1. Basic concepts

I/O System

Input / The output is host centric , Transferring information from an external device to a host is called input , The opposite is called output . Input / The main problem solved by the input system is to control the input and output of various forms of information .
I/O Several basic concepts in the system are as follows ：

• Peripheral ： Including the input / Output device and through input / An external storage device that can only be accessed by the output interface .
• Interface ： A logical unit that performs various coordinated work when transmitting data between each peripheral and the host . Coordination includes speed matching during transmission 、 Level and format conversion, etc .
• input device ： Used to enter commands and text into a computer system 、 Data and other information components .
• Output devices ： It is used to output the information in the computer system to the external display of the computer 、 Exchange parts, etc .
• External storage device ： Except for computer memory and CPU Memory other than cache . Hard disk 、 CD is the most basic external storage device .

Generally speaking ,I/O System consists of I/O Software and I/O The hardware consists of two parts ：

• I/O Software ： Including drivers 、 User programs 、 Management procedure 、 Upgrade patches, etc . Usually used I/O Instruction and channel instruction implementation CPU And I/O Information exchange of equipment .
• I/O Hardware ： Including external equipment 、 Equipment controller and interface 、I/O Bus etc. . Control by device controller I/O Specific actions of the equipment , adopt I/O Interface with host （ Bus ） Connected to a .

The control mode

In the input / In the output system , A large amount of data transmission is often required , In the process of transmission, there are various I/O The control mode , The basic control methods mainly include the following 4 Kind of ：

1. Program query method ： from CPU Continuously query through the program I/O Whether the equipment is ready , To control I/O The device exchanges information with the host .
2. Program interrupt mode ： Only in I/O The equipment is ready and sent to CPU Respond only when an interrupt request is made .
3. DMA The way ： Main memory and I/O There is a direct data path between the devices , When main memory and I/O When the device exchanges information , There is no need to call the interrupt service program .
4. Way of access ： A channel control part is set in the system , Each channel is connected with several peripherals , The host is executing I/O On command , Just start the relevant channel , The channel will execute the channel program , To complete the I/O operation .

One way is 1 and 2 It is mainly used for external equipment with low data transmission rate , Method 3 and 4 It is mainly used for equipment with high data transmission rate .

2. Peripheral

External devices are also called peripherals , It is a device other than the host that can exchange information with the computer directly or indirectly . The input device has a keyboard 、 Mouse, etc. , The output device has a display 、 Printers, etc .

3.I/O Interface

I/O Interface （I/O controller ） It is the interface between the host and peripherals , Through the interface, the information exchange between the host and peripherals can be realized . The host and peripherals have their own working characteristics , They have great differences in information form and working speed , The interface is set up to solve these differences .

I/O Function of interface

I/O The main functions of the interface are as follows :

• Realize the communication control between the host and peripherals . Solve the timing matching problem between the host and peripherals , Coordinate the exchange of information between peripherals and hosts at different working speeds , To ensure that the whole computer system can be unified 、 Work in coordination .
• Address decoding and device selection .CPU After sending the address code of the selected peripheral , The interface must decode the address to generate device selection information , Enables the host to exchange information with designated peripherals .
• Realize data buffering .CPU And the speed between peripherals often does not match , To eliminate speed differences , The interface must set the data buffer register , Used for temporary storage of data , To avoid data loss due to inconsistent speeds .
• Conversion of signal format . The level of both peripheral and host 、 There may be differences in data formats , The interface shall provide the conversion function of signal format between computer and peripheral equipment , Such as level conversion 、 and / String or string / And convert 、 Count / Modulo conversion or modulo conversion / Number conversion, etc .
• Transmit control commands and status information .CPU To start a peripheral , Send the start command through the command register in the interface ; When the peripherals are ready , Then the status information is sent back to the status register in the interface , And feedback to CPU.

