Fixed Block Architecture
Fixed Block Architecture (FBA) is a disk layout in which each addressable record (block) on disk is of the same size. The term fell out of use, since nearly all modern disk drives use this principle, termed logical block addressing and usually having a constant addressable block size of 512 bytes (even though the underlying physical block might be 4096 bytes).
Count Key Data
IBM mainframe systems' disks, called DASD, had traditionally been addressable in a form MBBCCHHR (Extent (M)-Bin (BB)-Cylinder (CC)-Head (HH)-Record (R), which was capable of storing records of varied size, up to 255 such records per track, with the zeroth record (R0) being reserved for certain error correction information, such as "skip defects"). Besides direct addressing, programmers were able to search by key, using the underlying count-key-data (CKD) structure.
Hard Disk Drives
- The "extent" can be 0x00 through 0xff, with 16 extents being reserved for each device in a "concatination" (association) of up to 16 physically separate devices, even of different device types, although each physical device may have only 16 such "extents".
- Applies only to "Data Cell" devices, and is 0x00 otherwise.
- IBM System/360 Operating System: System Control Blocks 
- IBM System 360 Model 20 Functional Characteristics
- IBM System/360 model 44 Functional Characteristics