How the Linux Kernel Uses CPU Rings and Syscalls to Isolate Processes

Modern operating systems prevent processes from accessing arbitrary hardware or other processes' memory using built-in CPU architecture features, not software checks on every instruction. On x86, processors operate in privilege levels called rings: the kernel runs in Ring 0 with full hardware access, while user programs run in Ring 3 with restricted permissions. Whenever an interrupt or a system call occurs, the CPU automatically switches to Ring 0, returning control to the kernel's pre-configured handlers. When a user-space program needs hardware access — such as reading a file — it issues a syscall instruction, prompting the kernel to verify permissions and perform the operation on the process's behalf. Memory isolation between processes is enforced through virtual memory, another hardware-level feature that maps each process to its own address space without exposing others.