Compiling and Booting the
			      Nsys UNIX Kernel

			       Warren Toomey

			     15th January, 1999


The `nsys' UNIX kernel was recently donated to the PUPS Archive by Dennis
Ritchie. This file describes how you can boot a slightly-modified version of
this kernel on a 5th Edition RK05 root filesystem.


In January 1999, Dennis Ritchie sent in a copy of the `nsys' UNIX kernel for
inclusion in the PUPS Archive. In the accompanying README, he says:

     So far as I can determine, this is the earliest version of Unix
     that currently exists in machine-readable form. ... The dates on
     the transcription are hard to interpret correctly; if my program
     that interprets the image are correct, the files were last touched
     on 22 Jan, 1973. ...

     What is here is just the source of the OS itself, written in the
     pre-K&R dialect of C. The file u is evidently a binary image of
     the kernel. It is intended only for PDP-11/45, and has setup and
     memory-handling code that will not work on other models (it's
     missing things special to the later, smaller models, and the
     larger physical address space of the still later 11/70.) It
     appears that it is intended to be loaded into memory at physical
     0, and transferred to at location 0.

     I'm not sure how much work it would take to get this system to
     boot. Even compiling it might be a bit of a challenge, though I
     imagine that circa V5-V6 compilers would handle the language
     (maybe even V7). It is full of =+ and use of char * as unsigned
     integers, and integers used as pointers in locations like p->x.

     So far as I can determine, the disk format it expects is
     compatible with the layout of other earlyish research systems (V5,
     V6) for example. But perhaps not, and it's not certain that the
     source is complete. Even the compilation mechanism is a bit
     unclear, though it certainly used the shell script ken/rc, which
     appears to depend on having the *.o files from ken/* and dmr/* and
     also slib.a, which doesn't exist on the tape. My guess is that
     this was an archive of already compiled .o files, so that (for
     example) to test-build a system one would edit a file, compile it,
     and run ken/rc to load it. The 'ustr' routine referred to in
     ken/rc evidently stripped off the a.out header and the symbols
     from the a.out file.

     Best wishes with this. I'd be interested to hear whether anyone
     gets the u image to run. If you're in luck, all you need is an
     11/45 processor or emulator and a V5/6/7 disk image.

I decided to try and compile the `nsys' code, and get it to boot on a
PDP-11/45 emulator. A secondary goal was to use the `nsys' compilation
effort to find any remaining bugs in my Apout PDP-11 user-mode emulator.

Tools Required

Because the `nsys' kernel is dated 1973, I decided to use a 5th Edition
development enviromnent to compile it. This minimised the changes I had to
make to the source code.

There are several ways you can obtain a 5th Edition development enviromnent:

   * Load a 5th Edition filesystem on an RK05, and boot it on a PDP 11/45;
   * Load a 5th Edition filesystem on an RK05 disk image, and boot it on a
     PDP 11/45 emulator; or

   * Unpack the 5th Edition root filesystem tree on a 32-bit little-endian
     Unix system, and use the Apout emulator.

For convenience, I chose to use the Apout emulator (version 2.2alpha8) as my
development environment, and Bob Supnik's PDP-11 simulator (version 2.3d) as
the booting environment. With the latter, I used the 5th Edition RK05 disk
image Distributions/research/Dennis_v5/v5root.gz (from the PUPS Archive) as
the filesystem.

Regardless of the method you choose, the modified `nsys' source must be
unpacked in the directory /sys/nsys, relative to the top of the 5th Edition
root directory, i.e the file u (size 26,266 bytes) becomes /sys/nsys/u. This
file is the original `nsys' kernel image, as supplied by Dennis; we will
build our own kernel image.

Changes to the Nsys Source

Several files in the `nsys' source had to be modified so as the `nsys'
kernel would work with a 5th Edition filesystem & boot environment. The
discovery of these changes took me several days of fiddling with C code,
assembly code, single-stepping machine code, and perusing the Lions'
commentary. Email me if you really want to know the gory details.

The changes to the ten `nsys' source code files are described below:

     The 5th Edition filesystem is laid out slightly differently to that
     which the `nsys' kernel is expecting. The filsys struct needs an extra
     field, s_ronly, and the inode struct needs an extra field, i_lastr. The
     two files affected are filsys.h and inode.h.

C language
     The C language changed slightly from the `nsys' kernel to the 5th
     Edition. Sub-structures defined within a structure were delimited by
     parentheses in `nsys', but by braces in 5th Edition. The only file
     affected is user.h.

     Several lines in the psig() routine in ken/sig.c were rearranged
     because the 5th Edition C compiler refused to parse them. The actual
     operations performed are unchanged.

