Correction to Fortran-77 OTS for
                   I/D Tasks Using Virtual Arrays


                           Gary L. Maxwell
                          Lawrence M. Baker

                       U.S. Geological Survey
                       Menlo Park, California


In   the  course  of  our  separate  investigations  on  using  PLAS
directives and in developing  Supervisor  mode  libraries,  we  have
discovered a bug in the Fortran-77 V5.0-12 OTS.  Symptoms of the bug
include  a  "Virtual  array  initialization  failure"  message  from
Fortran  or  a  segment  fault  caused when Virtual arrays are first
accessed by an I/D space task.  This  problem  can  occur  when  the
Instruction  Space  of  the  task  is  between  28K and 32K words in
length, or when a resident library (such as FCSRES) is  linked  into
the I/D task using APR 7.  

The following program will exhibit the problem:  

File TSTBUG.FTN:  

           Program TSTBUG
     C
           Virtual Ivfill(8192)
     C
           Do 1000 I = 1, 8192
              Ivfill(I) = I
      1000 Continue
           Do 2000 I = 1, 8192
              If (Ivfill(I) .ne. I) Stop 'Verify error'
      2000 Continue
           Stop
           End

File FILLIS.MAC:  

             .title  fillis
     ;
     ; Build a 28 KW instruction space array
     ;
             .psect  fcode,i,ro
     
             .blkw   28.*1024.       ; Load it up, folks
     
             .end
     

Compile, assemble, and build as follows:  

     >F77 TSTBUG=TSTBUG/TR:ALL
     >MAC FILLIS=FILLIS
     >TKB TSTBUG/ID=TSTBUG,FILLIS

The  above  program  will  fail  with a Virtual array initialization
error.  

The  problem  is  due to an error in the Fortran OTS routine $VINIT.
When  the  Virtual  array  PLAS  window  is  created,  Fortran  uses
information  provided  by the Task Builder to determine which APR to
use to map the Virtual array window.  For I/D space tasks, the  Task
Builder  correctly  determines  which  D-Space APR should be used by
Fortran for Virtual arrays.  However,  Fortran  does  not  make  any
distinction  between  I/D  and  non-I/D  tasks.  Fortran will always
attempt to map the Virtual array in I-Space.  

For  I/D  tasks  that  are not using APR 7 for instructions or data,
Virtual arrays still work correctly.  This is because the  Executive
will automatically map the D-Space APR along with the I-Space APR if
the D-Space APR is not currently in use.  However, if the task  uses
APR  7  for instructions, or is mapped to a resident library through
APR 7, Fortran will try to remap APR 7 I-Space, causing an  eventual
failure.  


The  correction  to  $VINIT  which  follows  will work regardless of
whether a task is or is not built as an I/D  space  task.   This  is
possible  by  the use of an undocumented bit mask in the status word
of the Window Definition Block, WS.EDS (Effective Data Space).  This
mask  is  the sum of WS.UDS (User Data Space) and WS.SIS (Supervisor
Instruction Space).  When used, the M-Plus Executive  will  map  the
window  based  on whether the task uses Data Space:  if it does, the
window is mapped in D-Space;  otherwise, the  window  is  mapped  in
I-Space.  

When  the  OTS  is  corrected,  the  program example above will work
correctly.  


Correction file VINIT.PAT:  

             .Title  $VINIT
             .Enabl  Lc
             .Ident  /F7705Z/
     
     ;
     ;  VINIT.PAT -- Correction for F77 OTS module $VINIT
     ;               (Original Ident: F77005)
     ;
     ;  Corrections:
     ;
     ;       GMLB01  --  Use WS.EDS bit mask to set up mapping
     ;                   in I-space for non I/D tasks and D-Space 
     ;                   for I/D tasks.
     ;
     ;  Original   file checksum:  25372
     ;  Correction file checksum:   5712
     ;
     
             .Mcall  Wdbdf$
     
             Wdbdf$          ; Window Definition Block Offsets
     
             .Psect  $$OTSI,CON,RO,REL,LCL,I
     
     $$$ = .
     . = $$$ + 62
     
             Mov     #<WS.64B!WS.MAP!WS.EDS!WS.WRT>,(R0)+ 
                             ; Store window status
     
             .End

To  correct  the  OTS,  follow  the  instructions  in the Fortran-77
Installation Guide, or use the following command sequence:  

     >LBR VINIT.OBJ;1=SYSLIB/EX:$VINIT
     >MAC VINIT.POB=VINIT.PAT
     >PAT VINIT.OBJ;2=VINIT.OBJ;1/CS:025372,VINIT.POB/CS:005712
     >LBR SYSLIB/RP=VINIT.OBJ;2
     Module "$VINIT" replaced
     
     >