barryg@sdcrdcf.UUCP (Lee Gold) (11/04/85)
Here is a brief technical summary OS-Merge and how it works. OS-Merge
was designed and written by Locus Computing Corporation in Santa
Monica, California.
OS-Merge is a system that allows a DOS process to "run under UNIX".
OS Merge can be implemented in multiple ways. Some examples are (1)
run DOS and UNIX on the same 80286 processor, (2) run UNIX on some
processor and run DOS on a 8086/8088/80186/80286 coprocessor. The
particular implementation of OS-Merge on the AT&T PC 6300 PLUS has DOS
and UNIX running on the same 80286 processor, with DOS running in
real-address mode and UNIX running in protected mode. On the 6300
PLUS, there is additional hardware support, called the CONTROL
hardware, that prevents the DOS program (which is running in an
otherwise unprotected environment) from exceeding its bounds and
possibly damaging the UNIX environment. By running the DOS
environment in real address mode, the application can run at full
speed without the system having to perform any address mode emulation.
At the heart of the system is Locus's PC-Interface system, which is a
"bridge" from DOS to UNIX. The Bridge intercepts DOS system calls,
examines them, and passes some of them on to DOS, and passes the rest
of them to a Server process which is running on the UNIX system. The
Server interprets the DOS system calls, and services them by executing
UNIX system calls to the UNIX kernel.
.-----------------.
| DOS Application |
`-----------------'
|
.-----------------. network .-----------------.
| Bridge |-------------| Server |
`-----------------' `-----------------'
| |
.-----------------. .-----------------.
| MS-DOS | | UNIX Kernel |
`-----------------' `-----------------'
To the application, the entire UNIX filesystem appears as an
additional DOS drive. For example, if the MS-DOS system had a floppy
and no hard disk, the UNIX filesystem would probably appear as drive
C:, and you could "cd \usr\dave" on that drive. This is transparent
at the DOS system call level, so DOS applications can automatically
take advantage of the UNIX file service.
In OS-Merge, the "network" above becomes shared memory, and both the
DOS enviroment (with the Application, the Bridge, and MS-DOS) and the
UNIX environment (with the Server and the Kernel) run on the same
machine. When the Bridge needs service from UNIX, it changes the
processor to protected mode and passes control to the Server. After
the Server has serviced the request, it changes the processor back to
real address mode and returns control to the Bridge. Of course,
interrupts can also require processor mode switches. Also, UNIX
timesharing is in effect, so the UNIX scheduler can preempt the
Server/DOS process to run other UNIX processes in the normal way.
The CONTROL hardware prevents the DOS application from writing on
memory outside its bounds, and also prevents it from writing
inappropriate values to I/O ports. In addition, a Sanity Timer
prevents the application from disabling interrupts for a long period
of time For example, the following code fragment will not harm the
system:
cli
mov ax, 0ffh
mov dx, 021h
out dx, ax
hlt
Note however that the DOS application may mask off interrupts for
devices that DOS currently controls.
The user can choose to run DOS programs either from the UNIX shell
prompt (actually the UNIX exec system call) or from the DOS command
interpreter (actually the DOS exec system call). The UNIX exec system
call recognizes DOS programs and causes an OS-Merge Server to be
invoked. The Server sets up the DOS environment and then passes the
necessary information to it so that it will run the program requested
by the user. Execing a DOS program is transparent at the UNIX system
call level; the shell is unmodified. In fact, no UNIX programs except
the kernel were modified to support OS-Merge. Shell scripts and
makefiles may freely execute dos programs.
For programs that are "well behaved" (by which I mean that they do
their I/O through DOS system calls) the standard output of the DOS
application program is passed to the standard output of the Server.
The standard input is similarly redirected. This means that pipes and
redirection work with UNIX programs. For example, the command
$ dir a: | grep FOO
works as expected.
Programs that write directly to the screen also work, but cannot feed
a UNIX pipe (or a DOS one for that matter).
OS-Merge is currently only available on the AT&T PC 6300 PLUS.
David Butterfield
Locus Computing Corporation
3330 Ocean Park Blvd
Santa Monica, California 90405
lcc!dave@ucla-csgemini@hou2h.UUCP (R.GBADAMOSI) (11/08/85)
Lest we forget, Uniform has had a product available ("The Connector")
for XENIX and VENIX which does the same thing as OS-MERGE for the
last year. I've been using it on my AT under Venix.
-Rick Richardson
..!houxm!vaximile!rer
[ I have no interest in Uniform ]