ively. A General-purpose registers that are used for performing arithmetic functions. A Current instruction registers that contain both the operator and the operand of the current instruction. The Program Counter is the register that holds the address of the next instruction to be carried out these instruction are automatically incremented to the next 'instruction. But when the current instruction is a branch or jump instruction, then that address is copied from the instruction to the Program Counter. The Program Counter is copied to the Memory Address Register which hold the address of the memory locations from which information will be read or to which data will be written and occasionally. It will hold the address of the instruction in the fetch cycle and the information to be used in an instruction in the execute cycle.Memory data registers are used to temporarily store information read from or written to the memory. Data goes here before it goes to the Current Instruction Register where it is decoded. Once the instruction has been decoded the operand of the instruction is put in the MAR and the data will then be copied to the MDR. Any transfers of data from memory go via the MDR. The MDR and the MAR serve the system as screen registers, this allows for the difference in speed between the CPU and the memory.The CIR or Current Instruction Register is where the instruction is copied to it holds both the operator and the operand of the current instruction If the Fetch / Execute cycle is interrupted by more information then it will "stack" the cycle between the fetch and execute phase then deal with the new data and return to the interrupted cycle. The test for interrupts is only carried out at the end of each instruction cycle.When the item in the MDR is added to the Accumulator the whole operation carries on returning to the fetch cycle. The Accumulator is the register that carries out arithmetical functions.The status registers co...
