Situation:

The variable x is stored at location 200 in memory
The variable y is stored at location 204 in memory
The variable z is stored at location 206 in memory

How does the CPU execute z = x + y using its registers ?

Pre-requisite: CPU must fetch the operands into a register before it can perform any operation

In order to compute x + y:
      We must fetch the variable x from location 200 in memory into a register
      We must fetch the variable y from location 204 in memory into another register

Step 1: CPU executes a "load from memory 200" operation and fetches the variable x into a register:

The CPU sends out control signals to fetch x from memory:
      CPU sends out a "Read" operation to memory
      CPU sends out the location 200 (x) (in binary) to memory

The memory will send the value in x which the CPU will save it in a register (R0)

Step 2: CPU executes a "load from memory 204" operation and fetches the variable x into a register:

The CPU sends out control signals to fetch y from memory:
      CPU sends out a "Read" operation to memory
      CPU sends out the location 20y (y) (in binary) to memory

The memory will send the value in x which the CPU will save it in a (different) register (R1)

Step 3: Only now can the CPU executes an "add" operation and add the copies of x and y together:

The result of the operation is (always) stored in some register (R2 in the example) !!!

Step 4 (final): CPU executes a "store to memory 206" operation and save the result into the variable z:

The CPU sends out control signals to write (update) z in memory:
      CPU sends out a "Store" operation to memory
      CPU sends out the location 206 (z) (in binary) to memory

The memory will update the value in z (location 206)