This paper presents a new two-levels page-based memory bus protection scheme. A trusted Operating System drives a hardware cryptographic unit and manages security contexts for each protected memory page. The hardware unit is located between the internal system bus and the memory controller. It protects the integrity and confidentiality of selected memory pages. For better acceptability the processor (CPU) architecture and the software application level are unmodified. The impact of the security on cost and performance is optimized by several algorithmic and hardware techniques and by a differentiated handling of memory pages, depending on their characteristics.