OS interrupt mechanism and interrupt handler

Interrupt plays a particularly important role in the operating system , It is the basis for the realization of multiprogramming , Without interruption , It is impossible to implement multiprogramming , because Switching between processes is accomplished through interrupts . On the other hand , Interruption is also the basis of device management , In order to improve the utilization of processor and realize CPU And I/O The device executes in parallel , There must also be support for interruptions . The interrupt handler is I / O The lowest layer in the system , It is the whole I / O Foundation of the system .

Interrupt profile

1． Break and fall into

1） interrupt

Interruption means CPU Yes I / O A response to an interrupt signal sent by a device . CPU Pause the running program , Retain CPU After environment , Automatically go to execute the I / O Device interrupt handler . After the execution , Back to the breakpoint , Continue with the original procedure . I / O The device can be a character device , It can also be a piece of equipment 、 Communication equipment, etc . Because the interrupt is caused by an external device , So it is also called external interrupt .

2） fall into

There's another one from CPU Caused by internal events interrupt , For example, the process has overflow or underflow in the operation , Another example is a program error , Such as illegal instructions 、 The address is out of bounds , And power failure . This kind of interrupt is usually called Internal interruption or trapping （ trap ). Like an interrupt , If the system finds a trap event , CPU The executing program will also be suspended , Go to execute the handler of the trapped event . The main difference between interrupt and trap is the source of the signal , That is, from CPU external , still CPU Inside .

2． Interrupt vector table and interrupt priority

1） Interrupt vector table

For the convenience of handling , Usually, each device is equipped with a corresponding interrupt handler , And put the entry address of the program in an entry of the interrupt vector table , And specify an interrupt number for the interrupt request of each device , It directly corresponds to an entry in the interrupt vector table . When I / O When the device sends an interrupt request signal , The interrupt controller determines the interrupt number of the request , Look up the interrupt vector table according to the interrupt number of the device , Get the entry address of the device interrupt handler , In this way, you can go to the interrupt handler to execute .

2） Interrupt priority

But the reality is ： There are often multiple interrupt sources , Each interrupt source has different urgency to service requirements , for example , The interrupt request of the keyboard terminal is less urgent than that of the printer , And the urgency of the printer interrupt request is not as good as that of the disk . So , The system needs to specify different priorities for them .

3． How to deal with multiple interrupt sources

For the case of multiple interrupt sources , When the processor is processing an interrupt , Here comes a new interrupt request , How to deal with it . for example , When the system is processing a printer interrupt , A higher priority disk interrupt signal has been received . In this case , There are two ways to deal with ：

• shielding （ prohibit ） interrupt
• Nested interrupts

1） shielding （ prohibit ） interrupt

When the processor is processing an interrupt , All interrupts will be masked , That is, the processor requests any new interrupt , Are temporarily ignored , And let them wait . Until the processor has finished processing this interrupt , The processor then checks whether an interrupt has occurred . If you have any , Then deal with the new interrupt , If there is no , Then return to the interrupted program . In this method , All interrupts will be processed sequentially . Its advantage is simplicity , But it cannot be used for interrupt requests that require high real-time performance . chart 6-9( a ） It shows a case when multiple interrupt sequence processing is prohibited .

2） Nested interrupts

In the system with interrupt priority set , Priority control is usually carried out according to such rules ：

• (1） When there are multiple interrupt requests with different priorities at the same time , CPU Give priority to the interrupt request with the highest priority ;
• (2） High priority interrupt requests can preempt the processor of running low priority interrupts , This method is similar to priority based preemptive process scheduling . for example , The processor is processing a printer interrupt , When a disk outage comes , You can pause the processing of printer interrupts , Turn to disk interrupt . If the new one is keyboard interrupt , Because its priority is lower than that of the printer , So the processor continues to process the printer interrupt . chart 6-9( b ） Shows the case when multiple interrupts are possible .

