02. Computer System Structure
Reference: Ewha Womans University Operating System Course - Prof. Ban Hyo-kyung
Types of Operating Systems
Open/Closed Source by Operating System
Windows: Closed Source Software
Linux: Open Source Software
Android: Open Source Software (includes Linux Kernel at bottom)
How is Open Source Possible?
Because software doesn't have material costs except labor costs
Market where monopoly is possible, so 1st place software sells well and dominates market, but competitors other than 1st place become obsolete
2nd place software opens source before becoming obsolete, so users can adapt source code to various environments and gradually increase market share
What is an Operating System?
What is an Operating System?
Software layer installed directly above computer hardware that connects users and all other software with hardware
Meaning of Operating System
Narrow meaning of OS (Kernel)
Core of OS, part that resides in memory
Broad meaning of OS
Concept including not only kernel but also various peripheral system utilities
Purpose of Operating System
Provide interface to use computer conveniently
Efficiently manage computer system resources (both software and hardware)
OS handles so each user doesn't know how hardware operates
CPU's Role
CPU is computer's brain, but has no judgment ability, just calculates and executes quickly as written
Therefore, computer's brain could be considered OS rather than CPU
CPU calculates, memory remembers
OS's Role
OS can be considered as ruler's role of efficiently managing and operating
Role of appropriately allocating when multiple programs are loaded simultaneously in limited memory space
Classification of Operating Systems
Concurrent Task Capability
Single tasking
Process only one task at a time
e.g. In MS-DOS prompt, couldn't execute another command before finishing one command
Multi tasking
Process two or more tasks simultaneously
Need to solve security/permission issues for multiple users
e.g. In UNIX, MS Windows, can execute other commands or programs before one command finishes
Number of Users
Single user
e.g. MS-DOS, MS Windows
Multi user: simultaneous access
UNIX, NT server
Need to solve security/permission issues for multiple users
Processing Method
Batch processing system
Collect certain amount of job requests and process all at once
Must wait until job is completely finished
e.g. Early punch card processing system for I/O
e.g. 'I won't show results immediately thinking of user, I'll do it my way!'
Time sharing system
Divide computer processing power into fixed time units when performing multiple tasks
Has shorter response time compared to batch processing system
Interactive method: thinking of user, shows immediate results
Even when using computer simultaneously, divides time to make it seem like using alone
e.g. UNIX, systems used by ordinary users
Real-time system
OS for real-time systems where certain work must be guaranteed to finish within fixed time
Concept of Deadline exists and must be satisfied. Violation causes very fatal results
e.g. Nuclear reactor/factory control, missile control, semiconductor equipment, robot control
Hard real-time system
Violating deadline is quite fatal
Soft real-time system
Violating deadline is relatively less fatal
e.g. When video player frame doesn't render, video cuts off
Terminology Summary
Below terms have same meaning of performing multiple tasks simultaneously in computer, but emphasize slightly different aspects.
Multi-tasking
Multi-process
Multi-programming
Emphasizes memory aspect
Meaning that multiple programs are loaded on memory simultaneously
Time sharing
Emphasizes CPU aspect
Mainly meaning of dividing and using CPU time
Below term has completely different meaning
Multi-processor
Meaning that multiple CPUs (Processors) are attached to one computer
However, not common environment, dealt with in high-performance computing, cloud computing fields
Examples of Operating Systems
UNIX
History
Originally hardware management was also needed, development was very difficult with assembly language
Later with C language emergence, development and modification became easier and more understandable for people
Features
Most code written in C language
C language is compatible across various architectures, so has high portability
OS for servers → Therefore needs to manage multiple programs, multiple users
Minimal Kernel structure
Open source
Easy program development and expansion
Various versions (e.g. Linux case, being developed based on this since it's open source)
DOS (Disk Operating System)
History
Developed by MS for IBM-PC in 1981
Features
Single user OS
Limited memory management capability (main memory: 640KB)
MS Windows
Features
MS's multi-tasking GUI-based OS
Plug and Play: Can use immediately without separate user operation or program installation when connecting hardware to computer
Enhanced network environment
Provides compatibility with DOS applications
Rich support software
Operating System Structure
Computer System Structure
Mode Bit
What is Mode Bit?
