SELF-CONTAINED RELOCATABLE MEMORY SUBSYSTEMAbstract of the DisclosureAn apparatus for use as part of a computer system'sexternal memory for storing a subprogram used by the computerand modifying selected data words read from the apparatusprior to transmission to the computer. The apparatus includesan addressable read only memory (ROM) having a word sizewhich is greater than the instruction word size used by thecomputer. The apparatus also includes a plurality of baseregisters which may be loaded under control of the computer'soperating system program. The read only memory contains asoftware subprogram written assuming it is stored at anabsolute location in the computer's memory system other thanits actual location. Each ROM location contains one instructionor word of data and a plurality of non-data bits which mayspecify one of the plurality of base registers. As a wordis read out of the ROM, a decoding circuit decodes thenon-data bits to determine which, if any, of the base registersis specified. If a base register is specified, the decodingcircuit triggers a gating circuit connected to the specifiedbase register. The triggered gating circuit gates the contentsof the specified base register into an adder, the other inputto the adder coming from the data bits read from the ROM. Theresulting data word, modified by the addition of the specifiedbase register's contents, is fed into a driver circuit whichtransmits it to the computer. If no base register is specifiedby the non-data bits, the decoder circuit does not trigger anyof the gating circuits, and a pattern of all zeros is providedas one input to the adder, the second input being the data-1-bits read from the ROM. In such case, the resulting dataword sent to the computer represents the unmodified databits read out of the ROM.
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