# Trusted Runtime System
The Intel(R) SGX trusted runtime system (tRTS) is a key component of the Intel(R) Software Guard Extensions SDK.
It provides the enclave entry point logic as well as other functions to be used by enclave developers.
**Intel(R) Software Guard Extensions Helper Functions**
**CustomExceptionHandling**
# Intel(R) Software Guard Extensions Helper Functions
The tRTS provides the helper functions for you to determine whether a given address is within or outside
enclave memory.
The tRTS provides a wrapper to the RDRAND instruction to generate a true random number from hardware.
enclave developers should use the rsgx_read_rand function to get true random numbers.
# CustomExceptionHandling
The Intel(R) Software Guard Extensions SDK provides an API to allow you to register functions, or exception handlers,
to handle a limited set of hardware exceptions. When one of the enclave supported hardware exceptions occurs within
the enclave, the registered exception handlers will be called in a specific order until an exception handler reports
that it has handled the exception. For example, issuing a CPUID instruction inside an Enclave will result in a #UD fault
(Invalid Opcode Exception). ISV enclave code can call rsgx_register_exception_handler to register a function of type
sgx_exception_handler_t to respond to this exception. To check a list of enclave supported exceptions, see Intel(R)
Software Guard Extensions Programming Reference.
**Note**
Custom exception handling is only supported in HW mode. Although the exception handlers can be registered in simulation mode,
the exceptions cannot be caught and handled within the enclave.
**Note**
OCALLs are not allowed in the exception handler.
**Note**
Custom exception handing only saves general purpose registers in sgx_ exception_info_t. You should be careful when touching
other registers in the exception handlers.
**Note**
If the exception handlers can not handle the exceptions, abort() is called.
abort() makes the enclave unusable and generates another exception.