The role of machine code in the execution of a program

Machine code and CPU operation
Instruction format

  • An instruction is the command given to the central processing unit by a computer program
  • Instructions are made up of an operation code and a memory address
  • An operation code is a machine lanugage command for a single operation
  • The control unit inteprets this code to determine the appropriate action
  • The CPU contains permenant microdcode instructions which cannot be modified

Use of registers and accumulators

  • A register is a temporary storage location within central processing unit
  • Any data leaving the main CPU goes through the register buffer
  • An accumulator is a register which stores the result of the latest computation carried out by the CPU or the last data that is about to be computed
  • After being in the accumulator, the data must be moved to a different memory location

The fetch-execute cycle

  • The fetch-exexute cycle is sperated into four steps:
    1. Fetch instruction from primary memory and place in instruction register
    2. set program counter to the next instructions adress
    3. Decode the instruction (done by control unit)
    4. Load operands from memory into a register
    5. Execute the instruction and store result in accumulator (generally the ALU will execute the task)
    6. Reset the cycle to recommence the cycle

Use of a program counter and instruction register

  • The program counter is a register that is part of the computers CPU
  • The program counter stores the address of the next instruction to be executed
  • Instructions are generally stored sequentially unless the instruction is to change the program counter
  • The instruction register is where the instruction either being decoded or executed is stored

Execution of called routines

Linking, including use of DLLs

  • Linking alllws for a machine code program to be combined with another machine code porgram
  • Because larger programs are modular in nature, the linker is used to join these modules
  • Since programs must be in the RAM to be executed, linkers save resources because only the needed modules are loaded
  • DLL’s or Dynamic link libraries is a library of machince code programs which can be accessed by others to perform specific tasks
  • DLL files allows programs common acess to resources and provide efficent use of the memory

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