separate frame buffer memory zone for security this is then a security frame buffer considered as part of the security processor 10 this in particular greatly facilitates the task of this processor 10 generally it may occur that a display command coming from the central unit 8 non secure can potentially compromise the integrity or the security of the secure display risking a modification of the secure display in this case it is up to the security processor 10 to detect and react to this situation for example by exiting the secure display mode or by blocking the command at fault in the example embodiment given hereinabove according to which a security frame buffer memory zone is used this reverts in particular to verifying that no one other than the security processor 10 writes in this zone and that no one other than the security processor 10 and display subsystem 5 reads in the zone in this case of implementation it is further considered that the main frame buffer is part of the display subsystem and that the security frame buffer is part of the security processor as such the updates of the main frame buffer will be considered as display commands that are inoffensive for security while the updates of the security frame buffer must be carried out only by the security processor this buffer will form to a certain extent a portion of the internal state of this security processor an attempt to update the security frame buffer by the central processing unit or any component other than the security processor 10 will typically be considered as an illegitimate security command to which the security processor 10 must react the work.