+.sp
+.ce 100
+A Package to Support VAX Compatability Mode on UNIX-32V
+.sp
+Arthur W. Wetzel
+.br
+735 LIS Bldg
+.br
+Interdisciplinary Department of Information Science
+.br
+University of Pittsburgh
+.br
+Pittsburgh, Pa. 15260
+(412)-624-5203
+.ce 0
+.sp 2
+This is a brief description of a
+package to support the execution of PDP-11 programs on
+VAX UNIX-32V
+or Berkeley VMUNIX
+in compatability mode.
+The major functions are to
+.in +5
+.sp
+.ti -5
+1) allocate a block
+of memory as the PDP-11 memory space (this must start at location 0),
+.ti -5
+2) read compatability
+mode program images into memory and lay them out properly (with arguments etc),
+.ti -5
+3) actually handle the change to and from compatability mode,
+.ti -5
+4) simulate system calls for what ever operating system is being simulated
+and
+.ti -5
+5) simulate floating point (FPU and FIS) instructions.
+.sp
+.in -5
+Unfortunately programs requiring separated I/D space can not be run.
+Loading of the package is rather slow since the entire process is about
+80K bytes
+(64K is the PDP-11 space).
+Once execution begins however, the speed is similar to a PDP-11/70.
+There is considerable overhead for each exception condition
+so that programs with a lot of system calls or especially with
+floating point will be greatly slowed down.
+Note that the text segment must be writable since the PDP-11
+memory space is there.
+.sp
+Three quick changes to UNIX-32V and Berkeley VMUNIX
+were made in the course of constructing this package.
+.in 5
+.sp
+.ti -5
+First, it is necessary to patch a bug in the original mchdep.c.
+The bug in the sendsig routine is that the condition codes are masked out
+of the psl before it is stacked when catching signals.
+This affects all
+programs not just compatability mode ones although is is not usually a frequent
+problem execept in this application.
+The mask which was 0xfff1
+should be changed to 0xffff.
+If this is not done, the condition codes
+after a signal trap routine returns will always be cleared which can result
+in many strange problems when condition codes are being checked in loops
+or in this case after an "illegal instruction" trap.
+This same bug remains in the
+Third Berkeley Software Tape version of Virtual Memory UNIX.
+.sp
+.ti -5
+Second, although it is easy to get into compatability mode one also
+needs a way to get back when an exception condition arises.
+This can be done
+by changing another mask in the last line of the same routine.
+The 0x1f
+mask should be changed to 0x8000001f.
+This clears the compatability mode bit
+so that all signals are necessarily caught in native mode where native code
+can do something about the situation.
+.sp
+.ti -5
+Finally, if one wants compatibility
+mode programs to have SETUID and SETGID status, there must be a way to change
+the effective uid or gid without clobbering the real uid or gid.
+This is
+easily done by adding seteuid and setegid system calls to UNIX-32V.
+My method
+of doing this was to modify setuid and getuid so that the high order 16 bits
+of the argument in the actual system calls is a flag (uids and gids are only
+16 bits in the low order part of the word) to indicate either a regular
+setuid or getuid function or alternately a seteuid and setegid function.
+Appropriate functions seteuid() and setegid() have been added to our libc.a
+which automatically set up the flags while setuid() and setgid() insure
+that the flags are zeroed.
+.in -5
+.sp
+Most of the programming was done in late August 1979 with additions being made
+occasionally thru August 1980.
+Compilation procedures are specified in Makefile.
+An effort was made to minimize the amount of assembly language coding
+so that only two small assembler routines are found here.
+One of these (memsiz.s) simply specifies
+how much memory is being allocated for PDP-11 images
+and makes it available through certain global variables.
+The other assembler file (compat.s) handles the protocol for getting
+into compatability mode at a certain pc and with a certain ps.
+It also includes a getreg function which copies machine registers into
+known places.
+The heart of the entire package is runcompat.c which is used for all RTSs
+(Run Time Systems).
+The function main here simply checks for the existence of the
+file to be executed and sets the required uid and gid.
+The execute function actually copies the file to memory
+and sets trap conditions.
+Finally illtrap() catches illegal instructions and goes to the
+code appropriate for what is found as the illegal instruction.
+The bulk of the lines of C code are in unixtraps.c and dofloat.c
+which do UNIX system calls in either version 6 or 7 format
+and simulate floating point operations.
+(Since PWB-UNIX is upward compatable with version 6, the version 6
+system support also includes PWB sys calls.)
+There are probably a number of bugs in the floating point simulation
+code just waiting to be found.
+If you are running programs which already include the PDP-11
+floating point interpretation code, you may want to disable
+dofloat as the illegal instructions can be caught and simulated
+in the PDP-11 code.
+To do this just make dofloat.o with "cc -c -O -DNOFPSIM dofloat.c".
+.sp
+A shell which will automatically invoke compatability mode programs
+is in the modshell directory as difference listings from the original
+UNIX-32V shell.
+Most of the new code is in a new function compat.c.
+The automatic recognition of PDP-11 UNIX version 6/7 programs
+relies on the fact that the second word (16 bit) of a PDP-11
+a.out file (text size) is nonzero whereas it is 0 for 32V a.outs.
+No easy distinction can be made between version 6 and version 7 a.outs
+so that a shell variable RTS sets up the name of a default Run
+Time System.
+On our system version 6 a.outs have been patched so that word 6 of the header
+which is unused is a 1.
+This hoaky? method seems to work just fine.
+A program v6flag.c is in the modshell directory to do this.
+.sp
+One possible use of this package is to get programs like INGRES running
+on the VAX without going through what appears to be a nontrivial
+conversion effort.
+There are two ways of running such programs.
+Firstly if the shell is patched to automatically recognize and
+run compatability mode a.outs (as in modshell), the PDP-11 a.out files
+for the program can be just put on the system with their
+normal names and run as usual.
+Note however that you will be using the UNIX-32V shell so that
+any shell files from PDP-11 version 6 will have to be modified
+for this to work correctly with something like INGRES.
+The second approach
+is to make a directory hierarchy somewhere which corresponds
+to what would be on a PDP-11 including the appropriate PDP-11 shell.
+In that case just execute that shell in compatability mode with
+the root directory set to the top of the PDP-11 hierarchy.
+This is the quickest way to get something going in a hurry since no
+changes are required to existing PDP-11 code or shell files.
+.sp
+Emulation of RT-11 system calls provided by Dan Strick are not being
+distributed at this time.
+.sp
+Please foreward any comments, bug fixes or quick questions to the author
+at the above address.
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