Which one or more of the following need to be saved on a context switch from one thread (T1) of a process to another thread (T2) of the same process?

Threads belonging to the same process share common resources, such as:

• Address space (code, data, and heap segments)

• Open files and I/O devices

• Page table / Page table base register (PTBR)

However, each thread maintains its own private execution state, which includes:

• Program counter (PC): Tracks the next instruction to execute for that thread.

• Stack pointer (SP): Points to the top of the thread's own execution stack.

• General purpose registers: Stores active local computation states.

• State and priority.

When a context switch occurs between two threads $T_1$ and $T_2$ of the same process:

1. The CPU register set—including the Stack pointer, Program counter, and General purpose registers—must be saved into the Thread Control Block (TCB) of $T_1$ and restored from the TCB of $T_2$.

2. The Page table base register does not need to be saved or changed because both threads share the identical address space and page tables.

Hence, the correct options are B, C, and D.