External Media

There are two media from which data can be read into the SPA data file. These are magnetic tape and disk archive files. At present, only SPA formatted magnetic tape I/O is supported. A magnetic tape is

1. initialized with the SPI program (see Chapter 2),
2. mounted and updated with the SPA TAPE function,
3. read from and written to with the SPA READ and WRITE functions.

The tape format consists of an information file which includes a directory listing of scans and in the most recent version, a record containing tape information such as initialization date, tape density and floating point convention. This information file is followed by one or more data files in which each record is a data scan. See Chapter 3 for more detailed information on the content of these files and tape format.

Under UNIX and VAX/VMS operating systems, a disk archive file is created and augmented with the DAR program. When optionally attached to the program at startup, the data resides in the readonly T subdirectory. To access this data within the program, the current subdirectory must be the T subdirectory.

The disk archive files are useful to transport data from the telescope to home institutions, to move data from one SFILE to another, or simply as a backup to reduced data since the disk file can be subsequently written to some external media such as exabyte, 1/2" or cartridge tapes. In addition, unlike the S files, the T file does not fragment, i.e. empty records located between records which contain data. Therefore, the T file is generally more disk efficient in the storage of data.