The creation of data blocks (physical records on tape) that can be handled in most computer systems by a FORTRAN formatted READ statement is prohibitively costly to the Bureau in producing a number of different summary tape files for the entire United States. Therefore, blocks will be of a larger size which can be divided into sub-blocks by the user with relative ease. The sub-block, which has been uniformly defined at 120 characters, is a convenient size for listing the tapes on line printers. The sub-blocks are not true logical records within a physical record because the content of the sub-blocks does not have a repetitive pattern.
The large block-size of the physical record must be small enough to the handled conveniently in a majority of computers. One size for all summary tape files proves wasteful, however, since certain files would require an excessive amount of padding. Therefore, block-size for each summary file has been fixed for the entire file at one block-size which is an even multiple of 120 characters. The minimum block-size for any file will be 720 characters; the maximum will be 2040 characters. Blocks have been so constructed that they can be sub-divided into sub-blocks of 120 characters each. Therefore, by padding, no item will be split across 120-character sub blocks or across physical records.
A full logical record consists of the geographic identification and all data fields for that geographic area. The logical record may consist of multiple physical records of the size specified for the particular count file. For example, a First Count logical record consists of all the geographic codes necessary to identify an enumeration district, or other area, plus the 409 data items for the area. Therefore, this logical record with padding consists of two physical records of 1800 characters each, or 30 sub-records of 120 characters each.
In the Fourth and Sixth Counts, a tape file will contain logical records of different sizes; however, the physical record size will remain the same. For example, there may be five physical records for data and two physical records for allocation counts in one logical record for one geographic area.