TAPE: Extensive cleanup of tape operations (gap, end of medium, format change, etc.) from Dave Bryan

256. ENHANCEMENT: Add tape runaway support to the simulator tape library.

     OBSERVATION:  The ANSI specifications for NRZI, PE, and GCR tape recording
     mandate a maximum length of 25 feet for erase gaps.  Currently, an erase
     gap of any length is ignored when reading or spacing.  To allow detection
     of non-compliant tape images, the simulator tape library is enhanced to
     halt positioning and return tape runaway status if a gap of 25 feet or more
     is encountered.

     Runaway detection is enabled by calling the tape library to set the tape
     density in bits per inch.  If this call is not made, erase gaps present in
     a tape image are effectively ignored.  Also, with the addition of a
     separate "set density" call, it is no longer necessary to supply the
     density when writing erase gaps.

     RESOLUTION:  Modify "sim_tape_rdlntf" and "sim_tape_rdlntr" (sim_tape.c) to
     detect tape runaway, and add a new MTSE_RUNAWAY status to sim_tape.h.  Add
     new "sim_tape_set_dens" and "sim_tape_show_dens" functions to set and show
     the bits per inch for a unit, respectively, and eliminate the "bpi"
     parameter to "sim_tape_wrgap" in preference to using the density
     established by a previous "sim_tape_set_dens" call.  Add named constants
     to "sim_tape.h" that specify the density.

257. ENHANCEMENT:  Improve performance when reading or spacing over erase gaps.

     OBSERVATION:  Performance when reading or spacing over erase gaps is poor,
     especially in the reverse direction.  Currently, each 4-byte gap marker is
     read individually, and in the reverse direction, each read is preceded by a
     seek to move the file pointer backward.  This combination causes stream
     cache invalidation and a physical disc access for each gap marker.  As a
     single gap consists of over 1000 markers, performance is far worse than if
     a gap was read as a block.

     RESOLUTION:  Modify "sim_tape_rdlntf" and "sim_tape_rdlntr" (sim_tape.c) to
     buffer reads of gap markers.  Using a 128-element buffer, performance
     improves about thirty-fold.

258. PROBLEM:  Writing an end-of-medium positions the tape image after the mark.

     OBSERVATION:  The "sim_tape_wreom" simulator tape library function writes
     an end-of-medium marker on the tape image.  The intent is to erase the
     remainder of the tape.  The "SIMH Magtape Representation and Handling"
     document states that the tape position is not updated by this function.
     However, the function leaves the tape positioned after the marker.

     A subsequent read would stop at the EOM marker.  However, writing a new
     marker over that one would then allow reading of the data following the EOM
     that supposedly had been erased by the original "sim_tape_wreom" call.

     CAUSE:  The tape position is updated by the internal "sim_tape_wrdata" call
     that is used to write the EOM marker, but it is not reset afterward by the
     function.

     RESOLUTION:  Modify "sim_tape_wreom" (sim_tape.c) to reset the tape
     position to point at the EOM marker before returning.  This prevents
     reading past an EOM marker, and a subsequent write will overwrite the
     marker rather than embed it between data records.

259. PROBLEM:  Reading through an erase gap in reverse may return EOM status.

     OBSERVATION:  A reverse read or spacing operation through an erase gap may
     return end-of-medium status.  Reading or spacing forward through the same
     gap works properly.

     CAUSE:  Writing an erase gap over existing records may produce a gap that
     is longer than requested.  This occurs when truncating the last record to
     be overlaid by the gap would leave a record that is shorter than the
     minimum size allowed (eight bytes for the length words plus two bytes for
     the data).  In this case, the gap is lengthened to overlay the entire
     record.  If the new gap size is not evenly divisible by four, a half-gap is
     metadata marker of value 0xFFFF added to the beginning of the gap.

     If a gap that begins with a half-gap marker is written immediately after
     a previous gap, the "seam" between gaps will contain the bytes FE FF FF FF
     ( FF FF ) FE FF FF FF....  Reading forward across this seam will yield a
     metadata value of 0xFFFEFFFF, which is recognized and handled by seeking
     two bytes back to resynchronize reading.  However, reading in reverse will
     yield the value 0xFFFFFFFF, which is interpreted as end-of-medium.

     RESOLUTION:  Modify "sim_tape_rdlntr" (sim_tape.c) to recognize 0xFFFFFFFF
     as a half-gap marker and resynchronize in response.  End of medium cannot
     occur when reading in reverse, as it is impossible to position the tape
     image beyond an EOM marker.  Therefore, any 0xFFFFFFFF value encountered
     must be a half-gap "seam" originating as above.

260. PROBLEM:  sim_tape_wrgap fails when format is changed from SIMH format.

     OBSERVATION:  The HP 2100 magnetic tape simulator supports erase gaps and
     calls sim_tape_wrgap when commanded to write a gap.  However, if a tape
     format other than SIMH format is selected, the call fails with MTSE_FMT.

     CAUSE:  Erase gaps are not supported in formats other than SIMH, but the
     call should not fail.  Instead, the call should be a "no-operation" if the
     underlying format does not support gaps.

     RESOLUTION:  Modify "sim_tape_wrgap" (sim_tape.c) to return MTSE_OK with no
     action performed if a tape format other than SIMH is selected.

261. PROBLEM:  The magnetic tape format of an attached unit may be changed.

     OBSERVATION:  The magnetic tape library supports several tape image
     formats.  The format to use may be specified either by an "ATTACH -F"
     command or by a "SET <unit> FORMAT" command.  The latter calls the
     "sim_tape_set_fmt" function, which allows the format of a file currently
     attached to be changed.  However, the format is an intrinsic property of
     the tape image file, so changing it once the file has been attached makes
     no sense.

     CAUSE:  Oversight.

     RESOLUTION:  Modify "sim_tape_set_fmt" (sim_tape.c) to return an error
     (SCPE_ALATT, "Unit already attached") if the unit is attached.
HP2100: Add CRCC and LRCC support to the HP 2100 MS simulation. from Dave Bryan

     OBSERVATION:  The HP 2100 MS device simulates the HP 7970 B/E magnetic tape
     drives.  The 7970B uses NRZI format, and its associated controller supports
     returning the cyclic redundancy check character and longitudinal redundancy
     check character (CRCC and LRCC) to the CPU.  Typically, these are used only
     by the controller diagnostic, but support is easy to add.

     RESOLUTION:  Modify "hp2100_ms.c" to add a function that calculates CRCC
     and LRCC for reads and writes, and call that function when the CPU requests
     that the controller deliver the values.