GATE CS 1999 Q2.10 — Memory Management and Virtual Memory

To implement a secure and functional multi-user, multi-processing operating system, the underlying hardware must provide mechanisms to ensure isolation, protection, and security among different users and processes.

1. Address Translation (Option A): In a multi-user system, multiple processes reside in memory simultaneously. To prevent one user's process from accessing or corrupting the memory space of another user or the operating system, memory protection is mandatory. This requires hardware-level address translation (such as base/limit registers or page tables) to map logical addresses to isolated physical addresses.
2. At least two modes of CPU execution (Option C): To maintain system integrity, user programs must be restricted from executing sensitive instructions (like halting the machine, modifying memory protection registers, or directly accessing I/O devices). The hardware must support at least two modes—privileged (kernel) mode for the OS and non-privileged (user) mode for user applications.

Why other options are not strictly necessary:

• DMA for disk transfer (Option B): While Direct Memory Access (DMA) significantly improves performance by freeing up the CPU during I/O, an OS can still function using programmed I/O or interrupt-driven I/O.
• Demand paging (Option D): Demand paging is a virtual memory technique. A multi-user OS can be implemented using simpler memory management schemes, such as contiguous allocation with swapping or entire-process paging, where the complete process is loaded into memory before execution.