System Architecture

System Architecture

1. FDE CYCLE

   • Fetch instructions
     • Essentially, all a CPU does is carry out instructions, one after another, billions of times a seconds. The Fetch-Decode-Execute cycle describe how it does it.
       • 1)Copy memory address from the program counter to the MAR
         • 2)Copy the instructions stored in the MAR address to the MDR.
           • 3)Increment (increase) the program counter to the point to the address of the address of the next instructions, ready for the next cycle
     • Decode instructions
       • The instruction in the MDR is decoded by the CU. The CU may then prepare for the next step, e.g by loading values into the MAR or MDR
     • Execute instruction
       • The instructions is performed. This could be: Load data from memory, write data to memory, do a calculation or logic operation (using the ALU), change the address in the PC, or halt the program

2. Registers

   • Accumulator - The part of the ALU that stores the intermediates results when doing a calculation