EL0

EL0, or Exception Level 0, is the lowest privilege level in ARMv8-A architecture. It is the execution level for user-space applications, separate from the privileged levels used by the operating system and firmware. In typical systems, software running at EL0 cannot directly perform privileged operations or access sensitive resources.

Code at EL0 executes in the AArch64 execution state (or AArch32 compatibility state) but with restrictions. It cannot access system registers or execute privileged instructions; attempts to do so trigger exceptions that are handled by higher exception levels, usually EL1 or EL2 depending on configuration.

When an EL0 program needs an OS service, it uses a system call, which traps to EL1 via the SVC instruction (supervisor call). The kernel then handles the request, performs the operation, and returns to EL0. Memory management, device I/O, and scheduling are controlled by the operating system running at higher levels.

In systems with virtualization or secure computing features, additional levels exist: EL2 hosts the hypervisor, EL3 (secure monitor) handles secure state transitions, and EL0 remains the least privileged run-time for applications. Hypervisor calls use HVC, and secure monitor calls use SMC to move between EL3 and lower levels.

Overall, EL0 provides isolation and protection for user applications, enabling multiple processes to run with restricted access within a system that enforces privilege separation at the hardware level.