Abstract: 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.