This is the kernel source tree, where new versions of the operating system
can be made.

Machine dependent sources are segregated into a directory
heirarchy (i386/), within which hardware drivers are isolated in a per-bus
directory (i386/isa), processor specific archetecture files may be found
in a per-processor directory (i386/i386), and externally visiable machine
dependant include files may be found also in a seperate directory
(i386/include -- note: these are referred to explicitly via the "machine"
include file symbolic link, or, implictly via the master include files in
located in /usr/include -- thus <sys/types.h> will include
i386/include/types.h, for example). System configuration files, are located
in a configuration directory (i386/conf), from where the config program
*must* be run as the first step in making a new system kernel.

Generic machine-independant code is located in the various directories
(kern, ufs, nfs, isofs, vm, net, netinet, stand, ddb, sys, conf).
The central kernel is in the "kern" directory, and all of it's include files
and shared definitions with other kernel subsystems are located in the
"sys" include file directory. Filesystem dependant code (ufs, nfs, isofs)
are located in their respective directories, and interface to the kernel
via a virtual filesystem interface (vfs), definied within the kernel proper.
The virtual memory subsystem is located in the "vm" directory. It's interface
is subject to considerable revision. The optional in kernel debugger is
present in two directories (ddb, i386/ddb), to use it you must compile the
system with the "ddb" device, and load the symbol table with the "dbsym"
command. Networking interface code is in the "net" directory, while "netinet"
holds the INTERNET core protocol implementation. Shared standalone bootstrap code
is in the "stand" directory.

The directory "compile" holds all per-host compilation directories. In use,
one goes to the i386/conf directory, runs the config program on a host
description file (e.g. config SUMNER), which builds the "compile/SUMNER"
directory and all files. Then, in the "compile/SUMNER" directory, the
command "make depend" is used to build the dependency graph (.depend)
for make. Next, the system is compiled from scratch by typing "make".
The system may be tested by copying into a floppy that holds a minimal
set of utilies (e.g. fixit floppy), and when satisfactory, can be copied
into the root as /386bsd on the hard disk, and the system rebooted.

To debug the system, one can use the "gdb" debugger to examine post-mortum
dumps, or to modify kernel memory on a running system (e.g. setting
debug flags in drivers). To read a postmortum (in /var/crash), type something
like "gdb -k /var/crash/system.N /var/crash/ram.N", for the Nth dump you
wish to examine. To look at a running system, "gdb -k /386bsd /dev/mem".
For in-kernel debugging, one can compile in the "ddb" debugger and enter
it by typing CTRL, ALT, and ESC simulatineously. Finally, printfs can be
scattered in the kernel for debugging, just like user programs.