A thread is the smallest unit of execution within a process in an operating system. It represents a sequence of executable instructions that can run concurrently with other threads within the same process. Threads share the process’s resources, such as code, data, and files, but each thread has its own stack, program counter, and set of CPU registers, which allow it to maintain its execution state independently.

A thread is defined as a lightweight, sequential flow of control within a process that shares the process’s resources, including its memory space, code segment, and open files, but operates independently with its own execution context. Formally, a thread can be represented as a tuple:T=(PC,SP,R)T = (PC, SP, R)T=(PC,SP,R)

where:

• PCPCPC is the Program Counter, indicating the current instruction to be executed.
• SPSPSP is the Stack Pointer, maintaining the stack information specific to the thread.
• RRR represents the set of CPU registers associated with the thread’s execution state.

In multithreaded systems, threads within the same process can run concurrently, sharing resources while maintaining individual execution contexts. Threads can be categorized based on their management level into user-level threads (managed by a thread library) and kernel-level threads (managed by the operating system). Threads improve computational efficiency and responsiveness but require synchronization mechanisms to handle concurrency issues such as race conditions and deadlocks.

Threads enable efficient multitasking and parallel execution within a process. They are categorized as user-level threads and kernel-level threads. User-level threads are managed by a user-level thread library without direct OS intervention, while kernel-level threads are managed by the operating system, allowing better integration with system resources but incurring higher overhead. Multithreading improves program responsiveness and resource utilization by enabling tasks to be divided into smaller concurrent operations.

Operating systems that support multithreading implement various models, such as one-to-one, many-to-one, and many-to-many, depending on how user threads map to kernel threads. Threads reduce context-switching overhead compared to processes, making them ideal for performance-critical applications like web servers and real-time systems. However, they introduce challenges like race conditions and deadlocks, which require synchronization mechanisms such as mutexes, semaphores, and monitors to ensure safe execution.