1. A secure demand paging system comprising: a processor operable for executing instructions; an internal memory for a first page in a first virtual machine context; an external memory for a second page in a second virtual machine context; a security circuit coupled to said processor and to said internal memory for maintaining the first page secure in said internal memory; the processor operable to execute sets of instructions representing: a central controller; an abort handler coupled to supply to said central controller at least one signal representing a page fault by an instruction in said processor; a scavenger responsive to said central controller and operable to identify the first page as a page to free; a virtual machine context switcher responsive to said central controller to change from the first virtual machine context to the second virtual machine context; and a swapper manager operable to swap in the second page from said external memory with decryption and integrity check, to said internal memory in place of the first page.