1. Operating Systems Structure
External Structure
Operating System Interfaces
System Calls
Device Drivers
Internal Structure: Layering and Monolithic Kernels
Alternative Structures:
Microkernels
Exokernels and Library Operating Systems

2. External Structure of an OS
applications programs/
processes
system call
interface
scheduler
file system
kernel
memory manager
etc.
device drivers
hardware

3. Example: Windows NT User mode Integral subsystems
Application
POSIX
Application
OS/2
Application
User mode
Integral subsystems
Environment subsystems
Workstation service
Server service
Security
Win 32
POSIX
OS/2
Kernel mode
Executive
Executive Services
I/O Manager
Security Refernce Monitor
IPC
Manager
Virtual Memory Manager (VMM)
Process Manager
PnP Manager
Power Manager
Window Manager
GDI
Object Manager
Kernel mode drivers
Microkernel
Hardware Abstraction Layer (HAL)
Hardware

4. Internal Structure of an OS
applications programs/
processes
system call
interface
scheduler
file system
kernel
memory manager
etc.
device drivers
hardware

5. Internal Structure: Layered Services
The “Onion View” of the OS.
user programs
file system
intermachine network communication
device manager and device drivers
real-time clock manager
interprocess communication
process coordinator
process manager
memory manager
hardware
Xinu [Comer 1984]

6. Internal Structure: Monolithic vs. μ-Kernels
Layered Kernel
Microkernel
...
user process
device drivers
file server
process server
virtual memory
user
user
user
file system
user
IPC
kernel
I/O and device management
virtual memory
process management
μ-kernel
kernel
hardware
hardware
Hierarchical decomposition.
Leads to monolithic kernels.
Kernel has only core operating system functions.
Other functions run in server processes in user space.

7. Operations in a μ-Kernel
user
file system server
sys_open()
file handle
μ-kernel
open_file request
Non-kernel components of the OS are implemented as server processes.
Communication between user and servers using messages through kernel.
“Client-server computing on a single computer.”
Examples: Mach, Windows NT/2000/XP/7/8/10/…, Chorus, L4, …

8. In micro-kernels, both users and servers run
in user-mode. This includes the File System Server.
The file server does not need to execute any
privileged instructions (i.e., instructions that need to
be run in kernel mode.)  
Of course, IO operations to read/write from/to disks
need to run in kernel mode, but these usually are
managed by the microkernel.
(For example, IO port operations are privileged
on the x86.)

9. Client-Server Computing with μ-Kernel
user
file system server
file handle
Network
μ-kernel
μ-kernel
μ-kernel
open_file req
open_file request
Non-kernel components of the OS are implemented as server processes.
Communication between user and servers using messages through kernel.
“Client-server computing on a single computer.”
Examples: Mach, Windows NT/2000/XP/7/8/10/…, Chorus, L4, …

10. Benefits of μ-Kernels Extensibility:
New services can be added by adding server processes.
Flexibility:
Services can be customized.
Portability:
Kernel small, with well-defined interface.
Robustness:
Servers can fail “locally”
Distributed System Support:
Interface between users and services is message-based.

11. μ-Kernels: Performance is Problem
user
server
reply
μ-kernel
request
Request traverses user/kernel boundary twice, same for reply.
Solution approaches:
Move critical services back into the kernel (“make kernel bigger”)
Make kernel “smaller”

12. Bigger: Windows NT User mode Integral subsystems
Application
POSIX
Application
OS/2
Application
User mode
Integral subsystems
Environment subsystems
Workstation service
Server service
Security
Win 32
POSIX
OS/2
Kernel mode
Executive
Executive Services
I/O Manager
Security Refernce Monitor
IPC
Manager
Virtual Memory Manager (VMM)
Process Manager
PnP Manager
Power Manager
Window Manager
GDI
Object Manager
Kernel mode drivers
Microkernel
Hardware Abstraction Layer (HAL)
Hardware

13. Smaller: Exokernel
Claim: Providing abstractions of resources or virtual machines for applications to execute is the wrong approach!
The Exokernel approach: Export hardware resources securely (!) instead of emulating them.
Expose everything about resources, but do it securely, e.g.:
Allow application to access HW resources as efficiently as possible.
Export physical names of resources (e.g., addresses)
Allow applications to allocate specific resources

14. Exokernel: Secure Binding
Web Application
High-Performance Num. Application
Application
http
Application libraries
DSM
POSIX
TCP
IPC
Traps VM
“Library Operating System”
Exokernel
Secure bindings
Hardware
Frame Buffer
TLB
Network
Memory
Disk
Library OSs manage hardware resources. Exokernel protects Library OSs from each other.

15. Summary: Operating Systems Structure
External Structure
Operating System Interfaces
System Calls
Device Drivers
Internal Structure: Layering and Monolithic Kernels
Alternative Structures:
Microkernels
Exokernels and Library Operating Systems