The von Neumann Model – Chapter 4 COMP 2620 Dr. James Money COMP 2620

The LC-3 as a von Neumann Machine

Instruction Cycle Each instruction is handled in a systematic way through a sequence of steps call the instruction cycle Each step is called a phase There are six phases to the cycle

Instruction Cycle The six phases of the instruction cycle are: Fetch Decode Evaluate Address Fetch Operands Execute Store Result

Instruction types There are three instruction types: Operate instruction – processes data, such as ADD Data movement instruction – moves data from one place to another, such as LDR Control instruction – alters the flow of execution in a program, such as JMP

Instruction types The control instruction changes the sequence of execution in the processing unit Normally, the MAR is loaded from the PC at the beginning of processing Thus, the control instruction must affect the PC value during the EXECUTE phase We use this generate loops and branches

Example: LC-3 JMP Instruction Set the PC to the value contained in a register. This becomes the address of the next instruction to fetch. “Load the contents of R3 into the PC.”

Control of Instruction Cycle We’ve described the six phases, each having a number of steps For example, the FETCH instruction requires loading the MAR issuing a read memory command reading MDR -> IR Recall the finite state machine is overseeing all the operations

Control of Instruction Cycle

Control of Instruction Cycle Remember, processing starts in state 1 The FETCH takes three clock cycles MAR is loaded with PC and PC=PC+1 Finite state machine asserts GatePC and LD.MAR PCMUX selects the +1 line and assert LD.PC signal for output MDR is loaded with instruction from memory MDR -> IR Asserts GateMDR and LD.IR

Control of Instruction Cycle DECODE takes one cycle in state 4 IR is used as input to for high order 4 bits(IR[15:12]) to finite state machine Execution continues until each instruction completes and the next state is set to state 1 again

Stopping the Computer The processing unit continues to run forever We need a way to stop processing so we can turn off the computer(that is, besides using the power switch itself) Even if a program finishes, others are waiting to run by the operating system To do this, we must stop the clock

Stopping the Computer The clock defines one machine cycle This allows the finite state machine to run precisely We must stop the clock to stop the instruction cycle

Stopping the Computer The clock generator is a crystal oscillator, which is a piezoelectric device For our purposes, we consider it a black box that produces oscillating voltage

Stopping the Computer The clock circuit is shown below. It have a RUN latch which controls the clock

Stopping the Computer If the RUN latch is in the 1 state, that is Q=1, the clock output is sent out If the run latch is 0 (Q=0), then the clock circuit outputs 0 So we need only clear the RUN latch to stop execution Typically this is done with a HALT instruction