.  ----------  ----------
==================                  . | security || whatever |
Linux architecture                  . |          ||          |
==================                  .  ----------  ----------
                                    .          ^    ^
                                    .          |    |
                                    .         --------      ---------
 ------------            ---------  .        |        |    |  file-  |
| C-compiled | fopen()  | shared  | syscalls |        |--->| systems |
| user prog  | fread()  | std C   | INT 80h  | Linux  |     ---------
| e.g. shell |--------->| library |--------->| kernel |     ---------
| or editor  | printf() |         | .        |        |    |  block  |
|            | malloc() | libc.so | .        |        |--->| devices |
 ------------            ---------  .        |        |     ---------
                                    .         --------
                                    .              | |
                               USER . SYSTEM       |  ---------
                          PRIVILEGE . PRIVILEGE    v           v
                                    .            ---------  ---------
                                    .           | network ||  char   |
                                    .           | devices || devices |
                                    .            ---------  ---------

1. Linux usually uses dynamic linking (though it can also use static
   linking). If user prog uses malloc() function, object code for
   malloc() remains in the libc.so file, and is linked to user prog at
   run-time. libc code pulled into memory during dynamic linking is
   shared with many programs running at once (saves memory). DOS does not
   multitask, so it does not need dynamic linking.

2. The syscalls are implemented with software interrupt 80 hex (INT 80h).
   The value in the EAX register when INT 80h is called tells the kernel
   what service is desired (open file, read file, get time, change
   directory, etc).

3. User prog can NOT go directly to hardware or device driver: it must
   go through kernel.

4. User prog and libc run in "user mode". When the CPU is in this mode,
   certain instructions are forbidden, mainly the instructions that
   disable interrupts. Timer interrupt returns control from user prog to
   kernel, and switches to system mode. This allows safe and fair
   multitasking.

5. Many Linux services can either be compiled into the kernel, or built
   as loadable modules, linked to the kernel at run-time using 'insmod'.

6. Linux supports many filesystems besides DOS (CD-ROM, Minix, OS/2,
   Macintosh, Amiga, other versions of UNIX, Windows NT, network
   filesystems, etc).

7. UNIX requires certain filesystem features like device files and file
   ownership/mode, so DOS filesystem alone can't be used with Linux/UNIX.

8. Linux does not use the BIOS. This is because Linux runs in 32-bit
   protected mode, and BIOS code is meant to be used in 16-bit real mode.