Burroughs B6500

Further improvements to the B5500 resulted in the B6500, announced in 1966 but not delivered until 1969, almost a year behind the original schedule, as a result of problems with electronic signal interference in its circuitry. Even then, hardware troubles continued for another year before the system settled down. The B6500 continued the development of some of the unique features of the B5000/B5500 architecture. One of these features was the memory tag, which categorized the contents of each word in memory. The designers of the B6500 expanded it to three bits (from one on the B5000), so that it could indicate whether a word contained an instruction, control information, or data. This was used as an additional memory protection feature, preventing an erring program from trying to execute data as an instruction or treating an instruction as data. Furthermore, for a data word the tag identified the data format: fixed-point, floating-point, or character/decimal. This made possible a very compact instruction set, because a single add instruction could determine from the tag what type of data was involved. (The B5000 had two add instructions, since a one-bit flag did not distinguish between single and double floating.) Most other computers had multiple add instructions: one for fixed-point, one for single floating, one for double floating, and so on.