Scenaroi #2: ALU & Memory

So far, we have only been using data from a limited number of CPU registers. Actual computers have much more storage in their main memory. Data gets from memory into the registers through additional communication channels.

Open the Knob & Switch Datapath and Memory Simulator in a new browser window.

Two notable components here are the memory and bus switches. The memory values on the left of the simulator represent RAM. The button next to each memory location indicates which location will be read (R) from or written to (W) in one cycle of the machine.

The bus connections from the ALU to the register bank now have switches on them. You can click these connections to toggle (open and close) the switches. Note that an open switch looks like an open door. No signal will flow across an open switch. A closed switch allows the signal to flow across it.

We also have two new connections along the C bus. The C bus is the pathway into the register bank. The connection pointing up from the Main Memory Bus into C bus allows us put memory values into registers, rather than ALU results.

Open the circuit from the ALU into the C bus, so that there is no longer a connection.

Click to close the connection from the Main Memory bus to the C bus. The connection from the C bus to the memory bus should be open.

Put the value 42 into memory location 0, and select it as the RW location.

Set the connection switch along the C Bus into the register bank so that it is closed. Select the C Bus address to be R0.

Q1: Based on these settings, what do you think will happen when you click Execute?

Q2: Test your prediction using the simulator. Did your hypothesis hold true? Why or why not?

Q3: What settings (bus addresses, ALU operation, switches, and memory RW) would result in R0 minus R3 being stored in memory location 4?

Q4: Set R0 = 23 and R3 = 16 to test your Q11 answer. Did your hypothesis hold true? Why or why not?