Editing Transaction Processing Facility (section)

====Data records====
Historically, all data on the TPF system had to fit in fixed record (and memory block) sizes of 381, 1055 and 4K bytes. This was due in part to the physical record sizes of blocks located on DASD. Much overhead was saved by freeing up any part of the operating system from breaking large data entities into smaller ones during file operations, and reassembling the same during read operations. Since IBM hardware does I/O via the use of '''channels''' and '''channel programs''', TPF would generate very small and efficient channel programs to do its I/O&nbsp;— all in the name of speed. Since the early days also placed a premium on the size of storage media&nbsp;— be it memory or disk, TPF applications evolved into doing very powerful things while using very little resource.

Today, much of these limitations are removed.  In fact, only because of legacy support are smaller-than-4K DASD records still used.  With the advances made in DASD technology, a read/write of a 4K record is just as efficient as a 1055 byte record.  The same advances have increased the capacity of each device so that there is no longer a premium placed on the ability to pack data into the smallest model as possible.