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Homework Help: Can someone check this

  1. Dec 31, 2009 #1
    1. The problem statement, all variables and given/known data

    If the content of d0 is $30 state in words what each of the following numbered instruction pairs will do.

    2. Relevant equations

    a. cmpi #$30,d0

    beq dan

    b. cmp.l unknown, d0

    bne dan

    c. cmp.w d1,d0

    bgt dan


    3. The attempt at a solution

    a. cmpi #$30,d0 = Z flag gets set.

    beq dan = [go to label dan] [if d0] [equal to zero]


    b. cmp.l unknown, d0 = so no flags get set.

    bne dan = [go to label dan] [if d0] [not equal to zero]


    c. cmp.w d1,d0 = N flag gets set

    bgt dan = [go to label dan] [if d0] [greater than zero]
     
  2. jcsd
  3. Dec 31, 2009 #2

    Mark44

    Staff: Mentor

    This is probably closer to how your answers should be "stated in words."
    Since d0 is $30, the compare instruction sets the Z flag.
    The beq instruction causes a branch to label dan, because the Z flag is set.
    You can't say that no flags get set, because you don't know what value is stored in unknown. You should look at two cases: unknown == $30 and unknown != $30 (meaning that unknown > $30 or unknown < $30).
    The branch instruction will do one thing if the Z flag is set and another if the Z flag is clear. You need to have both cases covered in your explanation.
    You can't say that the N flag gets set, because you don't know what value is stored in d1. You should look at two cases: d1 <= d0 and d1 > d0.
    You need to have both cases covered in your explanation.
     
  4. Jan 1, 2010 #3
    cmp.l unknown,d0 = I don’t know what value stored in unknown. Therefore their will be three cases (unknown > d0 or unknown < d0 and unknown = d0)

    Unknown > d0 = N flag gets set.
    Unknown < d0 = no flags get set
    unknown = d0 = Z flag gets set

    bne dan = [go to label dan] [if d0] [not equal to zero]


    cmp.w d1,d0 = I don’t know what value stored in d1. Therefore their will be three cases (d1 < d0 or d1 > d0 and d1 = d0)

    d1 > d0 = N flag gets set.
    d1 < d0 = no flags get set
    d1 = d0 = Z flag gets set

    bgt dan = [go to label dan] [if d0] [greater than zero]

    is this correct
     
  5. Jan 6, 2010 #4

    Mark44

    Staff: Mentor

    This is not correct. You are writing your answers as if you were writing code, which is not what is asked for. You should not be writing your answers like this: bne dan = [go to label dan] [if d0] [not equal to zero]

    Write your answers without brackets and in English sentences.

    bne dan causes a branch to label dan if the Z flag is clear (not set). Which possibility/possibilities cause this branch to be taken? Note that d0 being zero or not has no influence on things.
    No it isn't.
    bgt dan causes a branch to label dan if neither the Z flag nor the N flag are set. Which of your three conditions causes neither of these flags to be set?

    Your answer should be something like this:
    The bgt instruction causes control to flow to label dan if <fill in the blank>. The branch instruction is ignored if <fill in the blank>.
     
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