Tag Handling for Pointers
Original Publication Date: 1982-Jul-01
Included in the Prior Art Database: 2005-Feb-09
Pointers in a computer system as described in U.S. Patent 4,241,396 are sixteen bytes and are protected by the use of tag bits. Tag bits cannot be accessed at the instruction interface and cannot be set on by the user. Any attempt by the user to modify the contents of a pointer causes the tag bits to be set off by the hardware, making the pointer invalid for addressing purposes. Tag bits pose a problem, however, because they can add to storage size. In U.S. Patent 4,241,396, storage words are 40 bits where one of the bits is a tag bit. The 40-bit and 72-bit storage words are conventional and available. The 40-bit storage word has 32 data bits, 7 ECC (Error Correcting Code) bits and 1 tag bit. The 72 bit storage word has 64 data bits, 8 ECC bits and none available for a tag bit.