0. 4/25/2017
OS Structures

1. Agenda OS design and implementation Simple structure
3. Layered structure
Microkernels
Modular structure

2. 1. OS design and implementation
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1. OS design and implementation
An art of balance
hardware vs software
efficiency vs flexibility
user vs system
convenience vs effectiveness
General design guidelines
separation of mechanisms and policies
Best current practices
CSc 360Overview

3. 2. Simple Structure (1) E.g., MS-DOS single user almost single process
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2. Simple Structure (1)
E.g., MS-DOS
single user
almost single process
direct access
almost flat memory
MZ linked list
executables
.COM: segment limit
.EXE: MZ file magic
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4. 2. Simple structure (2) MS-DOS
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2. Simple structure (2) MS-DOS
Load program
“shrink” interpreter
make room for program
Execute program
access to everywhere
even “kernel”/interpreter
Reload interpreter back
otherwise, “cannot find command.com…”
CSc 360Overview

5. 3. Layered structure (1) Layers Divide & conquer Cross-layer issues
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3. Layered structure (1)
Layers
L0: hardware
LN: user interface
Li: anything in btw
use Li-1 service
offer service to Li+1
Divide & conquer
Cross-layer issues
CSc 360Overview

6. 3. Layered structure (2) Unix
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3. Layered structure (2) Unix
Hybrid structure
CSc 360Overview

7. 4. Micro-kernel structure
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4. Micro-kernel structure
E.g.
Mach
Smaller kernel
only those “essentials”
e.g., handle hardware
More by system/application programs
message passing
Overhead between kernel and user spaces
file client
file server
kernel
CSc 360Overview

8. 5. Modular structure (1) Object-oriented methodology
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5. Modular structure (1)
Object-oriented methodology
not necessary implemented in OO languages
popular choices for modern OS, e.g., Linux
e.g., insmod fat|vfat|msdos
On-demand, loadable kernel modules
each module is a separate function/support
communicate through know kernel interface
module dependency
CSc 360Overview

9. 5. Modular structure (2): Mac OS X
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5. Modular structure (2): Mac OS X
Mach (CPU,memory) + BSD (file,network)
CSc 360Overview

10. 5. Modular structure (3): SunOS Solaris
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Modular design (high-level diagram)
CSc 360Overview