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Link registers

In AArch64 state, the Link Register (LR) stores the return address when a subroutine call is made. It can also be used as a general-purpose register if the return address is stored on the stack. The LR maps to register 30. Unlike in AArch32 state, the LR is distinct from the Exception Link Registers (ELRs) and is therefore unbanked.

There are three Exception Link Registers, ELR_EL1, ELR_EL2, and ELR_EL3, that correspond to each of the exception levels. When an exception is taken, the Exception Link Register for the target exception level stores the return address to jump to after the handling of that exception completes. If the exception was taken from AArch32 state, the top 32 bits in the ELR are all set to zero. Subroutine calls within the exception level use the LR to store the return address from the subroutine.

For example when the exception level changes from EL0 to EL1, the return address is stored in ELR_EL1.

When in an exception level, if you enable interrupts that use the same exception level, you must ensure you store the ELR on the stack because it will be overwritten with a new return address when the interrupt is taken.