Protection mechanism to prevent damage to other programs and operating system from user program's incorrect execution
Need for Mode Bit
Because when CPU goes to user program, OS cannot control
Need distinguishing entity to determine whether OS or user program is executing when CPU executes machine language
When CPU is not in controllable position (when doing other work), mode bit is used for permission control
That is, Mode bit plays role of protecting system by allowing only safe machine language execution
Types of Mode Bit
Mode bit supports two modes of operation through hardware
0 (Monitor mode, Kernel mode, System mode)
When OS is executing CPU (during OS code execution) can do anything
However, when OS hands over to user program, changes to 1 and hands over
Privileged instructions (dangerous instructions) can only be executed by OS when 0
1 (User mode)
User program execution
Allows user program to execute only safe machine language with CPU
Mode Bit Principle
Important instructions that can harm security are privileged instructions that can only be executed in monitor mode(0). That is, dangerous instructions
Conversely, when user program has CPU, if it's user mode(1) even though it's privileged instruction, after confirming execution of machine language without permission, CPU automatically goes to OS (→ called Interrupt and Exception)
When Exception occurs, switches back to monitor mode(0) and CPU automatically goes to OS
Must execute in monitor mode(0) until handing CPU to user program, and Mode bit is set to 1 during user program execution
Mode Bit Transition Method
Exception
When executing machine language without permission, Mode bit changes to 0 and CPU usage rights go to OS
This means Exception that occurs when trying to execute unauthorized work
Interrupt Overview
Check if Interrupt from I/O devices has come in line while reading machine language every moment, before reading next machine language
If Interrupt has come, CPU automatically goes to OS → Mode bit changes to 0 → OS catches CPU and responds to that Interrupt
This means CPU Interrupt (from Disk controller, I/O controller, etc.)
Registers
What are Registers?
Fast and small-sized storage for storing operation input and output, attached to CPU
Types of Registers
PC (Program Counter) Registers
Holds memory address of machine language to execute next → meaning pointing to address
CPU executes machine language at memory address PC Registers points to
When execution ends, executes machine language at next position
When CPU Interrupt occurs, PC Registers looks at OS memory address, and authority goes to OS (that is, mode bit becomes 0)
Timer
What is Timer?
Taking away CPU usage rights from program
Timer's Role
Generates Interrupt after certain time passes → Role of taking away CPU usage rights
OS cannot take away CPU usage rights alone
Generate Interrupt so control goes to OS after fixed time passes
Need additional hardware to prevent CPU monopoly → Timer
Widely used to implement Time sharing, also used for time calculation purposes
Timer Operation Method
Timer decreases by 1 every clock tick
When Timer value becomes 0, Timer Interrupt occurs
When OS hands over to user program, doesn't just hand over, but sets time in Timer and hands over
Therefore even if want to use CPU continuously in infinite loop, Timer interrupts CPU so CPU control goes to OS → OS hands over CPU usage rights to other program
Additional Interrupt Explanation
What is Interrupt?
Save Registers and PC Registers at interrupted point, then hand over CPU control to Interrupt handling routine
That is, Interrupt line attached to CPU is set so CPU control automatically goes to OS before executing next machine language
Two Meanings of Interrupt
Interrupt (Hardware Interrupt)
Interrupt generated by hardware, more general meaning of Interrupt
e.g. Timer, Disk Controller, etc.
Trap (Software Interrupt)
Interrupt generated by software for individual program to hand over CPU to OS
e.g. Exception, System Call
Interrupt Related Terms
Interrupt Vector
Holds address of corresponding Interrupt handling routine
Contains location of code to execute for each Interrupt type, which OS code to execute
Can be said to be kind of pointer to address
Interrupt handling routine (=Interrupt Service Routine, Interrupt Handler)
Kernel function that handles corresponding Interrupt
When going to fixed address, what to do is defined in machine language
System Call
What is System Call?
User program calling Kernel function to receive OS service
If want to do I/O but only OS can do it, interrupt itself → System Call
When want to hand over CPU to OS, cannot directly hand over PC(Program Counter), so program sets Interrupt line through its machine language → Interrupt occurs (here Software Interrupt)
System Call Principle
All instructions requesting I/O from CPU are bundled as privileged instructions
That is, user program cannot execute directly
Because privileged instructions cannot be executed when mode bit is user mode(1)
Therefore need to request OS to do it instead (System Call)
Machine language executes at user program location → hands over CPU usage rights to OS
At this time, jump occurs where machine language executes (jump across program's virtual memory)
Device Controller
What is I/O Device Controller?
