Pertec interface

Mouse writes In the CDC tape community, we have three almost-compatible variants of the SimH format. SimH itself acknowledges two of them. The first, totally naive format came about when someone heard the briefest description of it (32 bits of block length, that many bytes of data, and 32 bits of block length) and ran off to write a reader that produced it. Note that all blocks have that format, even tape marks are 8 bytes long. The second is pretty much as described in the SimH documents, meaning tapemarks are 4 bytes long. Odd-length blocks are not padded. This is E11 format. The third is identical, except that odd-length blocks are padded. This is SimH format. One theory about the padding is that this format congealed at a time when 16-bit I/O on minicomputers was prevalent. I've had to make my readers a bit heuristic to read all three on-the-fly, modulo duplicate tape marks (assuming that the presence of data after 8 bytes of tapemark indicates a single 8-byte tapemark instead of two 4-byte ones (signalling EOI) ignores the possibility (which I've encountered many times) of recovering some data past the shorter thing that overwrote the first part of it). Wiping out bastard formats is a lost cause. Data cannot be destroyed--you'll run across backups of it and not know which format you used that day, so handle all of them. SimH also describes bad blocks, which have the high-bit set in the length but otherwise look normal. Except that most tape drives, without heroics, will not give you any data for an unrecoverable read error (both that and a tapemark read as 0-length blocks; some gymnastics are needed to tell them apart), except that such a block cannot be empty. So what to put in it, if we're noting them at all? I'm playing with ANSI-style labels for this. Some history is at: http://neil.franklin.ch/Usenet/alt.folklore.computers/20001209_New_Tape_Con… , though in there it is asserted that tape blocks cannot be longer than 64K, implying that only two bytes are required for the length. However, I have a tape someone else wrote (HDR2 even says so) with 65536-byte blocks (not every O/S today can handle that--16-bit signed comparisons). To represent that along with 0-byte tapemarks requires 17 bits. Elsewhere someone asserted that a tape block can be as long as the entire tape, which seems unlikely and wouldn't always fit in three bytes. -- Jeff Woolsey {woolsey,jlw}(a){jlw,jxh}.com first.last at gmail.com Spum bad keming. Nature abhors a straight antenna, a clean lens, and unused storage capacity. "Delete! Delete! OK!" -Dr. Bronner on disk space management "Card sorting, Joel." -me, re Solitaire