Patched VMB.EXE to support boot block booting with the same logic that all of the ROM based VMB systems (MicroVAX I onward) handle interpreting the boot block.  Many alternate operating system installation media assumes that all VAX boot logic works this way.
Updated vmb_exe include file
Merge branch 'master' into VAX-11-BootBlock-Fixup
VAX 8600 Address map cleanup from Johnny Billquist
Merge branch 'master' into VAX-11-BootBlock-Fixup
TAPE: Corrected debug output of tape position to be independent of the size of t_addr
PDP11, VAX: Enrich the debug output for the TS device simulation.
SCP: Fixed potential out of bounds array reference while expanding command arguments.

Avoid race condition while producing debug header output from multiple threads.
SCP: Avoid duplicate output when run_cmd completes just as is done in sim_printf()
SCP: Stop calls VM-provided address printer for PC without REG_VMAD from Dave Bryan

OBSERVATION:  For a simulator stop, sim_vm_fprint_addr (if defined) is
     called to print the value of the program counter, regardless of whether or
     not the register was defined with REG_VMAD.  However, displaying the PC
     value with "examine" calls sim_vm_fprint_addr only if the REG_VMAD flag is
     present.  The displayed value of the PC should be the same in both cases.
SCP: Add a "-N" (new file) option to the SET LOG and SET DEBUG commands. From Dave Bryan.
SCP: Add a -Q option to ATTACH -R and ATTACH -N to suppress success status messages. From Dave Bryan
SCP: Fixes Cannot show radix, etc. for a device that has no modifiers and SET and SHOW responses for invalid entry are inconsistent. from Dave Bryan

001. PROBLEM:  Cannot show radix, etc. for a device that has no modifiers.

     OBSERVATION:  The default data radix for a device may be set with the SET
     <dev> OCT|DEC|HEX command.  However, if the device does not have a modifier
     table, SHOW <dev> RADIX is rejected with "No settable parameters".

     The same problem occurs for SHOW <dev> DEBUG and SHOW <dev> NAMES.  For a
     device that provides debug printouts, SHOW <dev> MOD will list "DEBUG,
     NODEBUG" among the modifiers, and the SHOW <dev> DEBUG command will display
     the current debug status.  However, if the device does not contain a
     modifier table, SHOW <dev> MOD and SHOW <dev> DEBUG will report "No
     settable parameters", even though SET <dev> DEBUG is accepted and works as
     expected.  For such a device, SHOW MOD will show "DEBUG, NODEBUG" as
     acceptable modifiers.

002. PROBLEM:  SET and SHOW responses for invalid entry are inconsistent.

     OBSERVATION:  Entering SET <dev> <mod> where <mod> is not defined in the
     device's modifier table displays "Non-existent parameter."  Entering SHOW
     with the same parameters displays "Invalid argument."

     Similarly, entering SET <dev> DEBUG for a device that does not have
     debugging capability displays "Command not allowed."  Entering SHOW with
     the same parameters displays nothing.

     In both cases, the messages displayed should be the same for the same
     error.
SCP: Fix bug when debug file is opened with -N option.
SCP: Allow the VM to print simulator stop message information in lieu of, or in addition to, the default message.

The current implementation of "run_cmd" in scp.c calls "fprint_stopped_gen" (via "fprint_stopped") to print the message associated with the "sim_instr" return status.  Messages associated with VM stops must be provided to the SCP via the "sim_stop_messages" array.

"fprint_stopped_gen" prints the status message in a rigid format: the message string, a comma, the program counter register name, a colon, the current PC value, and the instruction at that address in symbolic format.

For example:

       HALT instruction, P: 24713 (LDA 1)

Only the message string is under the control of the VM.  If additional information is needed, it can only be added before the first comma.

The HP2100 simulator does this for halt instructions, which contain device select code and flag hold/clear bit fields that, in practice, are used to communicate to the operator the significance of the particular halt encountered, rather than to affect the device interface:

       HALT instruction 102077, P: 24713 (LDA 1)

To implement this, the simulator must define the message as a variable and then copy the formatted octal value into the buffer at the appropriate location before returning from "sim_instr".

However, if the VM wants to display a different register value, e.g.:

       Self test #13 complete, STAT: 000020

...this cannot be done without also displaying the program counter, which may be irrelevant for the given stop condition.
PDP10, PDP11, VAX: Adjusted KDP, DUP and DMR/DMC devices to be better behaved when the simulated system is idling.

This may not completely fix the problem identified in issue #99, but it should help
PDP11: Simplified idle call logic to leave the idle decision to sim_idle().

Removed the unused references to wait_enable which the comments suggest was related to an old style idle.
SCP: Added missing declaration of sim_vm_fprint_stopped
DOC: Add missing "SIMH Magtape Representation and Handling" document
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.