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In more detail:

EX

• In arithmetic instructions, this is when the calculation is done
• In load or store instructions, the effective address is calculated
• In conditional branches, this is the natural point for the comparison
• For computed jumps, this is where the branch target address is calculated

MEM

• In a store, the calculated address and source register are sent to memory.
• In a load, the calculated is sent to memory
• In a branch, the PC is set
• For arithmetic operations, this stage is inactive

WB

• For arithmetic and load instructions, write the result to the register specified
• For branch, this stage is inactive