"Stracka, James (GTI)" <[EMAIL PROTECTED]> > In days of old, the volume label was written at the lowest density of > the tape. The data portion at that specified. I know not if this is > still true.
Not knowing which days you were talking about, that isn't what OS/360 did, and probably not what z/OS still does. If you have an SL tape with, say, a label at 800BPI and want to write a file on it, but (accidentally) forget DEN= on the DD card, the system: Verifies the label by reading it. The drive figure out the density and sends the label to OS. The system rewinds or backspaces to write the new label (including HDR1 and such records). System writes the new label at the default density, likely 1600BPI, and the data at the same density. (Yes, it happened to me.) If you have a tape with 800 BPI labels and some 800 BPI files, and want to add another file, the system will verify the label, skip to the appropriate point to start writing, and write at the specified (or default) density, which might be 1600BPI. Such dual density tapes can only be read (through to the 1600 BPI files) on drives supporting both densities. At least for OS/360, the label is always the same density as the first file, assuming you use (,SL). -- glen
