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Lecture 04 - February 10, 2003
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AGENDA:
0 Admin
1 P1: last chance for questions
2 P2: version 2 of the shell
3 Ch 2.1: introduction to processes
4 Ch 2.2: interprocess communication

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0 - ADMIN
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Startout Q: What is a process?

Wednesday:
* RI #2
* email to cos421gr@css.tayloru.edu
* Give you the grade/feedback on P1

Friday:
* HM #2
* Quiz #2

Bochs/MINIX installation: thoughts from Thomas Hruska
"I got it to work.  The following link points at a
series of floppy images of Minix and a few batch files. 
Once I figured out the partition problem (I did not
partition correctly), Bochs and Minix work flawlessly.

The following link only works ON CAMPUS:
 
\\10.5.228.98\Programs\SOURCE\OS\MINIX\2.0.3\BUILDIMG

The readme.txt file documents how to create an image.
I created the floppy images using a FreeDOS utility
called raread (the counterpart to the infamous utility 
rawrite).  The floppy images drastically improve the 
overall speed of installation (zoom, zoom!!!)."

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1 - P1: LAST CHANCE FOR QUESTIONS
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This is the time and place.

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2 - P2: OVERVIEW OF V.2 OF SHELL
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Go over shell description on web.

Describe file descriptors
* index into "open files" table for process
* fd 0 == stdin - by default the keyboard
  fd 1 == stdout - by default the terminal screen
  fd 2 == stderr - ""   ""
* open files and the corresponding file descriptors
  are preserved across a fork or exec command
* EOF is not detected until the reference count to
  the writing end of the pipe is 0. Until that time,
  it is possible that a program could write more
  data.
* read 1.4.3 for explanations and sample code

signals: check out 


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3 - CH 2.1 - INTRODUCTION TO PROCESSES
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process: an abstraction of a running program
* an executing program
* also all relevant supporting data
  - program counter, registers, variables
pseudoparallelism: faking multiprocessor behavior

process vs. program
process hierarchies (incl. init)
process states
* running, blocked, ready


Implementing processes
* process table fig 2-4 p, 53
* interrupt vector & interrupt service routine
  fig 2-5 p. 54

threads
* single vs. multiple threads of control and program
  counter
* also called lightweight processes
* do they share the same address space?
* user-space vs. kernel control
* issue of what all is blocked when a thread blocks

Considerations in thread management

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3 - CH 2.2 - INTERPROCESS COMMUNICATION
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aka IPC
can use interrupts, but very primitive

Interprocess communication
3 issues
* how can processes pass information to each other
* ensuring that processes do not get in each other's
  way during critical activities
* ensuring proper sequencing when have dependencies

RACE CONDITIONS
* Spooler directory example fig 2-7 p. 58.
* Two processes end up both trying to write a file to
  same slot.
* All sorts of similar problems occur. Only one time
  in 1000 (or greater) might there be a problem, but
  that will drive you crazy.

CRITICAL SECTIONS
* Times when need guarantee that nobody else will get
  in to the same data structure and mess around at
  the same time.
* Need mutual exclusion.
* Aka critical region.

Four conditions for good solution
1) no two processes may be simultaneously inside their
   critical regions
2) no assumptions may be made about speeds or the
   number of CPUs
3) no process running outside its critical region may
   block other processes
4) no process should have to wait forever to enter its
   critical region


COS 421 - Lecture Notes #4		SPRING 2003	
COS421-lect-2003-02-10.doc	Page 1	Printed 10.02.03