JVM | runtime data area; Program counter (PC register);

Run time data area

• Method area ： After the class is loaded, it is placed in the method area
• Program counter
• Native Method Stack
• Pile up
• Virtual machine stack

The relationship between data area and thread

Private to each individual thread ： Program counter , Stack , Local stack
Sharing between threads ： Pile up , Out of heap memory （ Method area （ Permanent generation or meta space ） Code cache ）

Threads

A thread is a running unit in a program ,JVM Allows an application to have multiple threads executing in parallel .
Once the local thread is initialized successfully , It's going to call Java In thread run() Method .

Program counter （PC register ）

• It's a small memory space , It's also the fastest running storage area .
• A thread has only one method executing at any one time , It's called “ The current method ”.
• The program counter stores what the current thread is executing Java Methodical JVM Instruction address .

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 .

Use examples

The program counter stores the address of the next instruction , The execution engine looks for the corresponding statement according to this address , To operate .

common problem

1. Use PC What's the use of registers storing bytecode instruction addresses ？
answer ：CPU You need to constantly switch threads , After switching back, you have to know where to start .
2. Why use PC Register records the execution address of the current thread ？
answer ：JVM The bytecode interpreter needs to be changed PC Register to determine what kind of bytecode instruction should be executed next .

The reason why threads are private

So called multithreading , The method of only one thread will be executed in a specific time period ,CPU I'll keep switching tasks , This inevitably leads to frequent interruptions or recoveries .
In order to accurately record the current bytecode instruction address that each thread is executing , The best way is to assign one to each thread PC register .
In this way, each thread can perform independent calculation .