These are the basic steps for mips processor on single cycle for add operation.
Single cycle means that all operations are performed in just one cycle of clock, where the machine control just need to be a single decoder machine activating or deactivating respective signals according to operation to perform.
When pc (program counter) perform its operation (pc=pc+4) the next instruction is fetched, and the complete word of 32 bits is split depending of the kind of operation to perform, in this case the add operation have six (31 to 26 ) bits to control machine to decode all control signals to perform this operation and another five bits (5 to 0) to know wich operation to do.
ALUCtrl: According to the opcode and function, the machine control generates the specific decodification to the ALU, that allow execute determinated operation, in this case ADD.
RegisterWrite: This signal allows write/read to/from the register bank. In this case the ADD operation takes two values from the register bank. In addi operation (Inmediate Adition) just take one value from the register bank and the other is constant.
RegisterDST: Basically is the control signal of a multiplexor that choose between write in register bank directly or put the address later of operation ADD.
ALUsource: Basically is a control signal of a multiplexor that choose between let to pass directly from the register bank or constant value from the complete word.