ALU

Abstraction

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• A digital circuit inside CPU that is responsible to carry arithmetic operation like addition and multiplication etc
• ALU takes in operand_1 and operand_2 that are required to carry out the arithmetic operation and it also takes in Opcode which tells the ALU what arithmetic operation to carry out
• Besides outputting the result of the arithmetic operation, the ALU also outputs a status bit, known as a flag, to summarise the operation, such as indicating overflow

MIPS ALU

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Non-branch Instruction

• 4 bits are used for ALUcontrol

Branch Instruction

• PCSrc is 0 for not taking a branch and 1 for taking a branch. isZero? flag is also used to determine the value of PCSrc. isZero? is 1 when a branch should take place

ALUSrc is 0, even though branch instruction is an I-type instruction.

The branch target address isn’t calculated by the ALU!