JVM memory model: PC program counters

JVM Program counter in （Program Counter Register） in ,Register The name of comes from CPU The register of , Registers store instruction related thread information , there PC Not a generalized physical register .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 .

 PC Register Introduce ：

* Memory space is very small , I can almost ignore it , 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 declaration cycle of the thread .

* There is only one method in a thread at any time , That is, the current method . The program counter stores what the current thread is executing Java Methodical JVM Instruction assignment . If executed native Method , It is undefined

*PC Register It's 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 .

* No, OOM and GC.

Why? PC Registers store instruction addresses ？

CPU You need to switch threads all the time , Now, after switching back , You need to know where to start and continue .

JVM Bytecode interpreter of passed PC The counter knows what bytecode instruction should be executed next .

Why? PC Registers are thread private ？

CPU Keep switching tasks when multithreading , This will cause interruption or recovery , In order to be able to prepare to record the current bytecode instruction address of each thread , The best way is to allocate each thread should PC register .