Fetch decode-execute presentation

Fetch decode-execute presentation

• 1.
  Fetch-Decode-Execute Cycle
• 2.
  THE FETCH –EXECUTE CYCLE Both the data and the program that acts upon that data are loaded into main memory (RAM) by the operating system. The CPU is now ready to do some work.
• 3.
  Steps of theFetch/Execute Cycle: • Get the next instruction • Figure out what to do • Gathering the data needed to do it • Do it • Save the result, and • Repeat (billions of times/second)!
• 4.
  Fetch Cycle • TheProgram Counter (PC) contains the address of the next instruction to be fetched • The address contained in the PC is copied to the Memory Address Register (MAR). • The instruction is copied from the memory location contained in the MAR and placed in the Memory Buffer Register (MBR). • The entire instruction is copied from the MBR and placed in the Current Instruction Register (CIR) • The PC is incremented so that it points to the next instruction to be fetched
• 5.
  Execute Cycle • Theaddress part of the instruction is placed in the MAR • The instruction executed is decoded and • The processor checks for interrupts (signals from devices or other sources seeking the attention of the processor) and either branches to the relevant interrupt service routine or starts the cycle again.
• 6.
  CU 100 101 100 PC Program Counter IR Multiply no. Instruction in 500,501 Register ALU Acc Accumulator 378 Multiply no. in 500,501 101 Store result in 502 500 21 501 18 502 1.The PC contains the address of location 100 2.CU fetches instruction in location 100 3. Make a copy of the instruction into the IR 4. Increment the PC by 1 5. Activate the right circuits to execute the instruction
• 7.
  CU PC 100 102 101 Program Counter Store result Multiply no. Instruction IR inin502 500, 501 Register ALU Acc Accumulator 378 Multiply no. in 500,501 101 Store result in 502 500 21 501 18 502 378 1. The PC contains the address of location 101 2. CU fetches instruction in location 101 3. A copy of the instruction is saved in the IR 4. Increment the PC 5. Activate the right circuits to execute the instruction