Re: [TUHS] Determining what was on a tape back in the day

Yes and no. To start with, one needs to differentiate three different levels; i) what's actually on the medium; ii) what the device controller presented to the CPU; and iii) what the OS (Unix in this case) presented to the users. With the exception of magtapes (which had some semantics available through Unix for larger records, and file marks, the details of which escape me - but try looking at the man page for 'dd' in V6 for a flavour of it), you're correct about what Unix presented to the users. As to what is preserved; for disks and DECtapes, I think you are broadly correct. For magtapes, it depends. E.g. SIMH apparently can consume files which _represent_ magtape contents (i, above), and which include 'in band' (i.e. part of the byte stream in the file) meta-data for things like file marks, etc. At least one of the people who reads old media for a living, when asked to read an old tape, gives you back one of these files with meta-data in it. Here: http://ana-3.lcs.mit.edu/~jnc/tech/pdp11/tools/rdsmt.c is a program which reads one of those files and convert the contents to a file containing just the data bytes. (I had a tape with a 'dd' save of a file-system on it, and wanted just the file-system image, on which I deployed a tool I wrote to grok 4.2 filesystems.) Also, for disks, it should be remembered that i) and ii) were usually quite different, as what was actually on the disk included thing like preambles, headers, CRCs, etc, none of which the CPU usually could even see. (See here: http://gunkies.org/wiki/RX0x_floppy_drive#Low-level_format for an example. Each physical drive type would have its own specific low-level hardware format.) So what's preserved is just an image of what the CPU saw, which is, for disks and DECtapes, generally the same as what was presented to the user - i.e. a pile of bytes. Noel