Logics definitions

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Logics definitions
2015-06-01 05:03:23
Definitions logics digital systems

logic and digital systems definitions
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  1. What is a program counter?(PC) (its a type of register btw)
    Contains a binary number representing a location in memory where the next instruction is to be fetched from. Once instruction fetched, program counter is incremented
  2. What is an accumulator?
    Stores the result of the latest operation from the ALU. Its output can also be used as an input to the ALU. 

    Rmb its a type of register
  3. What is an instruction register?
    contains a binary number to tell the ALU what operations to perform.

    Did you know its a type of register
  4. What is the ALU?
    A set of gates and flip flops, arranged as full - adders, shift registers and bit-testers. 

    Precise signal path through ALU is determined by the state of a set of control inputs which are set by the contents of the instruction register.
  5. What defines a computer? And how is it different from a calculator?
    A machine that can perform a sequence of arithmethic and logical operations by following a program of instructions stored within the machine.

    Different to a calculator because it can make decisions which alter the course of its own program.
  6. What is machine code?
    Sequence of instructions built up in RAM memory
  7. What is an OS?
    Commecial software which manages all hardware and gives programs convenient access to all services.
  8. What is the Operation code?(opcode)
    Specifies WHAT equation to perform
  9. What is an operand?
    part which specifies where the relevant data is to be found.
  10. What is the internal BUS?
    The way in which the 3 main registers communicate with one another. A set of time shared conducting paths. Only one register at a time can transmit data onto the bus. All other registers 'listen' even if they dont have to take action.
  11. What is inside the MPU?
    Numbers held for temporary storage in a set of registers. about 4-10 in a typical MPU.
  12. What is a subroutine?
    A section of code that can be 'called' several times by different parts of the main function.

    Normally needs some input data (via registers)

    Leaves its results in registers or memory locations.
  13. What is an interrupt vector?
    A pointer to an ISR (interrupt subroutine)
  14. What is an IRQ
    Interrupt request line. When PIA has new data, it asserts a logic 1.
  15. Pros of Subroutines
    • Changes facilitated
    • Error risk reduced
    • Memory space saved
    • Simplifies the program layout
  16. Cons of subroutines
    Slower to execute than in-line coding due to 'overhead' extra time associated with stack operations
  17. What are jump and branch instructions?
    Alter value in program counter to change program flow
  18. What does the Shell do in an OS and what are its advantages?
    • The user accesses only by the OS shell
    • It constantly creates manages and destroys discrete processes. 

    Advantage: user programmes are easily adapted to run on new hardware
  19. What happens to an OS process?
    • 1. Placed in queue
    • 2. In one of 3 states
    • Running: Has use of CPU
    • Ready: Could use a CPU if one were available
    • Blocked: waiting for some event to happen
  20. What is assembly code?
    a set of mnemonics to represent opcodes with rules to specify operands.