When MIPS instructions are classified according to coding format, they fall into four categories: R-type, I-type, J-type, and coprocessor. The coprocessor instructions are not considered here.

The classification below refines the classification according to coding format, taking into account the way that the various instruction fields are used in the instruction. The details of the execution activities and the required control signal values depend almost entirely on the instruction type in this classification.

• Non-Jump R-Type
• Immediate Arithmetic and Logic
• Branch
• Load
• Store
• Non-Register Jump
• Register Jump

In the remainder of this web page, the instruction fetch and instruction decode activities are omitted since they are the same for all instructions. The PC update activity only shows updates beyond the standard PC increment (PC ← PC + 4).

The summary at the end of this presentation is available as a PDF document here.

Non-jump R-type instructions include all R-type instructions except jr and jalr. This includes all of the integer arithmetic and bitwise operations, along with the non-branching compare instructions such as slt, sgt, and seq. They use the R coding format. The opcode bits are all 0.

• PC update

  There is no update beyond the normal increment.

• Source operand fetch

  The two source operands are rs and rt.

• ALU operation

  The ALU operation is determined by the function field.

• Memory access

  There is no memory access for data.

• Register write

  The result from the ALU is written to rd.

Most immediate operand instructions perform arithmetic or logical operations using one operand that is coded into the instruction. The immediate operand group also includes the comparison instructions slti and sltiu and the lui instruction. Immediate operand instructions use the I coding format.

• PC update

  There is no update beyond the normal increment.

• Source operand fetch

  The two source operands are rs and the immediate field. For all instructions except sltiu the immediate field is sign extended. For sltiu the immediate field is zero extended. This instruction is not considered in Patterson and Hennessey.

• ALU operation

  The ALU operation is determined by the opcode.

• Memory access

  There is no memory access for data.

• Register write

  The result from the ALU is written to rt.

Branch instructions conditionally branch to an address whose distance is coded into the instruction. Branch instructions use the I coding format.

• PC update

  If the branch condition is true (see ALU operation),
  PC ← PC + 4 + (sign-extended immediate field)<<2.

• Source operand fetch

  The two source operands are rs and rt.

• ALU operation

  The source operands are subtracted for comparison.

• Memory access

  There is no memory access for data.

• Register write

  There is no register write.

Load instructions move data from memory into a register. The address for the load is the sum of a register specified in the instruction and a constant value that is coded into the instruction. Load instructions use the I coding format.

• PC update

  There is no update beyond the normal increment.

• Source operand fetch

  The two source operands are rs and the sign extended immediate field.

• ALU operation

  The two source operands are added to get the memory address.

• Memory access

  A memory read control signal is sent to memory. The result from the ALU is sent to memory as the address.

• Register write

  The data from memory is written to rt.

Store instructions move data from a register into memory. The address for the store is the sum of a register specified in the instruction and a constant value that is coded into the instruction. Store instructions use the I coding format.

• PC update

  There is no update beyond the normal increment.

• Source operand fetch

  The two source operands are rs and the sign extended immediate field. The rt register is also fetched.

• ALU operation

  The two source operands are added to get the memory address.

• Memory access

  A memory write control signal is sent to memory. The result from the ALU is sent to memory as the address. The contents of rt are sent to memory as the write data.

• Register write

  There is no register write.

The only non-register jump instructions are j and jal. Non-register jump instructions use the J coding format.

• PC update

  jr and jalr:

  PC ← target address

  The target address is the concatenation of the high order 4 bits of PC + 4, the target field of the instruction, and two 0 bits.

• Source operand fetch

  There is no source operand fetch.

• ALU operation

  There is no ALU operation.

• Memory access

  There is no memory access for data.

• Register write

  j:	There is no register write.
  jal:	$ra ← PC + 4

The only register jump instructions are jr and jalr. Register jump instructions use the R coding format. The opcode bits are all 0.

• PC update

  jr and jalr:

  PC ← rs

• Source operand fetch

  The only source operand that is used is the rs register.

• ALU operation

  There is no ALU operation.

• Memory access

  There is no memory access for data.

• Register write

  jr:	There is no register write.
  jalr:	rd ← PC + 4