04 program counter (PC register)

1. PC Register Introduce

https://docs.oracle.com/javase/specs/jvms/se8/html/index.html

1.1 PC Register Introduce

• JVM Program count register in （Program Counter
  Register）,Register The name of comes from CPU The register of , Registers store field information about instructions .CPU Only loading data into registers can run .
• here , It's not a physical register in a broad sense , Maybe translate it into PC Counter （ Or instruction counter ） It will be more appropriate （ Also known as program hook ）, And it's not easy to cause some unnecessary misunderstandings .JVM Medium PC Registers are for physics PC An abstract simulation of registers .
• It's a small piece of memory , I can almost ignore it . It's also the fastest running storage area .
• stay JVM Specification , Each thread has its own program counter , Threads are private , The life cycle is consistent with the life cycle of the thread .
• There is only one method in a thread at any time , It's called The current method . The program counter stores what the current thread is executing Java Methodical JVM Instruction address ; perhaps , If it's execution native Method , The value is not specified （undefined）.
• It is an indicator of the flow of program control , Branch 、 loop 、 Jump 、 exception handling 、 Basic functions such as thread recovery rely on this counter
• The bytecode interpreter works by changing the value of this counter to select the next bytecode instruction to execute
• It is the only one in Java Nothing is specified in the virtual machine specification OutofMemoryError Area of situation
• Virtual machine stack 、 Local method area 、PC Register not GC; But the virtual machine stack and local method area may appear OOM
• The heap and method areas already exist GC There will be OOM

1.2 effect

PC Registers are used to store the address to the next instruction , That is, the instruction code to be executed . The execution engine reads the next instruction .

2. Illustrate with examples

public class PCRegisterTest {
    
    public static void main(String[] args) {
    
        int i = 10;
        int j = 20;
        int k = i+j;

        String s = "abc";
        System.out.println(i);
        System.out.println(k);
    }
}

Decompile instructions ：javap -v PCRegisterTest.class

PC Registers store instruction addresses , The execution engine is based on PC The address in the register finds the operation instruction , And execute the instruction

3. Two common questions

Use PC register What's the use of storing bytecode instruction addresses ？
or
Why use PC register To record the execution address of the current thread ？

• because CPU You need to switch threads all the time , Now, after switching back , You need to know where to start after switching back
• JVM The bytecode interpreter needs to be changed PC Register to determine what kind of bytecode instruction should be executed next
  

PC Why is the register set to x Thread private ?

• We all know that the so-called multithreading will only execute one of the methods in a specific period of time ,CPU I'll keep switching tasks , This inevitably leads to frequent interruptions or recoveries , How to make sure there is no difference ？
• In order to accurately record the current bytecode instruction address that each thread is executing , The best way, of course, is to assign one to each thread PC register , thus , Independent calculations can be performed between threads , So that there will be no mutual interference
• because CPU Time slice wheel limit , Many threads are executing concurrently , Any definite moment , A core in a processor or multi-core processor , Only one instruction in a thread will be executed
• This inevitably leads to frequent interruptions or recoveries , How to make sure there is no difference ？ After each thread is created , Will produce their own program counter and stack frame , Program counters do not affect each other among threads

4. Time slice

• CPU Time slice is CPU Time allocated to each program , Each thread is assigned a time period , It's called a time slice
• On the macro ： We can open multiple applications at once , Each program runs parallel , Running at the same time
• But at the micro level ： Since there is only one CPU, Only part of the program's requirements can be processed at a time , How to deal with fairness , One way is to introduce time slices , Each program is executed in turn