Table of Contents
Easy Level Questions:
Describe three of the registers used in the fetch-execute cycle including their functions.
– MAR – This stands for memory address register where in the fetch-execute cycle, an address is copied from the PC to the MAR.
– MDR – This stands for memory data register where in the fetch-execute cycle, the contents/ instruction at the memory location contained in MAR are then copied temporarily into the MDR.
– CIR – This stands fro current instruction register where in the fetch-execute cycle, the current contents/instructions of the MDR are then copied and placed into the CIR.
Describe the role of buses and addresses in the Von Neumann software architecture .
– Buses move data (the data bus) around the computer and also send signals (address bus and control bus) to synchronize all operations. Buses can be unidirectional or bidirectional. An address is part of a memory location that uniquely describes where data can be found in the memory map.
If the value 1 1 1 0 1 1 1 0 is to be stored in location 1 1 1 1 0 1 1 1, describe the WRITE operation indicating the role of the MAR, MDR and buses.
– MAR will contain 11110111
– MDR will contain 11101110
– A write signal is sent to the computer memory along the control bus
Name two devices that do not need an operating system and why OS is not needed?
– Washing machine, fridge/freezer, cookers, microwaves/ovens.
– They carry out single tasks that do not vary. Input is a press on button or an option from a touch screen. This activates a simple hardware function which doesn’t need an OS to control it.
What is stored program concept?
– Both data and program instructions are stored temporarily in main memory, and then fetched, decoded and executed by the processor.
Memorize this fetch-execute cycle!
1. The program counter (PC) contains the address of the memory location of the next instruction which has to be fetched.
2. This address is then copied from the PC to the memory address (MAR), this is done using the address bus.
3. The contents (instruction) at the memory location (address) contained in MAR are then copied temporarily into the memory data register (MDR).
4. The contents (instructions) of the MDR are then copied and placed into the current instruction register (CIR).
5. The value in the PC is then incremented by 1 so that it now points to the next instruction which has to be fetched.
6. The instruction is finally decoded and then executed by sending out signals (via the control bus) to the various components of the computer system.
7. The process repeats itself from step 1.
What does the PC contain?
– THE PROGRAM COUNTER (PC) contains the address of the next instruction to be executed.
Medium Level Questions:
The CIR contains what?
– The CURRENT INSTRUCTION REGISTER (CIR) contains the current instruction being procesed.
How does a computer carry out a set of instructions?
– Do process known as fetch-execute cyle.
– To carry out a set of instructions, the processor first of all FETCHES some data and instructions from memory and stores them in suitable registers. Both the address bus and the data bus are used in this process. Once this is done, each instruction needs to be decoded before finally being EXECUTED.
What does Accumulator do?
– Holds the results of processing.
How does program counter work?
– Every instruction is fetched from external memory at the address in the program counter, and stored in the instruction register
– Program counter holds the memory address of the next instruction to be fetched from main memory
What is program counter?
– A program counter is a register in a computer processor that contains the address (location) of the instruction being executed at the current time. As each instruction get fetched, the program counter increases its stored value by 1.
Give important example of register and its use?
– One important register is the program counter. This keeps track of the running order of the instructions and shows which instruction in the program is due to be executed next.
What are registers?
– Registers are also called internal memory or immediate access memory stores.
– A register is a small amount of fast temporary memory within the processor where the ALU or the CU can store and change values needed to execute instructions.
How does control unit work?
– Essentially, 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)). This instruction is then interpreted. During that process, signals are generated along the control bus to tell the other components in the computer what to do.
What is the use of control unit?
– The CONTROL UNIT controls the operation of the memory, processor and input/output devices.
Explain the structure of MDR?
– The Memory Data Register is half of a minimal interface between a microprogram and computer storage, the other half is a memory address register.
What is MDR?
– The Memory Data Register (MDR) is the register of a computer’s control unit that contains the data to be stored in the computer storage (e.g. RAM), or the data after a fetch from the computer storage.
– It acts like a buffer and holds anything that is copied from the memory ready for the processor to use it.
Mastery Level Questions:
Explain the structure of MAR?
– The Memory Address Register is half of a minimal interface between a microprogram and computer storage. The other half is a memory data register. Far more complex memory interfaces exist, but this is the least that can work.
How does MDR work?
– When writing to memory, the CPU writes data from MDR to the memory location whose address is stored in MAR.
Define MAR?
– (Memory address register) MAR holds the memory location of data that needs to be accessed. When reading from memory, data addressed by MAR is fed into the MDR and then used by the CPU.
What are the different kinds of memory unit and registers?
– MAR memory address register
– MDR memory data register
– ALU arithmetic and logic unit
– PC program counter
– CIR current instruction register
Describe control bus and its direction?
– Carries signals relating to the control and coordination of all activities within the computer.
– This is regarded as being both unidirectional and bi-directional due to the internal connections within the computer architecture.
Describe data bus and its direction?
– Sends data between the processor, the memory unit and input/output devices.
– Bi-directional (data can travel in both directions).
Describe address bus and its direction?
– Carries signals relating to addresses between the processor and the memory.
– Unidirectional (signals travel in one direction only)
What are the three different types of bus?
– address bus
– data bus
– control bus
Give example how buffer is used?
– For example, when you download an audio or video file from the Internet, it may load the first 20% of it into a buffer and then begin to play. While the clip plays back, the computer continually downloads the rest of the clip and stores it in the buffer. Because the clip is being played from the buffer, not directly from the Internet, there is less of a chance that the audio or video will stall or skip when there is network congestion.
What is a buffer and what does it do?
– BUFFERS are used in computers as a temporary memory area.
