Andrew S. Tanenbaum (OS) Edition 4 Exercise 2 Question 14 (Page No. 175)

Question

In Fig. $2-12$ the register set is listed as a per-thread rather than a per-process item. Why? After all, the machine has only one set of registers.

Answer 1

The register set is listed as a per-thread rather than a per-process because of the following reasons:

• Multiprogramming threads behave in the same way as the multiprogramming processes. Due to this each thread require its own area for saving the register.
• When a thread stops, the register has the values of the threads which is needed to be saved, just like when the process halts then the register must be saved.

 

Answer 2

Registers store the thread data that the cpu was working with. If a thread wants to start right where it was context switched earlier, it needs that temporary data also. Hence we store it on per thread basis.

Else, if we are okay with repeating the most recent instruction, then a PC register will suffice and rest registers we can remove.