Chapter VI bus

Bus Overview

Basic concepts of bus

• Decentralized connection ： Separate wires are used between components
• Bus connection ： Connect the components to a set of public information transmission lines

A bus is a group of components that can be divided into several parts time-sharing share The public information transmission line

time-sharing ： It means that only one component is allowed to send information to the bus at the same time
share ： It refers to that multiple components can be attached to the bus , The information exchanged between various components can be shared time-sharing through this group of lines , Multiple components can receive the same information from the bus at the same time

Bus devices are divided into Main equipment and Slave device

Bus characteristics

1. mechanical properties ( Size 、 shape )
2. Electrical characteristics ( Transmission direction and effective level range )
3. features ( The function of each transmission line )
4. Time characteristics ( Relationship between signal and timing )

Bus classification

Structure of system bus

structure	form	advantage	shortcoming
Single bus structure	A set of buses	Simple structure , The cost is low , Easy access to new devices	Low bandwidth 、 Heavy load , Multiple components can only compete for a unique bus , And does not support concurrent transfer operations
Dual bus structure	Main memory bus + I/O Bus [1]	It's going to slow down I/O The device is separated from the single bus , The memory bus and I/O Bus separation	Need to increase the passageway Etc
Three bus structure	Main memory bus + I/O Bus + Direct memory access (DMA) Bus [2]	Improved I/O The performance of the equipment , Make it respond to commands faster , Improve system throughput	The working efficiency of the system is low
Four bus structure *	It can be understood as three buses + Bridge

[1] The main memory bus is used in CPU、 Transfer data between main memory and channel ;I/O The bus is used to transfer data between multiple external devices and channels
[2] The main memory bus is used in CPU And memory 、 Data and control information ;I/O The bus is used in CPU Communication between various peripherals ;DMA The bus is used to transfer data directly between memory and high-speed peripherals

Common bus standards ^*

Performance index of bus

• Bus transmission cycle ： Time required for one bus operation
• Bus clock cycle ： Machine clock cycle
• Bus operating frequency ： The reciprocal of the bus cycle
• Bus clock frequency ： The reciprocal of the clock period
• Bus width ： Number of data buses
• Bus bandwidth ： Bus operating frequency × （ Bus width * 8）
• Bus multiplexing
• Number of signal lines ： Address bus + data bus + Control bus

Bus transactions and timing

Bus transactions

Bus timing refers to the control of the time cooperation relationship in the process of data exchange between the two sides

The sequence of operations from the request bus to the completion bus is called Bus transactions

Synchronous timing mode

Asynchronous timing mode