Virtual memory system

Abstract

In a demand-paged virtual memory system, the pages are arranged in the virtual memory space in groups. In order to translate an address from the virtual address space to a physical memory address space, the virtual group address component is input to a contents addressable memory (767), which outputs a group code (767), and the group code and virtual page address component (768X,Y) are input to a RAM page table (750) which outputs the page address. When the physical memory capacity is substantially smaller than the virtual address space, the CAM provides a large saving in page table size. In the case where the data-elements provide a plural-dimensional representation, for example as in pixel data, the pages include data elements which are contiguous in each of the plural dimensions in order to reduce the amount of page-swapping between the physical memory and a paging memory. The data-elements in the physical memory are accessible in parallel as contiguous patches. In order to allow parallel access of patches which are not aligned with respect to a page and therefore may fall partly on the edge of one page and partly on the edge of a page which is contiguous with the first page in the virtual memory but not in the physical memory, the addressing system determines whether a patch to be accessed has data-elements in different pages, and may then modify the addresses to the physical memory.