Kind of small CPU that manages corresponding I/O device type
Has control register, status register for control information
Has local buffer (kind of data register)
Device Controller Principle
I/O happens between actual device and local buffer
Device Controller notifies CPU of that fact through Interrupt when I/O ends
Code executed in Device Driver is called 'firmware'
Works because pre-coded program called firmware of I/O device is included
Device Controller Terminology
Device Driver
OS code for each device processing routine → software
Machine language that CPU requests to device controller
Code that CPU executes inside computer
Device Controller
Kind of small CPU that controls each device → hardware
Not code that Device Controller executes, but code that CPU executes
Cases Where CPU Goes to OS
(1) True Interrupt
Cases where Hardware devices interrupt and CPU goes over
(2) System Call
Cases where user program Software directly sets Interrupt line and CPU goes over
(3) Trap
Cases where CPU goes over by program's own request (cannot be called Interrupt)
However, broad meaning Interrupt, that is, Trap interpretation
(4) Exception
Cases of executing machine language without permission
Cases where interrupt occurs by itself and CPU goes over
Cases Where CPU is Released
Cases where user program has CPU and hands over or releases to other program or OS
(1) Involuntary
Cases where want to use CPU but monopoly rights are taken away
e.g. Timer Interrupt
(2) Voluntary
Cases where no longer want to use CPU
e.g. During long I/O work (because must wait until I/O ends even if have CPU)
Modern OS is Driven by Interrupt
Meaning that OS does nothing and only works when Interrupt comes in
OS is also a program, so doesn't mean it can take away CPU with powerful authority, but can use as long as Interrupt comes in
When OS role is needed, always goes over by Interrupt to prevent CPU monopoly
Synchronous and Asynchronous I/O
(1) Synchronous I/O
Cases where work happening in I/O and work happening in CPU must match well in time
After CPU requests I/O, control goes to user program only after I/O work is completed
That is, after requesting I/O to Disk, when I/O ends controller Interrupts → look at results and coordinate with each other and proceed to next step
More common I/O method than asynchronous
Synchronous I/O Implementation Method 1
Waste CPU until I/O ends
Only one I/O can happen at each moment
Synchronous I/O Implementation Method 2
Take away CPU from that program until I/O is completed
Because CPU is wasted if just waiting
Line up that program in line waiting for I/O processing
Hand over CPU to other program while waiting
(2) Asynchronous I/O
Cases where work happening in I/O and work happening in CPU don't need to match
After I/O starts, control immediately goes to user program without waiting for I/O work to end
That is, when CPU requests to Disk, does next work regardless of request and result
Both synchronous and asynchronous notify I/O completion through Interrupt
DMA (Direct Memory Access)
Need for DMA
If Interrupt occurs too frequently → overhead
Therefore used to reduce Interrupt frequency
What is DMA?
Mechanism that allows Device Controller to communicate directly with Memory without CPU intervention
Sometimes used to process I/O devices with fast speed at speed close to Memory
That is, DMA directly copies data blocks between memory and device without CPU intervention, reducing Interrupt frequency and improving CPU efficiency
DMA Operation Principle
Since only CPU can access Memory, originally CPU moves every time by Device Controller's Interrupt → sometimes inefficient
Device Controller can transmit data blocks to Memory through DMA, and when data block transmission is completed, DMA Controller generates Interrupt to CPU
To summarize, put one more mechanism called DMA that can directly access Memory → Device Controller directly puts content read from Device into memory without CPU Interrupt → when work ends, interrupts CPU to notify at once
DMA Usage Advantages
If done as above, Interrupt occurs less frequently so CPU can operate efficiently
Different I/O Machine Languages
Two Methods of Performing I/O Through Machine Language
(1) Method of Having Machine Language Dedicated to I/O
When CPU executes machine language, there are separate machine languages for requests
Separate machine language for accessing Memory, separate machine language for performing I/O
(Left side in above picture) Device Addresses + Memory Addresses
(2) Memory Access Machine Language Also Accesses I/O
Extend memory address not only to actual memory but also to I/O devices, and access I/O through machine language that accesses memory
(Right side in above picture) Only Memory Addresses
Last updated