User's Manual

NBS Payment Solutions 21/60 User Manual
1.8.3 The isolation mechanism by electronic locking system
Isolation
Read/write isolation is obtained by USER confinement in the application software memory
space. Inter-software isolation is controlled by an MMU (Memory Management Unit).
MMU features
• Material protection
• Total inter-software protection (read/write)
• Code protection
Inviolable memory protection
The controller checks each access to the memory.
The OS is inviolable and protected
The Processor distinguishes two execution worlds:
- USER world: software domain.
-SUPERVISOR world: OS domain. No software running in the USER world can access the
SUPERVISOR world. This exclusion mechanism is ensured by the processor itself. Therefore the OS is
inviolable, even in the case of an application software bug. This system inviolability ensures that the isolation
mechanism supervised by the OS always remains operational.
The OS ensures the inter-inviolability of software applications
The OS can decide the access rights of the USER world at any time.
The MMU used by the processor enables the software's addressable space to be delimited.
A software application can only write to the memory space corresponding to the scope of its data field. Any
attempt to write or read in another space is immediately "trapped" by the controller, generating an exception.
Thus the OS keeps a track of this incident for future use with remote diagnostics. The other software
applications remain non-violated and operational. Furthermore, the software in question cannot even self-
destruct, because its write access to code is denied.