Question

In a multithreaded environment, there is still a single process control block, but now there are also separate control blocks for each thread. The thread control block contains register values.

i) What other information is stored in the thread control block?

ii) Why are the register values stored in the thread control blocks as opposed to the process control block given that the machine has a single set of registers?

Answer 1

i) The thread control block contains the thread's Register Values, other than register values the TCB also contains:

1. Thread Identifier: It is a unique ID assigned to every new thread.
2. Stack Pointer.
3. Program Counter: Stores the pointer to current program instruction.
4. State of the thread (running, ready, waiting, start, done).
5. Pointer to the PCB of parent process of the thread.

ii) Threads of the same process can run concurrently, this is made feasible because each thread have its own context. So in order to preserve this context the thread have to maintain its own resources that is independent of other threads. This information is stored in Register set and the stack.

To be more precise, the threads of the same process running o the same CPU actually share the same registers. The OS switches context so frequently and instantaneously that it gives an illusion that the threads are running parallel to each other. When context switching happens the registers are backed up and the back up that was made of the other thread is restored. Hence, the system behaves as if threads were using separate registers, even though they are not.