I/O The basic structure of the interface

Structure diagram is as follows ：

• I/O The interface passes through the host side I/O Bus and memory 、CPU Connected to a . Via data bus , In the data buffer register with memory or CPU Data transfer between registers . At the same time, the status information of the interface and equipment is recorded in the status register , Send information to the status line CPU.CPU Control commands to peripherals are also transmitted through data lines , Usually send it to I/O Interface control register . The status register and the control register are opposite in the transfer direction .
• The address line in the interface is used to give the address to be accessed I/O The address of the register in the interface , It and reading / The write control signal is sent to I/O Control logic unit of interface , The address information is used to select the register to exchange information with the host , A read transmitted through a control line / The write signal confirms whether to read the register or write the register , In addition, the control line will also transmit some arbitration signals and handshake signals .
• Interface I/O The control logic should also be able to decode the command word in the control register , And send the decoded control signal to the peripheral through the peripheral interface control logic , At the same time, send the data of the data buffer register to the peripheral or receive the data from the peripheral to the data buffer register . in addition , It also has the function of collecting peripheral status to status register .
• Data buffer register 、 state / The access operation of control register is completed through corresponding instructions , Such instructions are commonly referred to as I/O Instructions ,I/O Instructions can only be used at the bottom of the operating system kernel I/O Use... In software , Is a privileged instruction .
• It should be noted that , Interface and port are two different concepts . Port refers to the interface circuit that can be read / Write register , Several ports and corresponding control logic can form an interface .

I/O Type of interface

1. According to the way of data transmission , It can be divided into parallel interfaces （ All bits of a byte or word are transmitted simultaneously ） And serial interface （ Send... One by one ）, The interface shall complete the conversion of data format .
   （ The data transmission mode mentioned here refers to the transmission mode on the side of peripherals and interfaces , The interface is on the side of the host , Data is always transmitted in parallel .）
2. Access by host I/O The control mode of equipment can be divided into program query interface 、 Interrupt interface and DMA Interfaces, etc. .
3. According to the flexibility of function selection, it can be divided into programmable interface and non programmable interface .

I/O Port and its addressing

I/O A port is a port that can be used in an interface circuit CPU Direct access registers , There are mainly data ports 、 Status port and control port , Several ports and corresponding control logic circuits form the interface . Usually ,CPU Can read and write to the data port , However, only read operations can be performed on the status port , Only write operations can be performed on the control port .
I/O If the port wants to be able to be CPU visit , Each port must be numbered , Each port corresponds to a port address . And yes I/O There are two addressing modes of ports: unified addressing with memory and independent addressing .

• Unified addressing, also known as memory mapping , It means that I/O The port is used as a unit of memory for address allocation , This way, CPU There is no need to set up a special I/O Instructions , With a unified memory access instruction, you can access I/O port .
• Independent addressing is also known as I/O How to map ,I/O Port address space and main memory address space are two independent address spaces , Therefore, it is impossible to distinguish from the form of address code , Need to set up a special I/O Command to access I/O port .

4.I/O The way

common I/O The way is program query 、 Program interrupt 、DMA etc. .

Program query method

The control of information exchange is completely realized by the host execution program , Set a data buffer register in the program query mode interface （ Data port ） And a device status register （ Status port ）. The main engine is running I/O In operation , Send a signal first , Read the status of the device and decide whether to transfer data or wait according to the next operation of the device .

Program interrupt mode

Program interruption refers to the process of executing the current program by the computer , There are some exceptions or special requests that need to be handled urgently ,CPU Suspend the current procedure , And turn to deal with these exceptions or special requests , After processing CPU It automatically returns to the breakpoint of the current program , Continue with the original procedure .

DMA The way

DMA Mode is a control mode of group information transmission completely by hardware , It has the advantage of program interrupt mode , That is, in the data preparation stage ,CPU Work in parallel with peripherals .DMA Way to open a path between peripherals and memory “ Direct data channel ”, Information is no longer transmitted CPU, To reduce the CPU The cost of transmitting data , Therefore, it is called direct memory access mode . Because data transmission does not go through CPU, There is no need to protect 、 recovery CPU Site and other cumbersome operations .