Question

If I am given a MIPS assembly instruction and asked to write the corresponding machine code bit string representation how do I know if it is R Format, I Format, or J Format?

Answer 1

Answer:

R-Type Instructions: These instructions are identified by an opcode of 0, and are differentiated by their funct values. Except for the first 3 shift instructions, these operations only use registers. Note that in addition to arithmetic operations, these instructions also include jumps and the system call instruction.

These instrutcions 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 N/A) (5 bits)
• funct: function code (identifies the specific R-format instruction) (6 bits)

Example:

add $s0, $s1, $s2

J-Type Instructions: These instructions are identified and differentiated by their opcode numbers (2 and 3). Jump instructions use pseudo-absolute addressing, in which the upper 4 bits of the computed address are taken relatively from the program counter.

Example:

j Loop

I-Type Instructions: These instructions are identified and differentiated by their opcode numbers (any number greater than 3). All of these instructions feature a 16-bit immediate, which is sign-extended to a 32-bit value in every instruction (except for the and, or, and xor instructions which zero-extend and the lui instruction in which it does not matter). Branch instructions also effectively multiply the immediate by 4, to get a byte offset.

These instructions have:

• rs: register containing base address (5 bits)
• rt: register destination/source (5 bits)
• immediate: value or offset (16 bits)

Example:

lw $t0, 32($s3)

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