Episode 95: How Computers Work Part V - Assembly Language and the Operating System

Process Management

The operating system (OS) plays a crucial role in managing processes to ensure fair resource allocation. James Fodor explains that even simple devices like smartphones run dozens of processes simultaneously, requiring the OS to allocate CPU time and memory fairly among them. This prevents any single process from monopolizing system resources and ensures smooth multitasking 1.

The job of the operating system is to keep track of all the currently running processes and what their memory mappings are and where they're stored.

--- James Fodor

The OS achieves this by rapidly switching between processes, giving the illusion of multitasking, and using system calls to regain control when necessary 2.

   

Kernel & User Modes

Understanding kernel and user modes is essential for grasping OS functionality. James Fodor describes how the CPU boots in kernel mode to load the OS, which then runs processes in user mode to limit their access to system resources 3. This separation ensures that user applications cannot directly access critical system functions or memory, maintaining system stability and security.

User mode is the standard mode, and that's the mode that the process runs when it's running regular applications.

--- James Fodor

The OS manages transitions between these modes, allowing processes to request services through system calls, which temporarily switch the CPU back to kernel mode 4.

   

System Calls

System calls are vital for process interaction with the OS, enabling controlled access to hardware and resources. James Fodor explains that processes leave instructions for the OS via registers, which the trap handler interprets to execute necessary operations 5. This mechanism ensures that processes can perform tasks like input/output without directly accessing hardware, preserving system integrity.

A system call just works essentially by leaving the instructions for the operating system, handing over back to the operating system.

--- James Fodor

Timed interrupts and system calls allow the OS to maintain control, ensuring fair resource distribution and preventing any process from dominating the CPU 6.