MIPS Instruction Formats

• All MIPS instructions are encoded in binary.
• All MIPS instructions are 32 bits long.
  • (Note: some assembly langs do not have uniform length for all instructions)

  Examples:

              001000 10011010100000000000000100
              000010 00000000000000000100000001
              000000 10001100101000000000100000
              100011 10011010000000000000100000
              000100 01000000000000000000000101
  

• All instructions have an opcode (or op) that specifies the operation (first 6 bits).
• There are 32 registers. (Need 5 bits to uniquely identify all 32.)
• There are three instruction categories: I-format, J-format, and R-format (most common).

• • I-Format:

    op rs rt immediate   Example: addi $t2, $s3, 4 001000 10011 01010 0000000000000100

  • J-Format:

    op address   Example: j LOOP       (or j 1028) 000010 00000000000000000100000001

  • R-Format:

    op rs rt rd shamt funct   Example: add $s0, $s1, $s2 000000 10001 10010 10000 00000 100000

Typical Instruction Formats

Basic I-format Instructions

• Have 2 registers and a constant value immediately present in the instruction.
  • rs: source register (5 bits)
  • rt: destination register (5 bits)
  • immediate value (16 bits)

Instructions that follow typical format:

NOTE: Order of components in machine code is different from assembly code. Assembly code order is similar to C, destination first. Machine code has source register, then destination.

Example

        addi $t2, $s3, 4       (registers 10 and 19)

• Exceptions: beq, bne, lw, sw, lui (See format exceptions)

J-format Instructions

• Have an address (part of one, actually) in the instruction.

Example

        j LOOP

The address stored in a j instruction is 26 bits of the address associated with the specified label. The 26 bits are achieved by dropping the high-order 4 bits of the address and the low-order 2 bits (which would always be 00, since addresses are always divisible by 4).

    address = low-order 26 bits of (addrFromLabelTable/4)

In the example above, if LOOP is at address 1028, then the value stored in the machine instruction would be 257 (257 = 1028/4).

Basic R-format Instructions

• Have op 0. (all of them!)
• Also have:
  • rs: 1st register operand (register source) (5 bits)
  • rt: 2nd register operand (5 bits)
  • rd: register destination (5 bits)
  • shamt: shift amount (0 when not applicable) (5 bits)
  • funct: function code (identifies the specific R-format instruction) (6 bits)

NOTE: op is 0, so funct disambiguates

Example

        add $s0, $s1, $s2       (registers 16, 17, 18)

NOTE: Order of components in machine code is different from assembly code. Assembly code order is similar to C, destination first. Machine code has destination last.