Nine-track tapes and dual density feature

The operating system supports 9-track tape reels in densities of 800 bits per inch (bpi), 1600 bpi, and 6250 bpi.

The dual-density feature, available on some tape units, permits the unit to read and write in either of two densities. The density combinations available are 800/1600 bpi and 1600/6250 bpi. You specify the density with the DEN parameter of the DD statement or DCB macro instruction (Table 1 shows the DEN parameter codes). If the DEN parameter code is not specified, the greater density is assumed.

Table 1. DEN parameter codes for specifying tape density
DEN Value¹	Recording Density
DEN Value¹	7-track	9-track
1	556	—
2	800	800 (NRZI)²
3	—	1600 (PE)³
4	—	6250 (GCR)⁴
Notes: If the DEN parameter is not supplied by any source, the highest applicable density is assumed. Nonreturn-to-zero inverse mode Phase-encoded mode Group-coded recording mode

The DEN parameter is ignored for input. The system automatically sets the tape unit to the density of the tape.

For output with dual density, the tape is written in the density you specify or, if unspecified, the default density. If your request is for a standard labeled tape, and the label of the mounted volume is written in the wrong density, the system will rewrite the label to agree with your specification, if you are opening the first data set on the volume. If this is not the first data set on the volume, the system will change your density specification to agree with the density of the volume. Tapes created with the dual-density feature and those created without it are interchangeable.