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Division instruction

Division instruction. DIV Instruction The DIV (unsigned divide) instruction performs 8 bit, 16 bit , and 32 bit division on unsigned integers. Div r/m8 Div r/m16 Div r/m32 The following illustration shows EDX:EAX as the default dividend when a 32- bit divisor is used.

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Division instruction

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  1. Division instruction

  2. DIV Instruction • The DIV (unsigned divide) instruction performs 8 bit, 16 bit , and 32 bit division on unsigned integers. • Div r/m8 • Div r/m16 • Div r/m32 • The following illustration shows EDX:EAX as the default dividend when a 32- bit divisor is used. • EDX EAX = eax quotient • r/m32 edx remainder

  3. Example 1: • The following instruction perform 8-bit unsigned division (83h/2), producing a quotient of 41h and a remainder of 1. • mov ax, 0083h • mov bl, 2 • div bl • Example 2: • The following instruction perform 16-bit unsigned division (8003h/100h), producing a quotient of 80h and a remainder of 3. • DX contains the high pat of the dividend, so it must be cleared before the div instruction executes. • mov dx, 0 • mov ax, 8003h • mov cx, 100h • div cx

  4. Signed Integer Division • CBW, CWD, CDQ Instructions • Before discussing signed integer division, we need to look at three instructions that perform integer signed extension. • The CBW instruction extends the sign bit of AL into the ah register. This preserves the number’s sign • .data • ByteVal sbyte -101 ; 9Bh • .code • mov al, ByteVal ; AL = 9Bh • cbw ; AX = FF9Bh • In order words, 9Bh and FF9BH both equal -65. The only difference between the two is their storage size

  5. The CWD (convert word to doubleword) instruction extends the sign bit of AX into the DX register • .data • WordVal sword -101 ; FF9Bh • .code • mov ax, WordVal ; AX = FF9Bh • cwd ; DX:AX = FFFFFF9Bh • The CDQ (convert doubleword to quadword) instruction extends the sign bit of EAX into the EDX register • .data • DwordVal sword -101 ; FFFFF9Bh • .code • mov eax, DwordVal • cdq ; EDX:EAX = FFFFFFFFFFFFF9Bh

  6. The IDIV instruction • The IDIV (signed divide instruction) performs signed integer division, using the same operands as the DIV instruction. • When doing 8-bit division, you must sign-extend the dividend into AH before IDIV executes, (The CBW instruction can be used). • Example: • We divide -48 by 5. After IDIV executes, the quotient in AL is -9 and the remainder in AH is -3. • .data • ByteVal sbyte -48 • .code • mov al, ByteVal ; dividend • cbw ; AL into AH • mov bl, 5 ; Divisor • idiv bl ; AL = -9, AH = -3

  7. Example: Divide -5000 by 256 • Similarly, 16-bit division requires that AX to be signed extended into DX • .data • WordVal sword -5000 • .code • mov ax, WordVal ; Dividend, low • cwd ; extend AX into DX • mov bx, 256 ; divisor • idiv bx ; quotient AX = -19 • ; remainder DX = -136

  8. Example: Divide -50000 by 256 • Similarly, 32-bit division requires that EAX be signed extended into EDX. • .data • dwordVal SWORD -50000 • .code • mov eax, dwordVal ; dividend, low • cdq ; extend EAX into EDX • mov ebx, 256 ; divisor • idiv ebx ; quotient EAX = -195 • ; remainder EDX = -80 • For both DIV and IDIV, all of the arithmetic status flags are • undefined after the operations

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