A secure mode of a computer system is used to provide simulated devices. In operation, if an instruction executing in a non-secure mode accesses a simulated device, then a resulting exception is forwarded to a secure monitor executing in the secure mode. Based on the address accessed by the instruction, the secure monitor identifies the device and simulates the instruction. The secure monitor executes independently of other applications included in the computer system, and does not rely on any hardware virtualization capabilities of the computer system.