Fetch-Execute Cycle - Step 1

Inside the Computer Main Memory Address Contents 0000 00101011 0001 10001110 0010 10101001 0011 00000000 0100 10001011 0101 00011000 0110 11111111 0111 01100010 1000 00101000 1001 11110000 1010 10101011 1011 10111100 1100 11010000 1101 01000111 1110 00001100 1111 10001001 CPU     Control Unit Program Counter 0001 Instruction Register 01001011 Accumulator 00001110 Instruction Decoder     Arithmetic Logic Unit Add Circuit Subtract Circuit Compare to Zero Circuit etc...

Step 1: The Control Unit fetches the next instruction from main memory. It does this as follows: it looks in the Program Counter for the address of the instruction (in this example, the address is 0001, highlighted in light green), then it goes to that address and brings the contents of that memory location (highlighted in light blue) into the Instruction Register. (Note that the 0s and 1s you currently see in the Instruction Register would be the last instruction that was executed.) it adds one to the Program Counter (so the new value will be the address of the next sequential memory location which will usually contain the next instruction to be executed) Click here to see the result of step 1.