Introduction to Machine And Assembly Language

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Introduction to Machine And Assembly Language

• Data Transfers and Mathematics
• Dr. Richard Ford

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Okay

• We now really understand syntax and are
  pretending we understand conditional processing
• And were going to start looking at more
  interesting instructions
• Basically, were going to increase our vocabulary
  and see why ASM is very different from Java
• With great power comes great responsibility

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Direct Memory Operands

• Already used these
• Remember our very exciting program from last
  week?
• mov eax, var1

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The MOV instruction

• One of the most important instructions
• MOV destination, source
• Rules
• Both operands must be the same size
• Both operands cannot be memory operands
• CS, EIP and IP cannot be destination operands
• An immediate value cannot be moved to a segment
  register

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Memory to Memory?

• Cmon you tell me how to do it
• .datavar1 WORD ?var2 WORD ?.code
• Now what?

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Zero/Sign Extension

• How you move from a small register to a larger
  one
• Why is this a problem?
• Solution?
• .datacount WORD 1.codemov ecx, 0mov cx, count
• But what if count is SIGNED? Lets try it

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MOVZX

• Move with Zero eXtend
• .databyte1 BYTE 9Bhword1 WORD 0A69h.codemovzx
  eax, word1movzx edx, byte1movzx cx, byte1

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MOVSX

• Same idea but for signed numbers
• Remind me how signed numbers work?
• mov bx, 0A69hmovsx eax, bxmovsx edx, blmovsx
  cx, bl

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LAHF/SAHF

• Loads and saves the flags into AH
• .datasaveflags BYTE ?.codelahfmov saveflags,
  ahmov ah, saveflagssahf

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XCHG

• Swap two operands of equal size
• xchg ax, bxxchg ah, alxchg var1, bxxchg eax,
  ebx
• How to swap two memory contents?

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Direct-offset Operands

• Pretty straightforward
• .dataarrayB BYTE 10h,20h,30h,40h.codemov al,
  arrayBmov al, arrayB1mov al, arrayB2
• What about mov al, arrayB20 ?

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Addition and Subtraction

• The simplest method is to add or subtract one
• INC EAX
• DEC BX

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Adding More than One

• ADD instruction
• ADD destination, source
• ADD eax, var1

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Subtraction is the same way

• SUB destination, source
• sub eax, 12345h
• Note ADD and SUB both can change the flags this
  is important

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Negation

• Very simple indeed
• NEG reg
• NEG mem

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Flags and Arithmetic

• Zero and Sign Flags
• Zero set when result is ZERO
• Sign set when result is negative
• mov cx, 1sub cx, 1mov ax, 0ffffhinc axinc
  axsub cx, 1add cx, 2

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The Carry Flag

• Only meaningful on unsigned arithmetic, when the
  result is too large or too small
• mov al, 0ffhadd al, 1mov ax, 00ffhadd ax,
  1mov al, 1sub al, 2

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Overflow Flag

• Only meaningful on signed arithmetic
• When we would overflow a value
• mov al, 127inc al

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All Done

• We now understand how some of the flags work, and
  we can add and subtract numbers easily
• Next, well look at Data-related operators

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Assignment

• Make the bubblesort complete, so you can sort an
  arbitrary array of SWORDs
• Print out the array before you start and after
  using WriteInt
• For extra credit
• Stop sorting once youre done (i.e. dont do
  unnecessary passes through the array)
• Sort on the sequentially shorter unordered
  remainder of the array