1. The totality of protection mechanisms within a computer system , including hardware , firmware , and software , the combination of which is responsible for enforcing a security policy. Note: The ability of a trusted computing base to enforce correctly a unified security policy depends on the correctness of the mechanisms within the trusted computing base, the protection of those mechanisms to ensure their correctness, and the correct input of parameters related to the security policy. 2. The security protection mechanisms within a system's hardware, firmware and software which are responsible for enforcing the security policy. Note: In evaluation terms this means a region that contains no untrusted functions. 3. The totality of protection mechanisms within a computer system - including hardware, firmware and software - the combination of which is responsible for enforcing a security policy. A TCB consists of one or more components that together enforce a unified security policy over a product or system. The ability of a TCB to correctly enforce a security policy depends solely on the mechanisms within the TCB and on the correct input by the system administrative personnel of parameters (e.g., a user's clearance) related to the security policy.