Device table changes
     The `nsys' kernel is configured to have four block device drivers (rf,
     rk, tc and tm), and ten character device drivers (kl, dc, pc, dp, dn,
     mm, vt, da, ct, vs). To minimise debugging, I chose to remove as many
     of the drivers as possible. I left two block device drivers, rk and tm,
     and two character device drivers, kl and mm. The two main files
     affected were conf/c.c and conf/l.s.

     The device driver dmr/rk.c also had to be modified, as it was
     hard-wired to be block device number 1. It is now block device number

     In the putchar() routine in prf.c, a test is made on a register in the
     console KL device. This register isn't described in my PDP-11
     peripherals handbook (dated 1973), and it isn't implemented in Bob
     Supnik's simulator, so I removed the code.

     The declaration of the clist struct in tty.h need a semicolon to end
     the declaration; again, this could be due to a change in the C

     The machine code for location 0 in conf/l.s has the octal value 4 then
     the instruction br 1f. In 5th Edition, these two are transposed. It
     appears that the entry to the `nsys' kernel must have been at location
     2 (i.e the br instruction), whereas the 5th Edition kernel starts at
     location 0. Octal 4 is an IOT instruction, which causes an immediate
     hardware exception on the PDP-11. I have transposed these two lines in
     the `nsys' code.

Cosmetic changes
     While trying to get the `nsys' kernel to boot properly, I added the 5th
     Edition printf line to main() in main.c, which outputs the amount of
     physical memory available on the machine.

As well as these changes, I have moved a few files around so that those
parts of the kernel which must be tailored for each hardware configuration
are kept in the conf/ directory. Final linking of the `nsys' kernel is also
done in this directory. A few other files have been renamed or moved, again
to tidy up the layout of the source. The changes are:

   * dmr/malloc.c becomes dmr/mem.c
   * tables.c becomes dmr/partab.c
   * ken/45.s becomes conf/mch.s
   * ken/low.s becomes conf/l.s
   * ken/conf.c becomes conf/c.c

There are several new RCS directories, which hold the changes to the ten
`nsys' files listed above. Finally, three short shell scripts have been
created to make compilation of the `nsys' kernel relatively easy: ken/mklib,
dmr/mklib and conf/mkunix.

Compiling the Nsys Kernel

Here is a typescript of the commands required to compile the `nsys' kernel.
I have added some comments to the typescript.

% alias 5sh
setenv APOUT_ROOT /usr/local/src/V5; apout $APOUT_ROOT/bin/sh

% 5sh # Run Apout

# chdir /sys/nsys Move to the `nsys' directory
# chdir ken Start with the kernel code
# cat mklib
cc -c -O *.c
rm ../lib1
ar vr ../lib1 main.o alloc.o iget.o prf.o rdwri.o slp.o subr.o text.o trap.o \
 sig.o sysent.o clock.o fio.o malloc.o nami.o prproc.o sys1.o sys2.o \
 sys3.o sys4.o

# sh mklib Run the script to build lib1
../lib1: non existent
a main.o
a alloc.o
a iget.o
a prf.o
a rdwri.o
a slp.o
a subr.o
a text.o
a trap.o
a sig.o
a sysent.o
a clock.o
a fio.o
a malloc.o
a nami.o
a prproc.o
a sys1.o
a sys2.o
a sys3.o
a sys4.o

# chdir ../dmr Move to the devices directory
# cat mklib
cc -c -O *.c
as gput.s
mv a.out gput.o
rm ../lib2
ar vr ../lib2 *.o

# sh mklib Run the script to build lib2
../lib2: non existent
a bio.o
a cat.o
a dc.o
a dn.o
a dp.o
a draa.o
a gput.o
a kl.o
a mem.o
a partab.o
a pc.o
a rf.o
a rk.o
a tc.o
a tm.o
a tty.o
a vs.o
a vt.o

# chdir ../conf Move to the configuration directory
# cat mkunix
as mch.s
mv a.out mch.o
cc -c c.c
as l.s
mv a.out l.o
ld -x l.o mch.o c.o ../dmr/gput.o ../lib1 ../lib2
mv a.out unix
nm -n unix > namelist
ls -l unix
size unix

