ICS 51 Discussion 3

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ICS 51 Discussion 3

• Multiplication, Division, and Stack

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Multiplication Operation

• Unsigned multiplication MUL
• Code MUL Operand
• If Operand is a byte, e.g. MUL BL, thenAX
  ALOperand
• If Operand is a word, e.g., MUL BX, thenDXAX
  AXOperand
• If Operand is a dword, e.g., MUL EBX,
  thenEDXEAX EAXOperand
• Signed multiplication IMUL
• (look up the code table)

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MUL Examples

• Mov EAX, 0 // Clear EAX
• Mov AL, 5 //First number
• Mov BL, 10 //Second number
• Mul BL //Multiply 2 numbers
• Where and what is the result?
• AX ALBL 50

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MUL Examples (cont.)

• Mov EDX, 0 //clear EDX
• Mov EAX, 0 //clear EAX
• Mov AX, 4460 //first number
• Mov BX, 1000 //second number
• Mul BX //multiply 2 numbers
• Where and what is the result?
• DXAX AXBX 4,460,000
• 4,460,000 440DE0 in Hex
• DX has 0x0044, AX has 0x0DE0

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Division Operation

• Unsigned division DIV
• Code DIV Operand
• If Operand is a byte, e.g. DIV BL, thenAL
  AX/Operand AH Rest
• If Operand is a word, e.g., DIV BX, thenAX
  DXAX/Operand DX Rest
• If Operand is a dword, e.g., DIV EBX, thenEAX
  EDXEAX/Operand EDX Rest
• Signed division IDIV
• (look up the code table)

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DIV Example

• Mov EDX, 70001 //dividend
• Mov AX, DX //move lower 2 bytes to AX
• Shr EDX, 16 //DX has the higher 2 bytes
• Mov CX, 2 //divisor
• Div CX //divide 70001 by 2
• Result
• Quotient AX DXAX / CX 35000
• Remainder DX 1

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Why Using Stack

• What if we run out of registers?
• What if the registers you are going to use are
  currently used by other functions?
• Answers PUSH and POP

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Stack Push/Pop Examples

• run out of registersPush EAX //save EAX on
  the stackPush EBX //save EBX on the stack
  use EAX and EBX for some calculations Pop EBX
  //move top of the stack to EBXPop EAX //move top
  of the stack to EAX to restore
  its original value
• Note the orders of PUSH and POP are reversed

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Stack Push/Pop Examples (cont.)

• void functionXXX() __asm Push EAX Push
  EBX Pop EBX Pop EAX