The basic concept of the bus
1. Definition
A bus is a set of public information transmission lines that can be time-shared by multiple components.
Why use a bus?
When there were few external devices in the early computers, most of them used decentralized connection methods, and it was not easy to increase or decrease external devices at any time.
In order to better solve the flexibility problem of the connection between the I/O device and the host computer, the structure of the computer has developed from a decentralized connection to a bus connection.
2. Features
Sharing: Sharing means that multiple components can be connected to the bus, and the information exchanged between each component
can be time-shared through this group of lines.
Time-sharing: Time-sharing means that only one component is allowed to send information to the bus at the same time. If there are
multiple components in the system, they can only send information to the bus in time-sharing.
3. Characteristics
- Mechanical characteristics: size, shape, number of pins, arrangement order
- Electrical characteristics: transmission direction and effective level range
- Functional characteristics: the function of each transmission line (address, data, control)
- Temporal characteristics: the timing relationship of the signal
Classification of the bus
1. According to the data format,
it can be divided into serial bus and parallel bus .
Serial bus:
Advantages: Only one transmission line is needed, the cost is low, and it is widely used in long-distance transmission; when it is used in the computer, it can save wiring space.
Disadvantages: Disassembly and assembly are required when data is sent and received, and the issue of serial to parallel conversion must be considered.
Parallel bus:
Advantages: The logic timing of the bus is relatively simple, and the circuit is easier to implement.
Disadvantages: the number of signal lines is large, taking up more wiring space; the cost of long-distance transmission is high; due to the high operating frequency, there will be serious interference between parallel signal lines, and the requirement for the equal length of each line is higher. Therefore, the working frequency cannot be continuously increased. 2. On-chip bus
according to bus function : On-chip bus is the bus inside the chip. It is the public connection line between the internal registers of the CPU chip and between the registers and the ALU. System bus: The system bus is the bus that connects the various functional components (CPU, main memory, I/O interface) in the computer system. According to the different information content transmitted by the system bus, it can be divided into three categories: data bus, address bus and control bus. 1) The data bus is used to transmit data information between functional components. It is a bidirectional transmission bus, and its number of bits is related to the machine word length and storage word length. 2) The address bus is used to point out the address of the main memory unit or I/O port where the source data or destination data on the data bus is located. It is a one-way transmission bus, and the number of bits of the address bus is related to the size of the main memory address space. 3) The control bus transmits control information, including control commands sent by the CPU and feedback signals returned by the main memory (or peripherals) to the CPU. communication bus
A communication bus is a bus used for information transmission between computer systems or between a computer system and other systems (such as remote communication equipment, test equipment). The communication bus is also called an external bus.
Structure of the system bus
1. Single-bus structure
Structure: CPU, main memory, and I/O devices (via I/O interface) are all connected to a set of buses, allowing I/O devices, between I/O devices and CPU, or I/O Information is exchanged directly between the O device and main memory.
• Advantages: simple structure, low cost, and easy access to new equipment.
• Disadvantages: low bandwidth, heavy load, multiple components can only contend for the only bus, and does not support concurrent transfer operations.
2. Dual-bus structure
Structure: The dual-bus structure has two buses, one is the main memory bus, which is used for data transmission between the CPU, main memory and channels; the other is the I/O bus, which is used for multiple external devices Data transfer between channels.
• Advantages: Separate lower-speed I/O devices from a single bus to separate memory bus and I/O bus.
• Disadvantages: Need to add hardware devices such as channels.
3. Three-bus structure
Structure: The three-bus structure is to use three independent buses between the components of the computer system to form an information path. These three buses are the main memory bus, the I/O bus and the direct memory access DMA. bus.
• Advantages: Improves the performance of I/O devices, making them respond to commands faster and improving system throughput.
• Disadvantage: The system works less efficiently.
Introduction to Quad Bus Architecture
