Operating systems and computer architecture

Operating systems and computer architecture

1. All modern computers have some form of operating system - this makes it possible to communicate with the software and hardware making up a computer system

2. Operating systems (OS)

   • Software running in background of a computer system, managing many basic functions
     • Not every OS carries out all these function, but they're examples of what they can do
   • Functions
     • Human computer interface (HCI)
     • Multitasking
     • Multiprogramming
     • Batch processing
     • Error handling
     • Loading and running applications/software
     • Managing user accounts
     • File utilities (e.g copy, save, delete, sort, etc)
     • Processor management
     • Memory management
     • Real-time processing
     • Interrupt handling routines
     • Security (manages log on, passwords)
     • Input and output control
   • A common example is Windows, which is used on many PCs (personal computers). Other examples include Android, UNIX and DOS. Windows is an example of a single user multitasking OS - only one user can use the computer at a time, but many applications can be open simultaneously
     • When a computer is first powered up, the initiating programs are loaded into memory from the ROM (Read Only Memory) chip. These programs run a checking procedure to make sure the hardware, processor, internal memory and bios (basic input-out-put systems) are functioning correctly. If no errors are detected, the OS is loaded into memory
       • Simple devices with embedded microprocessors, such as cookers, microwave ovens and washing machines don't typically have operating systems

3. Interrupts and buffers

   • Interrupt: signal sent from a device or software to a processor. This causes the processor to temporarily stop what it's doom and service the interrupt
     • Interrupts can occur if: a disk drive is ready to receive more data, an error has occurred, such as a paper jam in a printer, the user has pressed a key to interrupt the current process or a software error has occurred, e.g an .exe file couldn't be found to initiate execution of a program
       • Once the interrupt signal is received, the processor either carries on, or stops to service the device/program that sent the interrupt signal.
         • Interrupts allow computers to carry out many tasks, or to have many windows open at the same time, e.g downloading a file from the Internet whilst listening to music from the computer library simultaneously. An interrupt handler saves the status of the current task being run when an interrupt us being serviced, and when the interrupt has been serviced, the status of the interrupted task is reinstated and continues from the point prior to being interrupted
   • Buffers
     • Used in computers as a temporary memory area. These are essential in modern computers since hardware devices operate at much slower speeds than the processor
       • Without buffers, processors would spend the majority of their time idle, waiting for the hardware device to complete its operation.
         • Buffers are basically filled from the processor or memory unit, and while these are emptied to the hardware device, the processor carries on with other tasks. Buffers are used when streaming a video from the Internet for example, which ensures that the video playback doesn't keep on stopping to wait for data from the Internet. Buffers and interrupts are typically used in conjunction with each other to carry out basic computer functions, which users take for granted

4. Computer architecture

   • Very early computers were fed data while the machines were actually running - they couldn't store programs, so couldn't run without human intervention. Roughly about 1945, John von Neumann developed the idea of a stored program computer, often referred to as the von Neumann architecture concept. He had the idea to hold programs and data in a memory, and data would then move between the memory unit and processor.
     • Buses are microscopic wires that move data round the computer and make sure everything is properly synchronised
   • Buses
     • Address bus: Carries signals relating to addresses between processor and memory. Unidirectional
     • Data bus: Sends data between processor, memory unit and input/output devices. Bi-directional
     • Control bus: Carries signals relating to coordination and control of all activities within the computer. Unidirectional and bi-directional
   • Addresses: indicate where the data is stored. Each address is unique, and is stored in the memory unit
     • Register: High speed storage area within the computer. All data must be represented in a register before it can be processed, e.g if two numbers are to be added, both numbers have to be stored in registers and the result of the addition must be stored in a register. Example registers: MAR, MDR, ALU, PC, CIR

5. Memory unit

   • Made up of a number of partitions - each partition consists of an address and its contents (contents are binary values)

6. Processor

   • Contains the arithmetic and logic unit (ALU). The ALU allows arithmetic (e.g add, subtract, etc) and logic (e.g AND, NOT, etc) operations to be carried out

7. Control unit

   • Controls operation of memory, processor and input/output devices
     • Basically, the control unit reads an instruction from memory (the address of the location where the instruction can be found is stored in the Program Counter (PC), which is then interpreted. During that process, signals are generated along the control bus, telling the other components in the computer what to do

8. Input and output devices

   • Main method of entering data and getting data out of computer systems. Input devices convert external data into a form the computer can understand and can then process (e.g keyboards, touchscreen, microphones). Output devices show the results of the computer processing in a humanly understandable format (e.g printers, monitors, loudspeakers)

9. Fetch-execute cycle

   • To carry out a set of instructions, the processor firstly fetches some data and instructions from memory and stores them in suitable registers. Both the address bus and data bus are used in this process. Once this is done, each instruction needs to be decoded, before finally being executed - this is all known as the fetch-execute cycle
     • The CIR (Current Instruction Register) contains the current instruction being processed, and the Program Counter (PC) contains the address of the next instruction to be executed
   • The PC contains the address of the memory location of the next instruction to be fetched
     • This address is then copied from the PC to the MAR (Memory Address Register), done using the address bus
       • The contents (instructions) at the address contained in MAR are then temporarily copied into the MDR (Memory Data Register)
         • The instructions in the MDR are then copied and placed into the CIR
           • The PC value is incremented by 1, so it now points to the next instruction to be fetched
             • The instruction is finally decoded and then executed by sending out signals via the control bus to the various components of the computer system

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