What are the main types of internal buses in computer systems?

The internal buses of computer systems can be mainly divided into 5 categories: 1. Data bus, which transmits data that needs to be processed or stored back and forth between the CPU and RAM; 2. Address bus, which is used to specify the location in RAM. The address of the data stored in it; 3. Control bus, which transmits signals from the microprocessor control unit to peripheral devices; 4. Expansion bus, which is a bus for data communication between external devices and the computer host, such as ISA bus and PCI bus. ; 5. Local bus, replacing the expansion bus for higher-speed data transmission.

The operating environment of this tutorial: Windows 7 system, Dell G3 computer.

Bus (Bus) is a public communication trunk for transmitting information between various functional components of the computer. It is a transmission harness composed of wires.

The bus is an internal structure. It is a common channel for CPU, memory, input and output devices to transmit information. The various components of the host are connected through the bus, and the external devices are connected to the bus through the corresponding interface circuits. , thus forming a computer hardware system.

In a computer system, the common channel for transmitting information between various components is called a bus. Microcomputers use a bus structure to connect various functional components.

Types of internal buses of computer systems:

Bus can be divided into five types according to their functions and specifications:

• Data Bus: Transfers data that needs to be processed or stored back and forth between the CPU and RAM.

• Address Bus: Used to specify the address of data stored in RAM (Random Access Memory).

• Control Bus: Transmits signals from the microprocessor control unit (Control Unit) to peripheral devices.

• Expansion Bus: A bus for data communication between external devices and the computer host, such as ISA bus and PCI bus.

• Local Bus: An expansion bus that replaces higher-speed data transmission.

The data bus DB (Data Bus), address bus AB (Address Bus) and control bus CB (Control Bus) are also collectively referred to as the system bus, which is commonly known as bus.

Data bus DB

"Data bus DB" is used to transmit data information. The data bus is a bidirectional three-state bus, that is, it can transmit data from the CPU to other components such as memory or I/O interfaces, and can also transmit data from other components to the CPU. The number of bits in the data bus is an important indicator of a microcomputer and is usually consistent with the word length of the microprocessor. For example, the word length of the Intel 8086 microprocessor is 16 bits, and its data bus width is also 16 bits. It should be pointed out that the meaning of data is broad. It can be real data, instruction code or status information, and sometimes even control information. Therefore, in actual work, what is transmitted on the data bus is not necessarily Just real data.

Common data buses are ISA (ISA bus), EISA, VESA, PCI, etc.

Address bus AB

"Address bus AB" is specially used to transmit addresses. Since addresses can only be transmitted from the CPU to external memory or I/O ports, So the address bus is always unidirectional and tri-state, unlike the data bus. The number of bits in the address bus determines the size of the memory space that the CPU can directly address. For example, the address bus of an 8-bit microcomputer is 16 bits, so its maximum addressable space is 2^16=64KB. A 16-bit microcomputer (x-bit processing The address bus refers to the number of bits [1, 0] that the microprocessor can process in one clock cycle, that is, the word size) is 20 bits, and its addressable space is 2^20=1MB. Generally speaking, if the address bus is n bits, the addressable space is 2^n bytes.

Control bus CB

"Control bus CB" is used to transmit control signals and timing signals. Among the control signals, some are sent by the microprocessor to the memory and I/O interface circuits, such as read/write signals, chip select signals, interrupt response signals, etc.; some are fed back to the CPU by other components, such as: interrupt application signals, reset signals, bus request signals, device ready signals, etc. Therefore, the transmission direction of the control bus is determined by the specific control signal, (information) is generally bidirectional, and the number of bits on the control bus is determined according to the actual control needs of the system. In fact, the specific situation of the control bus mainly depends on the CPU.

Bus characteristics

Because the bus is a set of signal lines connecting various components. Information is represented by signals on the signal line, and how operations are implemented can be agreed upon by agreeing on the sequence of different signals. The characteristics of the bus are as follows

(1) Physical characteristics: Physical characteristics are also called mechanical characteristics, which refer to some characteristics of the components on the bus when they are physically connected, such as the geometric size, shape, and pins of plugs and sockets. Number and arrangement order, etc.

(2) Functional characteristics: Functional characteristics refer to the function of each signal line, such as the address bus used to represent the address code. The data bus is used to represent transmitted data, and the control bus represents the commands, status, etc. operated on the bus.

(3) Electrical characteristics: Electrical characteristics refer to the signal direction of each signal line and the effective level range of the signal. Usually, the signal sent by the main device (such as CPU) is called the output signal (OUT) , the signal sent to the main device is called the input signal (IN). Usually data signals and address signals define high level as logic 1 and low level as logic 0. There is no conventional convention for control signals. For example, WE means low level is valid and Ready means high level is valid. There are no unified regulations on the level ranges of high and low levels of different buses, and they are usually consistent with TTL.

(4) Time characteristics: Time characteristics, also known as logic characteristics, refer to when the signal on each signal line is valid during the bus operation. Through this agreement on the timing relationship of the signal validity, the bus is ensured correct operation. In order to improve the scalability of computers and the versatility of components and equipment, in addition to the on-chip bus, each component or equipment is connected to the bus in a standardized form, and information transmission on the bus is implemented in a standardized manner. These standardized connection forms and operation methods of the bus are collectively called bus standards. Such as ISA, PCI, USB bus standards, etc. Correspondingly, buses using these standards are ISA bus, PCI bus, USB bus, etc.

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