A system for data processing a security critical activity in a secure management mode in a computer, which comprises a processor (10), handling devices (20, 28-38), memory storage resources (14, 42). The system comprises a security device (50) comprising a processor (52) and signal generators (SG.sub.PM, SG.sub.A), a number of control switches (60), with signal receivers (SR.sub.A, SR.sub.PM) arranged respectively between the security device and the pre-selected resources. The switches contain information regarding accessibility to and from the resources, or parts of the resources, hereafter named resource ranges, wherein the switch controls requests from the computer processor to the resources or resource ranges depending on the information contained in the switch, and wherein, in response to a call from the computer processor or the handling devices, the switches are activated by receiving a signal (SG.sub.PM) from the security device, enabling the security device access to and from the resources or resource ranges selected by the security device, and denying the computer processor access to and from the resources or resource ranges selected by the security device.