# sh mkunix Build config, link the kernel
-rwxrwxrwx 1 root 25322 Jan 14 22:02 unix
21286+888+15962=38136 (0112370)

# ls -l And see what other files we have
total 82
drwxr-xr-x 2 root 512 Jan 14 05:37 RCS
-r--r--r-- 1 root 307 Jan 14 00:31 c.c
-rw-rw-rw- 1 root 292 Jan 14 22:02 c.o
-rw-rw-rw- 1 root 1200 Jan 14 22:02 l.o
-r--r--r-- 1 root 2004 Jan 13 22:37 l.s
-rw-rw-rw- 1 root 1888 Jan 14 22:02 mch.o
-r--r--r-- 1 root 3896 Jan 10 18:19 mch.s
-r--r--r-- 1 root 161 Jan 14 19:37 mkunix
-rw------- 1 root 3995 Jan 14 22:02 namelist
-rwxrwxrwx 1 root 25322 Jan 14 22:02 unix

Installing the Nsys Kernel

Now that the `nsys' kernel is compiled, we have to install it in the root
directory of an RK05 5th Edition UNIX root filesystem. I used the bootable
5th Edition disk image v5root and Bob Supnik's emulator to do this.

5th Edition UNIX doesn't have tar, so I mounted v5root as RK pack 0, and I
mounted the new `nsys' kernel as RK pack 1, after doing some padding to the

% ls -l
-rwx------ 1 wkt wheel 117728 Jan 11 14:02 pdp Supnik simulator
-rw------- 1 wkt wheel 55 Jan 15 14:12 v5 Config file
-r-------- 1 wkt wheel 2494464 Jan 15 13:35 v5root V5 filesystem
-rw------- 1 wkt wheel 4096 Jan 14 12:06 zero File of zeroes

% cp ../V5/sys/nsys/conf/unix nsys.binary Copy the kernel here
% cat zero >> nsys.binary Pad it with zeroes

% cat v5 Here is the config file
set cpu 18b
att rk0 v5root
att rk1 nsys.binary
boot rk

% ./pdp v5 Run the simulator

PDP-11 simulator V2.3d
@unix Start V5 UNIX

login: root
# check /dev/rrk0 Check root filesystem
spcl 5
files 566
large 126
direc 28
indir 126
used 3790
last 3985
free 128

# ls -l /dev You may need to /etc/mknod
total 0 at least /dev/rrk1
cr--r--r-- 1 bin 1, 0 Nov 26 18:13 mem
crw-rw-rw- 1 bin 1, 2 Nov 26 18:13 null
crw-rw-rw- 1 root 2, 0 Mar 21 13:53 rrk0
crw-rw-rw- 1 root 2, 1 Mar 21 14:18 rrk1
crw--w--w- 1 root 0, 0 Mar 21 15:26 tty8

# dd if=/dev/rrk1 count=50 of=z Load in `nsys' kernel + pad
50+0 records in
50+0 records out

# dd if=z of=nsys bs=11761 count=2 Trim back to correct size
2+0 records in
2+0 records out

# rm z Remove temporary files

# ls -l nsys Check correct size
-rw-rw-rw- 1 root 23522 Mar 21 15:29 nsys

# size nsys Verify a.out values, should
21286+888+15962=38136 (0112370) be the same as before

# sync Shut down V5 UNIX
# sync
# ^E and exit the simulator
Simulation stopped, PC: 014116 (BNE 14150)
sim> q

You now have the `nsys' kernel stored in the root directory of the 5th
Edition root filesystem. You can now boot it and see that it works.

% ./pdp v5 Run the simulator

PDP-11 simulator V2.3d
@nsys Load the nsys kernel
mem = 64539 Printout of avail memory

login: root /etc/init works!

# ls -l So does /bin/ls
total 107
drwxr-xr-x 2 bin 944 Nov 26 18:13 bin
drwxr-xr-x 2 bin 112 Mar 21 14:21 dev
drwxr-xr-x 2 bin 240 Mar 21 12:07 etc
drwxr-xr-x 2 bin 224 Nov 26 18:13 lib
drwxr-xr-x 2 bin 32 Nov 26 18:13 mnt
-rw-rw-rw- 1 root 23522 Mar 21 15:29 nsys
drwxrwxrwx 2 bin 128 Mar 21 14:16 tmp
-rwxrwxrwx 1 bin 25802 Mar 21 12:07 unix
drwxr-xr-x 14 bin 224 Nov 26 18:13 usr
# sync
# ^E Exit the simulator
Simulation stopped, PC: 015140 (BLT 15050)

Final Notes

The `nsys' kernel can now boot and run some 5th Edition UNIX a.out binaries.
However, `nsys' is an earlier version than 5th Edition, so there will be
some V5 functionality which `nsys' does not support. In particular, `nsys'
does not have the pipe() system call. I have only just got the `nsys' kernel
to boot, so I have not had a chance to sit down and work out exactly what
functionality is missing.