EL1

EL1 stands for Exception Level 1, a privilege level defined by the ARMv8-A architecture. It is the software execution level used by operating system kernels and other privileged software. EL1 sits above EL0 (unprivileged user mode) and below EL2 (the hypervisor) in the typical ARM privilege hierarchy. ARMv8-A also supports a secure world through TrustZone, which can provide a secure variant of EL1 within the secure state.

In normal operation, applications run at EL0, while the OS kernel runs at EL1 and manages hardware resources, memory protection, interrupts, and system calls. If virtualization is enabled, a hypervisor runs at EL2 and hosts one or more guest operating systems, each with its own EL1 context inside a virtual CPU. The separation between EL1 and higher levels is central to enforcing security and isolation in modern ARM systems.

EL1 can execute code in either the AArch64 64-bit state or the legacy AArch32 32-bit state, depending on the platform and software. Transitions between exception levels are caused by events such as interrupts or system calls and are governed by a set of control registers and privilege rules. Memory access and translation are managed through the MMU (or MPU in some configurations), with policies configured from EL1.

Notes: The exact usage of EL1 can vary by implementation and product line. Some devices emphasize secure EL1 within TrustZone, while others rely on a non-secure EL1 for the main operating system. In virtualization contexts, EL1 is the operating system level inside each guest context.