SSC IMD SA 2017 – Question 138

The address of a page table in memory is pointed to by:

• Stack pointer
• Page table base register
• Page register
• Program counter

Detailed Explanation:

The correct answer is b) Page table base register (पृष्ठ टेबल आधार रजिस्टर – PTBR).

In paged virtual memory systems, the operating system maintains a page table for each running process to map logical page numbers to physical frame numbers.

• Page-Table Base Register (PTBR): A dedicated hardware CPU register that points directly to the starting physical address (base address) of the active process’s page table in main memory.
• During a context switch, the operating system simply reloads the PTBR with the base address of the incoming process’s page table, which allows fast switching without copying entire page tables.
• Along with PTBR, modern architectures often use the Page-Table Length Register (PTLR) to indicate the size of the page table and ensure memory access bounds checking.

Why Other Options are Incorrect:

• Stack Pointer (SP): Points to the top of the call stack in memory, managing local variables and function call return addresses.
• Page Register: A generic term; does not specifically denote the CPU register holding the page table base address.
• Program Counter (PC): Holds the memory address of the next instruction to fetch and execute.