WebEE

MUX5 selects between (7) and (10). Again, the control line for this MUX (not shown) would also come from the ID phase, and is be based on the instruction type.

The Execution phase "executes" the instruction. Any calculations necessary are done in this phase. These calculations are all done by the ALU (Arithmetic and Logic Unit). The internals of these unit is beyond the scope of this tutorial, but you should find an explanation of this unit in any elementary book on digital logic, see for example [2].

Conceptually this stage is very simple. The ALU needs two operands. These either come from the ID phase (8, 9) and thus in turn from the register file (or PC+4 for jump and link instructions), or one operand comes from the ID phase (8) and the other comes from the instruction register (6) to supply an immediate value.

MUX7 selects between (9) and (6). The control line for this MUX is not shown, but would come from the ID phase and is based on the instruction type (it will select (9) for instructions that operate on two registers, and (6) for instructions that work one one register and one immediate value).

(11) is used for the s (store instruction). Consider the following instruction:

s 10(r0), r2 // memory[10+r0] := r2

The calculation 10+r0 will be done by the ALU (so which source does MUX7 select?), and the result of this calculation will be passed on to the MA stage (12). In the MA phase, however, we will also need to know the contents of r2. This is why we need (11).

The purpose of the Memory access phase is to store operands into memory or load operands from memory. So, for all instructions except loads and stores, the MA simply passes on the value from the ALU on to the WB stage (12, 16).

For loads and stores, the MA needs to send the effective address calculated by the ALU to memory (13). For loads, the memory will then respond by sending the requested data back (15). For stores, we need to send the data to be stored along with the effective address (11, 14), and the memory will respond by updating itself.