Abstract: A pointer (86) has generate circuitry (90), propagate circuitry (90), carry circuitry (90) and detector circuitry (92). The pointer is for use with a buffer to designate one of a plurality of entries of the buffer. The generate circuitry receives a first and a second data word and generates a plurality of local generate functions. One bit of the first data word, second data word, and one of the local generate functions each corresponds to one of the entries of the buffer. Each data bit of the first data word is representative of the eligibility of the pointer to designate an entry. The second data word is representative of the pointer location at a previous time. The propagate circuitry receives the first and second data words and generates a plurality of local propagate functions. Each local propagate function corresponds to one of the entries of the buffer. The carry circuitry is coupled to the generate circuitry and to the propagate circuitry and generates a plurality of carry bits.

Abstract: Segmentation is added to a reduced instruction set computer (RISC) processor which supports paging. The arithmetic-logic-unit (ALU) is extended to allow for a 3-port addition so that the segment base can be added when the virtual address is being generated. Segment bounds checking is achieved by extending the paging system to allow for valid regions that are less than the full page size. Sub-page validity can mimic segmentation because a segment can be broken up into a number of full pages and one or more partially-valid pages at the segment boundaries. A page that is not wholly valid has an "event" on the page, and a memory reference to this page will either cause a software routine to be invoked to check the segment bound, or an extension to the TLB, called a sub-page validity buffer, is used to check if the reference was to a valid portion of the page. Events may also be defined for program watchpoints and defective memory locations.