Operating Systems (COMP30023 W1 L1,2)
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We need to understand the ecosystem of where our application is running. Running too slow? Know where to optimize it.
What hardware and software components are involved between recording a video and getting it to viewers?
- CPU memory to process and render the video
- Network protocol to get the recording to the colud
- Hosting the recording on the cloud
- Web application where a user connects
- User downloads and watches it
Topic
- The role of the operating system
- The memory hierarchy (caches, virtual memory)
- Where is it stored?
- Interrupt handling, processes and scheduling
- Multiple processes running at the same time?
- Introduction to multiprocessors and synchronization
- Conflict resolution
- Computer system security and cryptographic protocols
- Introduction to network protocols (OSI model)
- Development of client-server applications
Components of a modern computer
- 1+ processors
- Main memory
- Disks
- Printers
- Keyboard
- Mouse
- Display
- Network interfaces
- I/O devices
These are hardwares. OS seamlessly takes care of everything!
Operating systems
OS is a layer in between the hardware and the software. (The manager that makes sure that the recipe is done in the kitchen)
Examples: Windows, macOS, Unix, Linux, Android… GUI or a command line shell is not an operating system.
- OS Provides an **1. abstraction**
- Abstraction between the hardware resources and the software that is running on it. → The developer can ignore the underlying processes.
- i.e. The main cook in the kitchen that makes sure that the recipe is done
- OS as a **2. resource manager**
- Managing: ensuring that the software get their CPU time, all being run
- Top down view
- Provide abstractions to application programs
- Bottom up view
- Manage pieces of complex system
- Alternative view
- Provide orderly, controlled allocation of resources
- Isolation of the memory given to different programs?
- OS as an **3. extended machine**
- OS turns ugly hardware into beautiful abstractions.
History of OS
- Driven by computer architecture
- The first generation (1945–55) vacuum tubes
- The second generation (1955–65) transistors and batch systems
- The third generation (1965–1980) integrated circuits and multiprogramming
- The fourth generation (1980–present) personal computers
- The fifth generation (1990–present) mobile computers
OS concepts
- Processes (running program)
- Address spaces (memory)
- Files and file system
- Input and Output
- Protection (manage security)
Typical memory hierarchy
Processor/CPU
- Central processing unit (CPU)
- Fetches instructions from registers (memory); executes them (marks the job done)
- Registers keep some process state information
- Two modes of execution:
- Kernel (execute any CPU instruction and reference any memory address) i.e. initiate I/O
- User (only a subset) i.e. add, sub
- CPU: The cooks that are doing the job, Kernel: higher level chef that deals with important ingredients
Processes: program in execution
- Key concept in all operating systems
- At any given time, the number of processes running a given program can be 0, 1, 2, 3, ….
- Assocciated with an address space & set of resources
- A container that holds all info needed to run the program
- A process is dynamic (↔ a program is static)
- The state of a process consists of:
- The code / text of the program
- The values of all the variables in memory and registers
- Address of the current instruction
- Current directory, etc.
- Act of cooking (=process) is dynamic <-> A recipe is a static entity (=program)
- The state of a process consists of:
Typically, number of processes run by a machine > number of CPUs
Multiple processes
- Each process has its own address space
- The state of the process has to be remembered, which requires "complete isolation" between processes
- Process memory has three segments:
- text (program code, “read only”)
- data (constant data strings, global vars)
- stack (local vars like functions and state of the variable)
- Conceptually, each process has its own virtual CPU.
-
In reality, multiple processes share a CPU, each taking a small period of time to run. (multiprogramming)
OS as a process
- Another way of defining the OS: process
- OS ensures that different proesses do not get in each other’s way while they are active at the “same” time
- Provides services such as “read N bytes from this file”, which is used by application programs, utilities, and non-privileged parts of the OS.
User vs. Kernel mode
- The program status word (PSW) register gives the CPU its current mode.
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Code running in User mode
cannot issue privileged instructions, and can only access the parts of memory that is allowed by kernel mode code.
-
Code running in kernel mode
can issue all instructions, and can access all memory.
- Privileged instructions and memory locations: those which access anything that might interfere with another system (i.e. accessing I/O device)
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Code running in User mode
- User / kernel distinction builds the security mechanisms for OS.
System call
- Transition between user and kernel mode is made via system calls
- Allow user programs to ask the OS to execute privileged instructions / memory locations on their behalf.
- Preserves system integrity and security since OS checks the requst before executing them.
- Several privileged things are carried out as a single logical operation to make system calls more convenient to use.
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To the application programmer: system call = call to a privileged function
-
Mode bit (register) helps you distinguish which mode the system is running on.
- Example of a system call: Standard C library handling of write(). The library provides the system-call interface.
Typical system calls
- Process control (load/execute; create and terminate a process; get/set process attributes)
- File management (read/create/delete/open/close a file; get/set file attributes)
- Device management (request/release a device; read/write from a device)
- Information maintenance (get/set time or date)
- Communication (create/delete communication connections)
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