1. A computer system, comprising: a first hardware subsystem including a first hardware processor configured to run applications; a second hardware subsystem including a second separate hardware processor configured to run security firmware; and peripherals connected to the second hardware subsystem, wherein access to the connected peripherals by the applications is selectively prevented by the security firmware running on the second hardware processor which emulates corresponding peripheral connections of the first hardware subsystem, wherein the peripherals include a video display, the computer system further comprising: a video multiplexer having a first input connected to the first hardware subsystem, a second input connected to the second hardware subsystem, and an out put connected to the video display, the selection of content from the first input and the second input for driving the output being controlled by the security firmware running on the second hardware processor.