1. What does an Operating System Do?
Manages Resources
processes compete for CPU, memory, ...
can’t leave sharing to individual programs (why?)
protection
Provides services
Abstraction (hiding detail) e.g.:
filestore, not disk drive (geometry, capacity, etc)
variable size stack (hide memory shuffling, etc)
reliable network connection (not sumchecks, retries)
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CS2051 Lecture 2

2. Processes Process: A program in execution Process != Program
Why? (simplicity, speed; think of the real world!)
eg, time-sharing, multiprogramming (!=multiprocessing)
Thread: (process = thread + address space)
sequence of instructions in a sequential execution context
multithreading (what if two threads run a++ concurrently?)
process & thread? multiple threads per address space
more later on! See Bacon, Chapter 4.
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3. Example a.out 7 results in 7 - 0X80495a8 Int x;
main(int argc, char *argv[])
{ x=atoi(argv[1]);
while (1) printf(“%d - 0X%x \n”, x, &x); }
a.out 7 results in X80495a8
Different programs have the illusion that they use the same address!
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CS2051 Lecture 2

4. OS provides “Virtual Machine”
More convenient abstraction than real machine
(cf. with Java Virtual Machine)
User Application
Filestore
CPU
Network
4 Gbyte memory
Yahoo.com
User Application
Filestore
CPU
Network
4 Gbyte memory
Yahoo.com
User Application
Filestore
CPU
Network
4 Gbyte memory
Yahoo.com
User Application
Network
4 Gbyte memory
Filestore
CPU
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CS2051 Lecture 2
Yahoo.com

5. Virtual Machine enforces Protection
System against Program
Buggy (or malicious?) program cannot prejudice OS
Program against Program
Buggy program cannot interfere with other program
what if:
while(1)
fork();
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CS2051 Lecture 2

6. OS Services Provided E.g. User types “run myprog” (“% myprog” in Unix)
Read command (command interpreter - shell in Unix)
Find program file (how big?)
Allocate memory
Read file into memory
Find libraries
Start myprog running
Finish “clearly”
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7. Privileged Operations
OS/Hardware prevent unprivileged software from directly accessing facilities which may affect more than one program
file operations
disk I/O
memory allocation
CPU allocation
Network operations
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8. Network Device Manager
OS Structure
Application
Operating System
File Manager
Memory
Manager
Process
Manager
Network
Manager
Device Manager
Network Device Manager
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9. System Calls vs System Programs
Interface between a running program and the OS. Eg:
read(int fd, char *buf, int num_bytes);
not to be confused with the library function:
fread(void *ptr, size_t size, size_t n, FILE *stream);
not all languages allow system calls to be made directly
System programs provide a convenient environment for program development/execution (eg, file manipulation, status, etc…); most users’ view of the OS!
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10. Don’t always need all modules...
e.g. Network Gateway
Network B
Network A
Gateway
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11. But… Big OS Kernels have problems... Microkernels
Size: (1.8Mb for Sun; 1.2Mb for Linux)
Internal Complexity
Debugging
Microkernels
tailored for special-purpose applications
easier to maintain, tune, debug mK and services
real-time(?)
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12. In Summary... Operating System Components: Process Management
Memory Management
File Management
I/O Management
Secondary Storage Management
Network Management
Command Interpreter/GUI
Protection
Read Bacon, ; try the exercises at the end of the chapter.
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