– A buffer contains data that is stored for a short amount of time, typically in the computer’s memory (RAM).
– The purpose of a buffer is to hold data right before it is used.
What is second level interrupt handler? How does it work?
– The functionality of SLIH is they complete long interrupt processing tasks, just like processes. SLIHs either have a kernel thread which is dedicated for each handler, or are executed by a pool of kernel worker threads. Just like processes, these handlers sit on a run queue in the operating system until the processor time is available for them to perform processing for the interrupt.
– SLIHs, unlike FLIH, can be scheduled similarly to processes and threads.
What is first level interrupt handler? How does it work?
– This type of interrupt handler is the faster of the two, it also has more jitter while process is getting executed and they are mainly maskable interrupts.
– The functionality of FLIH is to quickly service the interrupt, or to record platform specific critical information (that is only available at the time of the interrupt) and then schedule SLIH execution for further (more slowly) interrupt handling.
What cause jitter?
– It can be caused by a number of different factors, such as collisions, signal interference, and network congestion etc.
Define jitter?
– Jitter refers to small intermittent delays during data transfers.
What are the two types of interrupt handlers?
– First level interrupt handler (FLIH)
– Second level interrupt handler (SLIH)
What are the functions of interrupts handler?
– The interrupt handler is the part of the operating system which is responsible for dealing with interrupt signals.
– The interrupt handler prioritises interruptions as they are received, placing them into a queue as necessary.
– For every interruption, the current task needs to be stopped, with it’s status saved (so it can resume later).
The functions and speed of interrupts handler depend on what?
– Interrupts have a number of functions, which ultimately depend on what triggered the interrupt in the first place, therefore the speed of which an interrupt handler completes its tasks also varies depending on what triggered the interrupt.
How does interrupts handler work?
– Hardware and software interrupts or software exceptions initiate the interrupt handler, furthermore the interrupt handler is used to implement device drivers or transitions between protected modes of operation, such as system calls.
Define interrupts handler?
– Interrupt handlers, also commonly known as Interrupt service routine (ISR), is a block of code that is associated with a specific interrupt condition.
Define asynchronous interrupt?
– Interrupts that are independent to the system clock and are not in phase to it.
Define synchronous interrupt?
– Interrupts that are dependent and in phase to the system clock.
Define aperiodic interrupt?
– Interrupts that cannot be predicted.
Define periodic interrupt?
– Interrupts that occur at a fixed interval in timeline.
Define race condition?
– Refers to an undesirable situation that occurs when a device or system attempts to perform two or more operations at the same time, though these operations must be done in a proper sequence to get executed properly.
Define spurious interrupts?
– These types of interrupt are categorised to be invalid, short-duration signal on an interrupt input. These types of interrupts are caused by glitches that are a result of electrical interference, race conditions, or malfunctioning devices.
Define non-maskable interrupt?
– The hardware interrupts which cannot be delayed, and therefore require the processor to process them immediately.
Define maskable interrupt?
– The hardware interrupts can be delayed when a high priority interrupt has occurred at the same time.
Examples of hardware interrupts?
– 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 – an example could be <CTRL><ALT><BREAK> keys pressed simultaneously
Give example of software interrupts handling an error while the program is running?
– For example, if a program expects a variable to be a valid number, but the value is null, an interrupt may be generated to prevent the program from crashing.
– It allows the program to change course and handle the error before continuing. Similarly, an interrupt can be used to break an infinite loop, which could create a memory leak or cause a program to be unresponsive.
– a software error has occurred – an example of this would be if an .exe file couldn’t be found to initiate the execution of a program.
Software interrupts are used for what?
– Used to handle errors and exceptions that occur while a program is running.
What are interrupts basic operations?
1. CPU issues a read command.
2. Whilst the CPU processes, the input/output module get the data from the peripheral.
3. Input/output request data.
4. The CPU will request the data,
5. Input/output transfers data.
Define interrupt signals?
– A response to software or hardware events in the system. These events are classified as software interrupts or hardware interrupts.
Define IRQ?
– Stands for Interrupt Request Line. Refers to a hardware signal sent to the processor that temporarily stops a running program and allows the interrupt handler to run instead.
A hardware interrupt is often created by what? Give example!
– A hardware interrupt is often created by an input device such as a mouse or keyboard.
– For example, if you are using a word processor and press a key, the program must process the input immediately. Typing “hello” creates five interrupt requests, which allows the program to display the letters you typed.
– Similarly, each time you click a mouse button or tap on a touchscreen, you send an interrupt signal to the device.
Define interrupt?
– A signal sent to the processor that interrupts the current process. It may be generated by a hardware device or a software program.
What are the 2 examples of user inferfaces?
– Graphical User Interface (GUI)
– Command Line Interface (CLI)
What is a user interface?
– The boundary between the user and a computer system. It allows us to communicate with the computer, and vice versa.
What are the functions of operating system?
– Provides an interface for computer interaction
– Control of inputs and outputs
– Interrupt and error handling
– Management of hardware and peripherals
– Management and loading of software
– File management (copy, save, sort, delete)
– Processor management for multitasking
– Memory management
– Security (access rights, user accounts, firewalls etc)
– Network communication
Give examples of operating systems?
– Microsoft Windows
– Linux
– iOS
– Mac OS
– Android
– UNIX
– DOS
An operating system usually provides?
– Provide a graphical user interface (GUI) to help users to interact with the computer, however successful operating systems can also function from a command line interface (CLI).
An operating system (OS) is the computer program (software) responsible for what?
– For the operation and smooth running of a computer.
Define operating system (OS)?
– Essentially software running in the background of a computer system. It manages many of the basic functions.
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