Bangladesh Competition Commission Programmer Written Exam: 2019 (DU)প্রশ্ন ও সমাধান

Multitasking: the OS's ability to run multiple programs/processes concurrently by rapidly switching the CPU between them, giving the illusion of simultaneous execution on a single CPU (or genuine parallel execution on multiple cores).

Multithreading: a single process divided into multiple threads (lightweight sub-units of execution) that run concurrently and share the same process's memory space (code, data, open files) — unlike multitasking's separate processes, which don't share memory by default.

graph TD
P[Process] --> T1[Thread 1]
P --> T2[Thread 2]
P --> T3[Thread 3]
T1 --- SM[(Shared Memory/Code/Data)]
T2 --- SM
T3 --- SM

Advantages of threads over processes:

• Faster creation & context switching: creating/switching a thread is cheaper than a process, since threads share the same address space.
• Responsiveness: if one thread blocks (e.g., waiting on I/O), other threads in the same process can continue running.
• Resource sharing: threads share code, data, and open file handles directly — no inter-process communication (IPC) overhead needed.
• Efficient multiprocessor use: multiple threads of one process can run truly in parallel across CPU cores.

Example: a web browser uses multithreading — one thread renders the page while another downloads images and another handles user input — all sharing the same process's memory, so switching between them is far cheaper than running three separate browser processes.

Exam takeaway: multitasking = multiple processes sharing CPU time; multithreading = multiple threads within one process sharing memory — threads trade isolation (a crashing thread can bring down the whole process) for speed and lower overhead.