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How can I translate this mips code into binary/hexadecimal

  1. Dec 26, 2011 #1
    Hello

    I have two doubts, In an exercise I have to convert a MIPS instruction into hexadecimal code. I understand a part of it.
    The instruction given by the professor is:
    Exercise 2 : bne $t2, $zero, previous instruction
    The solution is
    Co =0b0 00101, $rs =0b0 1010,$rt =0b0 0000, Imm16 = 0b1111 1111 1111 1110= 0xFFFE
    So the whole instruction is : 0x1540 FFFE.

    I understand that bne = 000101, and $t2 occupies the 10th position that $t2 is 01010. But I do not understand why Imm16 = 0b1111 1111 1111 1110= 0xFFFE
    Can anybody help me?
    The previous instruction (since it makes reference to the previous instruction it may be usefull) was :
    Exercice 1 addi $t3 , $t2 ,-1 and the solution :
    Co =0b0 01000,$rs =0b0 1010,$rt =0b0 1011,Imm16 =0b1111 1111 1111 1111
    And the instruction : 0x214B FFFF
    Another doubt is (more or less the same) :
    I am given this :
    Exercise 3: bne $t2, $zero, current instruction and the solution is :
    Co =0b0 00101, $rs =0b0 1010,$rt =0b0 0000, Imm16 = 0b1111 1111 1111 1110= xFFFF So the whole isntruction is 0x1540 FFFF. What I do not understand here is the same, why does Imm16 = 0b1111 1111 1111 1110 ??

    Anybody can help me?
     
  2. jcsd
  3. Dec 27, 2011 #2

    Mark44

    Staff: Mentor

    Imm16 means "immediate 16-bit address." This is in contrast to an indirect address.

    For this instruction, what I believe happens is that when this instruction is read, the program counter (PC) is at the beginning of the next instruction. If the number in $t2 isn't zero, control branches to the preceding instruction, which is 8 bytes (or 2 machine words) lower in memory. If the number in $t2 is zero, control continues from the next instruction, which is the address in the PC.

    The branch instructions use offsets in units of 32-bit machine words. So a branch to the previous instruction takes an offset of -2, which is 0xFFFE.
    Here 0xFFFF is not an address - it is -1. If I recall what this does, it adds -1 to register $t2, and then copies that to $t3.
    You have a mistake here. The number in binary should be 0b1111 1111 1111 1111.
    Again, no it's not. 0xFFFF = 0b1111 1111 1111 1111
     
  4. Jan 1, 2012 #3
    Thank you very much for your help!
     
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