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Interrupt handler

When a process requests I / O In operation , The process will be suspended , until I / O Equipment complete I/O After the operation , The device controller sends a message to CPU Send an interrupt request , CPU After responding, it turns to the interrupt handler , The interrupt handler executes the corresponding processing , After processing, remove the blocking state of the corresponding process .
The process of interrupt handler can be divided into the following steps ：

(1） Determine whether there are unresponsive interrupt signals

Whenever the device completes a character （ Words or data blocks ） Read in of （ Or output ）, The device controller sends an interrupt request signal to the processor . The request processor transfers the data read by the device to the buffer in the memory （ Read in ）, Or request the data to be output by the processor （ Output ） To the device controller . Every time the program finishes executing the current instruction , The processor should test whether there is any unresponsive interrupt signal . If there is no , Continue with next instruction . If you have any , Stop the execution of the original process , Prepare to transfer to execute interrupt handler , Prepare for transferring control of the processor to the interrupt handler .

(2） Protection of interrupted processes CPU Environmental Science

Before transferring control to the interrupt handler , You need to protect the interrupted process first CPU Environmental Science , So that the operation can be resumed later . The first thing to save is , Information needed to recover from the interruption site to the current process operation . The processor status word is usually automatically set by the hardware （ PSW ） And stored in the program counter （ PC ) The address of the next instruction in is saved in the interrupt reserve （ Stack ） in . then , Put the interrupted process CPU Site information , About to include all CPU The register of （ Such as general register 、 Segment register, etc ） The contents are pushed into the interrupt stack . Because these registers may be used in interrupt processing . chart 6－10 A simple schematic diagram of the protection interruption site is given . The program is instructed in N Position is interrupted , The contents in the program counter are N +1, The contents of all registers are kept on the stack .

(3） Go to the corresponding device handler

The processor tests each interrupt source , To determine the cause of this interruption I / O equipment , And send a confirmation signal to the equipment providing the interrupt signal . After the device receives the confirmation signal , Immediately cancel the interrupt request signal it sends . then , Load the entry address of the corresponding device interrupt handler into the program counter . such , When the processor is running , You can automatically turn to the interrupt handler .

(4） Interrupt handling

For different devices , There are different interrupt handlers . The program first reads the device status from the device controller , To judge whether this interrupt is a normal completion interrupt or an abnormal end interrupt . If the former , Interrupt the program and do the end processing . If this is the reading operation of the character device , The interrupt from the input device indicates that the device has read a character （ word ） The data of , And has been put into the data register . At this time, the interrupt processing shall transmit the data to CPU , Then store it in the buffer , And modify the corresponding buffer pointer , Make it point to the next memory unit . If there are orders , New commands can be sent to the controller , Carry out a new round of data transmission . If it is an abnormal end interrupt , Then deal with it according to the cause of the abnormality .

(5） recovery CPU And exit the interrupt

When the interrupt processing is completed , Recovery required CPU The scene of , Exit interrupt . however , Whether to return to the interrupted process at this moment , It depends on two factors ：

• ① Whether this interrupt adopts shielding （ prohibit ） Interrupt mode , if , Will return to the interrupted process .
• ② Interrupt nesting is adopted , If there is no higher priority interrupt request I / O , After the interrupt is complete , The interrupted process will still be returned ; conversely , The system will process higher priority interrupt requests .

If you want to return to the interrupted process , The field information of the interrupted process stored in the interrupt stack can be taken out , And load it into the corresponding register , It includes the address of the next instruction executed by the program N +1、 Processor status word PSW , And the contents of general registers and segment registers . such , When the processor executes this program again , From N +1 Start at , Finally return to the interrupted program .
The interrupt processing flow is shown in the figure 6-11 Shown .

I / O After the operation is completed , The driver must check this I / O Whether there is an error in the operation , And report to the upper software , Finally, report this to the caller I/O Implementation of . In addition to the above paragraph 4 Out of step , Other steps are for all I / O The equipment is the same , So for some operating system , for example UNIX System , Is to put these common parts together , Form interrupt master control program . Whenever interrupt processing is required , You must first enter the interrupt master program . And for the first 4 Step , Then different equipment interrupt processing procedures must be used for different equipment to continue execution .

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