CSS Optional — Paper-II, Section-A · P-II.III.I (cont.) · Previously missing from this vault — added because "Design and Components of OS" is named explicitly in the syllabus line for this topic. This page answers "what must an OS achieve, and what pieces does it break into" — the companion [OS Structures and Operations] page answers "how is it physically architected."

1. Design Goals

These two goal-sets are sometimes in tension (e.g. maximum safety/protection checks can cost raw speed) — OS design is a continual balancing act between them.

2. Mechanisms vs Policies (a classic separation-of-concerns principle)

• A mechanism is how something is done (e.g. the timer-interrupt hardware that CAN preempt a process).
• A policy is what/when decision is made using that mechanism (e.g. which scheduling algorithm decides WHEN to preempt and WHOM to run next).

Good OS design keeps mechanism and policy separate — the same underlying mechanism (a preemption timer) can then support many different policies (FCFS, RR, Priority, MLFQ) without redesigning the hardware/low-level support each time. This separation is why, e.g., Linux can offer several pluggable schedulers on the same kernel mechanisms.

3. Components of an Operating System

graph TD
    OS["Operating System"] --> PM["Process Management"]
    OS --> MM["Memory Management"]
    OS --> FM["File Management"]
    OS --> IO["I/O System Management"]
    OS --> SM["Secondary Storage Management"]
    OS --> NW["Networking"]
    OS --> PROT["Protection System"]
    OS --> CI["Command-Interpreter / UI"]

• Process Management: creating/scheduling/terminating processes and threads, synchronization, deadlock handling.
• Memory Management: allocating/deallocating main memory, tracking which parts are in use, deciding which processes/data move in and out of memory.
• File Management: creating/deleting files and directories, mapping files onto secondary storage, backup.
• I/O System Management: a buffer/caching system, a general device-driver interface, drivers for specific hardware devices.
• Secondary Storage Management: free-space management, storage allocation, disk scheduling.
• Networking: enabling communication and resource-sharing across machines (distributed-system support).
• Protection System: controlling which processes/users may access which resources (see Security below).
• Command-Interpreter System / User Interface: the shell or GUI through which a user's commands are read and acted upon.

Kernel responsibilities recap (frequently asked as a standalone short-note): the kernel is the privileged core implementing most of the above — process management/scheduling, memory management, file-system management, device management via drivers, IPC/synchronization primitives, system-call interface, and security/access-control enforcement.

4. Protection & Security as a Design Component