1. A data processing system comprising: a peripheral storage device having at least one recording medium and means for writing data onto and reading data from said recording medium; a controller coupled to said peripheral storage device and responsive to predetermined instructions and command data for controlling the writing of data onto and reading of data from said recording medium; a main memory storage device coupled to said controller and having a plurality of addressable storage locations, at least some of said storage locations capable of storing data to be written onto or that has been read from said recording medium, and at least some other of said storage locations capable of storing said command data, said command data including a portion thereof defining a verification word; and a central processing unit coupled to said controller and to said main memory storage device and including (a) means for supplying said predetermined instructions to said controller, (b) means for addressing the storage locations of said main memory storage device, (c) means for examining said portion of command data accessed from said main memory storage device in order to verify the validity of said accessed command data, said means for examining comprising constant memory means having a predetermined verification word stored therein and means for comparing said portion of said accessed command data with said predetermined verification word, said accessed command data being verified as valid only when the verification word in said first portion thereof matches said predetermined verification word, and (d) means for supplying to said controller command data verified as valid.