سوال ۹۸ کنکور ارشد مهندسی کامپیوتر ۱۴۰۵

What protects a process's address space? The Memory Management Unit (MMU) — a dedicated hardware block that sits inside the processor chip (the CPU), between the core and the memory bus.

On every memory reference the MMU:

1. Translates the virtual address the process emits into a physical address (via page tables / segment or base–limit registers), and
2. Enforces protection at the same time — it checks that the access falls within the pages/regions the process owns and that the requested operation matches the permission bits (read / write / execute / user vs. kernel). A violation triggers a hardware trap (page fault / protection fault) to the OS.

Because this check runs in hardware on each access, isolation between address spaces is enforced at full speed. The operating system's role is to configure the MMU (set up page tables, load the base/limit or page-table base register on a context switch); it does not perform the per-access check itself.

Why the other options are wrong

• Option 1: Protection is not purely software — the MMU hardware itself enforces it during translation. The OS only sets up the tables.
• Option 3: The MMU is a hardware unit, not a software module of the OS.
• Option 4: The MMU resides in the processor chip, not in the RAM chip.

Hence the correct choice is option 2.

Exam trick: "MMU" literally means Memory Management Unit — the U stands for a hardware unit inside the CPU. Any option that calls it software, part of the OS, or part of RAM is a distractor. Translation and protection are two faces of the same hardware step.