Context Switch

A context switch is the process of saving the current state of a process and restoring the state of another process. It occurs when the operating system decides to switch the execution from one process to another. This allows multiple processes to share the CPU and execute concurrently.

What needs to be saved on a context switch?

During a context switch, several components of the process need to be saved so that the state of the process can be restored later. These components include:

1. General Purpose Registers: General purpose registers store the values of variables and intermediate results during the execution of a process. These include registers like AX, BX, CX, etc. The values in these registers must be saved during a context switch to ensure that the process can resume execution from where it left off.

2. Program Counter: The program counter (PC) holds the memory address of the next instruction to be executed. It keeps track of the current position in the program. Saving the program counter is crucial during a context switch as it allows the process to resume execution from the correct instruction.

3. Translation Lookaside Buffer: The Translation Lookaside Buffer (TLB) is a cache used for storing recently accessed page table entries. It is a hardware component that helps in the translation of virtual addresses to physical addresses. The TLB is specific to each process and does not need to be saved during a context switch. It can be cleared or flushed as the new process starts execution.

Why Translation Lookaside Buffer (TLB) does not need to be saved?

The TLB is a hardware cache that is specific to each process. It is used to speed up the translation of virtual addresses to physical addresses by caching frequently accessed page table entries. However, the contents of the TLB are not part of the process's state that needs to be saved during a context switch. Here's why:

1. TLB is not part of the process's address space: The TLB is a hardware cache managed by the CPU, and it is not part of the process's address space. It does not store any of the process's data or instructions. Instead, it caches page table entries for faster address translation.

2. TLB entries are specific to virtual addresses: The TLB stores virtual-to-physical address mappings. Since the virtual addresses used by one process may not be valid for another process, there is no need to save and restore TLB entries during a context switch.

3. TLB can be cleared or invalidated: When a context switch occurs, the TLB can be cleared or invalidated, as the new process will have its own set of virtual-to-physical address mappings. This allows the TLB to be refreshed with the new process's page table entries when it starts executing.

In summary, the Translation Lookaside Buffer (TLB) does not need to be saved on a context switch because it is a hardware cache specific to each process and does not store any critical information that needs to be preserved.