In a data processing system in which the complete set of op-code signal groups are stored in a ROM unit, a programmable, non-volatile memory unit stores the address signal groups of the set of op-code signal groups currently controlling the operation of the data processing system. In addition, the programmable, non-volatile memory unit has error checking and correction signal groups associated with each op-code signal groups. To retrieve an op-code signal group from the ROM unit, the central processing unit applies a pointer/address signal group to the programmable, non-volatile memory. The op-code address and the error checking and correction signal group at the location specified by the pointer/address signal group is retrieved from the programmable, non-volatile memory unit, subjected to error checking and correction procedures and the resulting address signal groups is applied to the ROM unit. The op-code signal group identified by the address signal group is retrieved and transferred to the central processing unit. A net data compression is achieved by this configuration.