Overview of the MIPS Instruction Set

The MIPS instruction set consists of 32-bit fixed-length instructions, divided into several types, each with a specific field format.

Main Instruction Types:

• R-type Instructions: Used for operations between registers.

  • opcode | rs | rt | rd | shamt | funct

• I-type Instructions: Used for operations between registers and immediate values or for memory access.

  • opcode | rs | rt | immediate

• J-type Instructions: Used for jump operations.

  • opcode | address

• Branch Instructions: A subtype of I-type instructions, used for conditional jumps.

  • opcode | rs | rt | offset

R-type Instructions

R-type instructions are used for operations between registers, where both operands and the result are stored in registers. They are commonly used for arithmetic and logical operations.

Format:

| opcode (6 bits) | rs (5 bits) | rt (5 bits) | rd (5 bits) | shamt (5 bits) | funct (6 bits) |

• opcode: The operation code, fixed as 000000.
• rs: Source register 1.
• rt: Source register 2.
• rd: Destination register.
• shamt: Shift amount, set to 00000 for non-shift operations.
• funct: Function code, specifying the exact operation.

Example: add $s0, $t0, $t1

This instruction adds $t0 and $t1, storing the result in $s0.

• opcode: 000000
• rs: $t0 (01000)
• rt: $t1 (01001)
• rd: $s0 (10000)
• shamt: 00000
• funct: 100000 (Addition)

Machine code:

000000 01000 01001 10000 00000 100000

Hexadecimal representation: 0x01098020

I-type Instructions

I-type instructions are used for operations between registers and immediate values, or for loading and storing memory operations. These instructions contain a 16-bit immediate value or address offset.

Format:

| opcode (6 bits) | rs (5 bits) | rt (5 bits) | immediate (16 bits) |

• opcode: The operation code, specifying the instruction type.
• rs: Source register or base register.
• rt: Destination register.
• immediate: Immediate value or address offset.

Example 1: addi $t0, $0, 10

This instruction adds the immediate value 10 to $0 (which is always 0), storing the result in $t0.

• opcode: 001000 (addi)
• rs: $0 (00000)
• rt: $t0 (01000)
• immediate: 0000000000001010 (binary for 10)

Machine code:

001000 00000 01000 0000000000001010

Hexadecimal representation: 0x2008000A

Note: While the assembly format for addi $t0, $0, 10 places the destination register $t0 before the source register $0, the machine code is generated in the order opcode | rs | rt | immediate, where the source register (rs) comes before the destination register (rt). This design difference is based on architectural considerations.

Example 2: lw $t0, 4($t1)

This instruction loads a word from the address $t1 + 4 into $t0.

• opcode: 100011 (lw)
• rs: $t1 (01001)
• rt: $t0 (01000)
• immediate: 0000000000000100 (binary for 4)

Machine code:

100011 01001 01000 0000000000000100

Hexadecimal representation: 0x8D280004

J-type Instructions

J-type instructions are used for unconditional jumps to a specified address.

Format:

| opcode (6 bits) | address (26 bits) |

• opcode: The operation code, indicating a jump instruction.
• address: The target address (excluding the upper 4 bits and lower 2 bits).

Example: j 0x00400000

This instruction jumps unconditionally to the address 0x00400000.

• opcode: 000010 (j)
• address: 00000000010000000000000000

Machine code:

000010 00000000010000000000000000

Hexadecimal representation: 0x08000000

Branch Instructions

Branch instructions are used for conditional jumps and are commonly found in control flow for conditions and loops.

Format:

| opcode (6 bits) | rs (5 bits) | rt (5 bits) | offset (16 bits) |

• opcode: The operation code, indicating the branch type (e.g., beq, bne).
• rs: Source register 1.
• rt: Source register 2.
• offset: The offset, relative to the next instruction’s address.

Example: beq $t0, $t1, label

If $t0 equals $t1, this instruction branches to label.

• opcode: 000100 (beq)
• rs: $t0 (01000)
• rt: $t1 (01001)
• offset: 0000000000000010 (assuming an offset of 2)

Machine code:

000100 01000 01001 0000000000000010

Hexadecimal representation: 0x11090002