Thursday, June 08, 2006

The number of UNIX installations has grown to 10, with more expected.

The UNIX Programmer's Manual, 2nd Edition, June, 1972

Completion of I/O triggers an interruptHardware Trap:

Division by zero Invalid memory access

Request for OS service Trap/System call/Monitor call

Protection

Dual Mode Operation User mode Kernel mode

Mode bit (0/1) (MS-DOS did not have it)System call (switches from user mode to

kernel mode)Privileged instructions should be

executed in kernel mode only

Protection

Changing mode bit?

I/O Protection

User cannot issue I/O instructions directly

Changes to interrupt vector?

Memory Protection

Memory Protection

Memory Protection

Changes to base and limit registers?

CPU Protection

E.g. Infinite loop executed by a process

CPU Protection

Timer handler?

Process :Program in execution

Associated state (PC, SP, registers etc)

Address space

ProcessesThe Process Model

Multiprogramming of four programsConceptual model of 4 independent, sequential processesOnly one program active at any instant

Logical program counter

Execution order not reproducible

Process creation• System initialization

• User request to create a new process

Process Termination

Conditions which terminate processes

1. Normal exit (voluntary)

2. Error exit (voluntary)

3. Fatal error (involuntary)

4. Killed by another process (involuntary)

Process Hierarchies

Parent creates a child process, child processes can create its own process

Forms a hierarchy UNIX calls this a "process group"

Windows has no concept of process hierarchy all processes are created equal

As a process executes, it changes state new: The process is being created running: Instructions are being executed waiting: The process is waiting for some event to occur ready: The process is waiting to be assigned to a

processor terminated: The process has finished execution

Process States

ps in Unix

Background processes

Process Control Block (PCB)

CPU Switch From Process to Process

Process Scheduling Queues

Job queue – set of all processes in the systemReady queue – set of all processes residing in main

memory, ready and waiting to executeDevice queues – set of processes waiting for an I/O

deviceProcesses migrate among the various queues

Ready Queue And Various I/O Device Queues

Representation of Process Scheduling

I/O parallelism:

overlap execution: make 1 CPU into many (Real parallelism: > 1 CPU (multiprocessing))

Completion time:

B’s completion time = 100s (A + B) So overlap

Why processes? Speed

emacs (Wait for input) (Wait for input)

gcc

A B

A

B

20 s80 s

Completion time for B? A?10 s

Context Switch

When CPU switches to another process, the system must save the state of the old process and load the saved state for the new process

Context-switch time is overhead; the system does no useful work while switching

Time dependent on hardware support

Passing of Parameters As A Table

Process Creation (Cont.)

Address space Child duplicate of parent Child has a program loaded into it

UNIX examples fork system call creates new process exec system call used after a fork to replace

the process’ memory space with a new program