Processes

Quintessentially, computing systems are designed, developed, and often tweaked for running user applications efficiently. Every element that goes into a computing platform is intended to enable effective and efficient ways for running applications. In other words, computing systems exist to run diverse application programs. Applications can run either as firmware in dedicated devices or as a "process" in systems driven by system software (operating systems).

At its core, a process is a running instance of a program in memory. The transformation from a program to a process happens when the program (on disk) is fetched into memory for execution.

A program’s binary image carries code (with all its binary instructions) and data (with all global data), which are mapped to distinct regions of memory with appropriate access permissions (read, write, and execute). Apart from code and data, a process is assigned additional memory regions called stack (for allocation of function call frames with auto variables and function arguments) and heap for dynamic allocations at runtime.

Multiple instances of the same program can exist with their respective memory allocations. For instance, for a web browser with multiple open tabs (running simultaneous browsing sessions), each tab is considered a process instance by the kernel, with unique memory allocations.

The following figure represents the layout of processes in memory:

The illusion called address space

Modern-day computing platforms are expected to handle a plethora of processes efficiently. Operating systems thus must deal with allocating unique memory to all contending processes within the physical memory (often finite) and also ensure their reliable execution. With multiple processes contending and executing simultaneously (multi-tasking), the operating system must ensure that the memory allocation of every process is protected from accidental access by another process.

To address this issue, the kernel provides a level of abstraction between the process and the physical memory called virtual address space. Virtual address space is the process' view of memory; it is how the running program views the memory.

Virtual address space creates an illusion that every process exclusively owns the whole memory while executing. This abstracted view of memory is called virtual memory and is achieved by the kernel's memory manager in coordination with the CPU's MMU. Each process is given a contiguous 32 or 64-bit address space, bound by the architecture and unique to that process. With each process caged into its virtual address space by the MMU, any attempt by a process to access an address region outside its boundaries will trigger a hardware fault, making it possible for the memory manger to detect and terminate violating processes, thus ensuring protection.

The following figure depicts the illusion of address space created for every contending process:

Kernel and user space

Modern operating systems not only prevent one process from accessing another but also prevent processes from accidentally accessing or manipulating kernel data and services (as the kernel is shared by all the processes).

Operating systems achieve this protection by segmenting the whole memory into two logical halves, the user and kernel space. This bifurcation ensures that all processes that are assigned address spaces are mapped to the user space section of memory and kernel data and services run in kernel space. The kernel achieves this protection in coordination with the hardware. While an application process is executing instructions from its code segment, the CPU is operating in user mode. When a process intends to invoke a kernel service, it needs to switch the CPU into privileged mode (kernel mode), which is achieved through special functions called APIs (application programming interfaces). These APIs enable user processes to switch into the kernel space using special CPU instructions and then execute the required services through system calls. On completion of the requested service, the kernel executes another mode switch, this time back from kernel mode to user mode, using another set of CPU instructions.

System calls are the kernel's interfaces to expose its services to application processes; they are also called kernel entry points. As system calls are implemented in kernel space, the respective handlers are provided through APIs in the user space. API abstraction also makes it easier and convenient to invoke related system calls.

The following figure depicts a virtualized memory view:

Process context

When a process requests a kernel service through a system call, the kernel will execute on behalf of the caller process. The kernel is now said to be executing in process context. Similarly, the kernel also responds to interrupts raised by other hardware entities; here, the kernel executes in interrupt context. When in interrupt context, the kernel is not running on behalf of any process.