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INGRES (Interactive Graphics and Retrieval System) is a relational data base and graphics system which is being implemented on a PDP-11/40 based hardware configuration at Berkeley. INGRES runs as a normal user job on top of the UNIX operating system developed at the Bell Telephone Laboratories. The only significant modification to UNIX that INGRES requires is a substantial increase in the maximum file size allowed. This change was implemented by the UNIX designers. The implementation of INGRES is primarily programmed in "C", a high level language in which UNIX itself is written. Parsing is done with the assistance of YACC, a compiler-compiler available on UNIX.
Held et al. (Wed,) studied this question.