Sometimes it is necessary for software to request a function from a more privileged entity. This might happen when, for example, an application requests that the OS opens a file.
In A64, there are special instructions for making such system calls. These instructions cause an exception, which allows controlled entry into a more privileged Exception level.
SVC- Supervisor call
Causes an exception targeting EL1.
Used by an application to call the OS.
HVC- Hypervisor call
Causes an exception targeting EL2.
Used by an OS to call the hypervisor, not available at EL0.
SMC- Secure monitor call
Causes an exception targeting EL3.
Used by an OS or hypervisor to call the EL3 firmware, not available at EL0.
If an exception is executed from an Exception level higher than the target exception level, then the exception is taken to the current Exception level. This means that an
SVC at EL2 would cause exception entry to EL2. Similarly, an
HVC at EL3 causes exception entry to EL3. This is consistent with the rule that an exception can never cause the processor to lose privilege.