An apparatus for use as part of a computer system's external memory for storing a subprogram used by the computer and modifying selected data words read from the apparatus prior to transmission to the computer. The apparatus includes an addressable read only memory (ROM) (12) having a word size which is greater than the instruction word size used by the computer. The apparatus also includes a plurality of base registers (14) which may be loaded under control of the computer's operating system program. The read only memory (12) contains a software subprogram written assuming it is stored at an absolute location in the computer's memory system other than its actual location. Each ROM location contains one instruction or word of data and a plurality of non-data bits which may specify one of the plurality of base registers (14). As a word is read out of the ROM (12), a decoding circuit (24) decodes the non-data bits to determine which, if any, of the base registers (14) is specified. A triggered gating circuit (16) gates the contents of the specified base register (14) into an adder (18), the other input to the adder (18) coming from the data bits read from the ROM (12).
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