CS 3841 Computer Organization II Recitation LAB6

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CS 3841 Computer Organization IIRecitation LAB6

• - Debugging -

LI XU
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XSpim

• Registers
• function keys
• Text segment
• Data segment
• Messages

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Text segment

• MIPS instructions
• First column - hexadecimal address
• Second - encoded machine language
• Last - original assembly code

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Data segment

• MIPS memory data
• DATA, STACK, KERNEL DATA

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Registers

• Shows the contents of
• All 32 general purpose registers
• floating point registers, and a few others

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Function Keys

• Run
• Break points
• Step
• step ltNgt

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Function Keys

• Continue
• Set value
• Print

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Example

• Convert capital string to lowercase string.
• Read a string from keyboard
• Convert the capital letter to lowercase letter.
• Print the result
• Expect ABCD ? abcd

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Pseudo code

• i 0
• read_char string_inputi
• while (read_char ! EndOfString)
• lower_char read_char 32
• string_outputi lower_char
• i
• read_char string_inputi
• string_outputi EndOfString

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Assembly code (part)

• li t0, 0 t0 is index register
• lb t1, str_in(t0) read one character
• loop
• beq t1, 0, end if (string end)
  goto end
• add t2, t1, 32 a A 32
• sb t2, str_out(t0) write one charater
• add t0, t0, 1 increase index by 1
• lb t1, str_in(t0) read one character
• b loop repeat the loop
• end
• sb zero, str_out(t0) mark the end of
  string
• Wrong result ABCD ? abcd

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Trace (init status)
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Trace (after input AB)
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Trace (convert A to a)
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Trace (convert B to b)
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Trace (unexpected converting)
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ASCII Chart
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Trace (result)

• Reason
• Null ASCII cord 0
• New line ASCII cord 10

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Exercise

• Download the following code and run.
• Run the instructions one by one by using Single
  Step.
• Find the addresses that store A and B.
• Find the data bytes store a and b.
• Modify the code to fix the problem (ab)