmission line of the bus . It is usually stipulated that the signal sent by the CPU is called the output signal, and the signal sent to the CPU is called the input signal.
2.3 Features
Functional characteristics refer to the function of each transmission line in the bus . For example, the address bus is used to indicate the address code; the data bus is used to transmit data ; the control bus sends control signals , both from the CPU, such as memory read/write, 1 /0 device read/write, there are also 1/0 devices sent to the CPU, such as interrupt requests, DMA requests, etc. It can be seen that the functions of each line are different.
2.4 Temporal characteristics
The time characteristic refers to when any line in the bus is valid. Various signals on each bus have an effective timing relationship with each other , so the time characteristics can generally be described by a signal timing diagram.
3. Performance indicators of the bus
①Bus width: usually refers to the number of data buses, represented by bit (bit), such as 8 bits, 16 bits, 32 bits, 64 bits (ie 8, 16, 32, 64). ②Bus bandwidth: Bus bandwidth can be understood as the data transmission rate of the bus, that is, the number of bits of data transmitted on the bus per unit time, usually measured by the number of bytes of information transmitted per second, and the unit can be MBps (megabytes per second) express. For example, if the operating frequency of the bus is 33 MHz and the bus width is 32 (4 B), then the bus bandwidth is 33x(32+8)=132 MBps ③ Clock synchronous/asynchronous: A bus whose data on the bus works synchronously with the clock is called synchronous A bus that works asynchronously with a clock is called an asynchronous bus . ④Bus multiplexing: two signals are transmitted on one signal line in time division. For example, usually the address bus and the data bus are two physically separate buses, the address bus transmits address codes, and the data bus transmits data information. In order to improve the utilization rate of the bus and optimize the design, the address bus and the data bus share a set of physical lines, and the address signals and data signals are time-divisionally transmitted on this set of physical lines, which is the multiplexing of the bus. ⑤ Number of signal lines: the sum of the three bus numbers of address bus, data bus and control bus. ⑥Bus control mode: including burst work, automatic configuration, arbitration mode, logic mode, counting mode, etc. ⑦ Other indicators: such as load capacity, power supply voltage (whether 5V or 3.3 V is used), whether the bus width can be expanded, etc.
