A data processing system includes a disk drive, a disk drive controller, a main memory and a CPU. The main memory has disk command data stored therein in a chain of disk command blocks (DCB's). Each DCB contains a first word pointing to the next DCB in the chain, a second word containing status information and a third word containing command information. A portion of the command word contains a predetermined verification word when the DCB is valid. The CPU includes means for comparing this portion with the predetermined verification word as stored in a constant memory. If the two correspond, the DCB is valid. Each DCB also includes a fourth word pointing to a block of main memory in which header data is stored. Header data defines the address of the recording location of the disk. A fifth word points to a block of main memory in which label data is stored. Label data defines the name and/or number of the file, as well as itself including a pointer to the address location on the disk at which the next page of the file data is stored. A sixth word of the DCB points to a block of main memory in which the associated page of file data is stored. A seventh word specifies the recording location on the disk (drive, track, sector) where the actions specified in the command word are to be carried out. Each such recording location is capable of having stored therein the header, label and file data associated with a specific DCB.