Arithmetic Logic Unit (ALU)

Arithmetic logic unit

Mathematical operation

Add sub compare increment

2 inputs (example)

CU gets instruction from ram

Tells alu operation

Adds inputs

Outputs -> Register (like ram but nearby)

The register won't save until the CU turns on the set wire

When the set wire turns on it sets the  register = to the current output

Enable wire from CU to register, makes register output what ever is saved -> connects to CPU Bus

CPU Bus connects components inside cpu has more registers with set and enable, CU turns on set for whatever register it wants to save the output to CU then turns off enable wire to clear register of ALU

Some registers for just storing info nearby CU

ALU uses temporary register for input doesnt need enable because it only outputs ot ALU

Other input from cpu bus directly stays on bus until ALU is done

CU stores instruction in instruction register only outputs to CU

Flags turn on or off based on conditions

Flag register 1 input and output per flag

Ex flags are A is larger, equal, B is larger

Instruction address register used to know where the next instruction should come from in ram

Instruction address goes to Memory address register tells ram what memory address the cpu wants

CU turns on enable ram wire ram sends data at address instruction goes to instruction register

CU control wires: keyboard, display etc

Compare

Gives flags instead of output

From https://www.youtube.com/watch?v=cNN_tTXABUA

The combinational logic circuitry of the 74181 integrated circuit, which is a simple four-bit ALU… From wikipedia

1 bit alu from https://www.youtube.com/watch?v=WZAbZR4ef-E