C.11.6. - The index file
Reading a long ASCII data file takes time. When scans.4 first opens a file, it scans the whole file and saves a directory of the scans in a binary-format index file. The name of the index file is formed by appending
.I to the original data-file name.
As long as the index file is more recent than the data file, scans.4 will use the information in the index file.