The block count shown in the HDR1 label is always an ASCII 0. This 0 is recorded in the DCB and increased during processing for comparison to the block count shown in the trailer label (EOV1 or EOF1).

For reading backward, the block count shown in the trailer label (EOV1 or EOF1) is recorded in the DCB and decreased during processing for comparison to the 0 block count in the HDR1 label.

The block count is verified at the end of the data set or the end of volume. For more information, see Verifying block count and Verifying block count in label when reading.