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Converting Mips to binary

  1. Jan 31, 2011 #1
    Hey Guys!!!! Thank you for checking out my Thread.... I am taking a computer assembly class and i have homework due on Wed but i am stuck on a single problem. I have to convert MIPS instructions into binary, I want to learn how to do this because it will be on a test... and i could just go into mars and assemble the code and read it from there but i wanna know how to do it..... okay soo the instructions are:
    addi $sp, $sp, -8
    sw $ra, 4($sp)

    Now from looking into my book i see that addi has an Opcode of 001000 and it has a I instruction format...
    Also sw has an Opcode of 101011 and is an I instruction format

    How do i figure out what registers $sp?, $ra, the array 4($sp) are in binary?

    Please help... i would really appreciate it!
  2. jcsd
  3. Jan 31, 2011 #2


    Staff: Mentor

    See http://en.wikipedia.org/wiki/MIPS_architecture, in the section titled Compiler register usage. $sp is the stack pointer, and its value in MIPS instructions is $29. $ra is return address; its value is $31.

    4($sp) would be an address on the stack - $sp + 4*(size of a machine word).

    Here's another link to a site that lists the instructions and what they look like in binary - http://www.mrc.uidaho.edu/mrc/people/jff/digital/MIPSir.html.

    For your ADDI instruction, the constant to be added is -8, which will need to be written in two's complement form.
  4. Jan 31, 2011 #3
    Thank you for your reply... i have already checked those sites out... but they have not helped me at all....

    So how do i go about converting it into binary code? i think i am missing the simple concept...

    So i take Addi for instance and use its opcode... okay cool got that done but what about the registers?
    all it says on that web site is: Used to add sign-extended constants (and also to copy one register to another: addi $1, $2, 0), executes a trap on overflow
    001000ss sssttttt CCCCCCCC CCCCCCCC

    what the hell does that mean lol
  5. Jan 31, 2011 #4


    Staff: Mentor

    This -001000ss sssttttt CCCCCCCC CCCCCCCC - shows how the instruction op code and registers and operands are encoded to make up an ADDI instruction.

    First 6 bits - the instruction opcode (001000 for ADDI)
    Next 5 bits - the source ($2 in your example)
    Next 5 bits - the destination ($1 in your example)
    Next 16 bits - the immediate value

    ADDI effectively does this $t = $s + imm. value

    So if you wanted to add 5 plus the value in $t1 to $t2, the MIPS assembly instruction would be:
    ADDI $t2, $t1, 5

    Note that the numbers associated with $t0 and $t1 are 8 and 9.

    The binary form of this instuction would be
    Code (Text):

    001000 01001 01000 0000000000000101
    ......| $t1..|
    ............| $t0.|
  6. Jan 31, 2011 #5
    Thank you again for clarifying that...but what if there is no value for the registers i see that i have a constant of -8 which i would use two's complement but for the other $sp register is it defaulted at 0?
  7. Jan 31, 2011 #6


    Staff: Mentor

    Are you asking about ADDI $sp, $sp, -8?

    What do you mean "but for the other $sp register is it defaulted at 0"
  8. Jan 31, 2011 #7
    Yes i am

    addi $sp, $sp, -8
    would that in the end turn to be (opcode) 001000 (default 0 for $sp) 00000 (default 0 for $sp) 00000 1111111111110111 (-8)
  9. Jan 31, 2011 #8


    Staff: Mentor

    0 is not the number for $sp, so the parts with 00000 and 00000 are not right. This link - http://en.wikipedia.org/wiki/MIPS_architecture- tells you the numbers of all the registers. I also told you what it was in post #2.
  10. Jan 31, 2011 #9
    oohhh i think i may understand.... so the value of $sp ( the stack pointer) is $29 sooo
    it would be

    001000 11101 11101 111111111111011
    opcode $sp $sp -8 constant

    did i correctly get the value?
  11. Jan 31, 2011 #10


    Staff: Mentor

    Looks good!
  12. Jan 31, 2011 #11
    okay SWEET!!! soo and for the other one i got
    101011 11111 11111 how do i figure out the 4($sp) ?

    you stated before "4($sp) would be an address on the stack - $sp + 4*(size of a machine word)."

    so do i take 29 + 4 * ( idk what the size of word is) ?
  13. Feb 1, 2011 #12


    Staff: Mentor

    sw $ra, 4($sp)

    What this is doing is to store the word (== 32 bits) at $sp + 4 to the $ra register.

    The size of a machine word seems to be 32 bits, since that's the size of the encoded instruction. Let's go with that assumption.

    According to the reference I cited, the encoded instruction is
    101011 sssss ttttt iiii iiii iiii iiii.

    Here, the s bits hold the source register, the t bits the destination register, and the i bits the offset from the source register. I am thinking now that the offset is in bytes, so we use the offset as-is.

    So the number for $sp is $29, the number for $ra is $31, and the offset is 4. I think this is the encoded instruction for SW $RA, 4($SP)
    101011 11101 11111 0000 0000 0000 0100
    The first 6 bits are the SW instruction. The next 5 bits are the source register, $SP. The next 5 bits are the $RA register, and the last 16 bits are the offset, which is 4.
  14. Feb 1, 2011 #13
    Thank you so much for your help!! You have really helped me understand this!! I am very grateful!!!!
  15. Feb 1, 2011 #14


    Staff: Mentor

    You're very welcome. Thanks are always appreciated!

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