JVM Part 1 ： Memory and garbage collection – Runtime data area 4 - Program counter

1. Introduce

JVM Program count register in （Program Counter Register） in ,Register The name of comes from CPU The register of , register ** Store field information related to 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 JVM The fastest running storage area .

• stay JVM Specification , Each thread has its own program counter , yes Thread 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 the so-called current approach . 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 （undefned）.

  • Local methods need to call C/C++ Library, so it is undefned

• It's a program Indicator of control flow , Branch 、 loop 、 Jump 、 exception handling 、 Basic functions such as thread recovery rely on this counter .

• Bytecode interpreter When you work, you know Select the next bytecode instruction to be executed by changing the value of this counter .

• It is the only one in Java Nothing is specified in the virtual machine specification outotMemoryError Area of situation .

  • At the same time, it will not exist GC, Just store the address of the next instruction

2. effect

PC Registers are used to store the address to the next instruction , Also about to execute the instruction code . The execution engine reads the next instruction .

3. Code example

Write a simple code

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);

    }
}

Then compile the code into a bytecode file , Let's look at... Again , Find a line number to the left of bytecode , It's actually the instruction address , Used to point to where the current execution is .

    0: bipush        10
    2: istore_1
    3: bipush        20
    5: istore_2
    6: iload_1
    7: iload_2
    8: iadd
    9: istore_3
    10: ldc           #2 // String abc
    12: astore        4
    14: getstatic     #3 // Field java/lang/System.out:Ljava/io/PrintStream;

adopt PC register , We can know where the current program is going

The execution engine finds the operation command through the instruction address in the register , To manipulate the local variable table , Operand stack and direction CPU Send machine instructions

4. Two common problems of registers

4.1 Use PC What's the use of register to store bytecode instruction address ？/ Why use PC Register records 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 and continue .

JVM The bytecode interpreter needs to be changed PC Register value Specify what bytecode instructions should be executed next .

4.2PC Why are registers set to be 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 , In this way, each thread can be independently calculated , 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 .

5. CPU 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 the same time , Each program runs parallel , Running at the same time .

But in On the micro ： 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 .