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3.2.3  Multiplication Instructions

The 80386 has separate multiply instructions for unsigned and signed
operands. MUL operates on unsigned numbers, while IMUL operates on signed
integers as well as unsigned.

MUL (Unsigned Integer Multiply) performs an unsigned multiplication of the
source operand and the accumulator. If the source is a byte, the processor
multiplies it by the contents of AL and returns the double-length result to
AH and AL. If the source operand is a word, the processor multiplies it by
the contents of AX and returns the double-length result to DX and AX. If the
source operand is a doubleword, the processor multiplies it by the contents
of EAX and returns the 64-bit result in EDX and EAX. MUL sets CF and OF
when the upper half of the result is nonzero; otherwise, they are cleared.

IMUL (Signed Integer Multiply) performs a signed multiplication operation.
IMUL has three variations:

  1.  A one-operand form. The operand may be a byte, word, or doubleword
      located in memory or in a general register. This instruction uses EAX
      and EDX as implicit operands in the same way as the MUL instruction.

  2.  A two-operand form. One of the source operands may be in any general
      register while the other may be either in memory or in a general
      register. The product replaces the general-register operand.

  3.  A three-operand form; two are source and one is the destination
      operand. One of the source operands is an immediate value stored in
      the instruction; the second may be in memory or in any general
      register. The product may be stored in any general register. The
      immediate operand is treated as signed. If the immediate operand is a
      byte, the processor automatically sign-extends it to the size of the
      second operand before performing the multiplication.

The three forms are similar in most respects:

  þ  The length of the product is calculated to twice the length of the
     operands.

  þ  The CF and OF flags are set when significant bits are carried into the
     high-order half of the result. CF and OF are cleared when the
     high-order half of the result is the sign-extension of the low-order
     half.

However, forms 2 and 3 differ in that the product is truncated to the
length of the operands before it is stored in the destination register.
Because of this truncation, OF should be tested to ensure that no
significant bits are lost. (For ways to test OF, refer to the INTO and PUSHF
instructions.)

Forms 2 and 3 of IMUL may also be used with unsigned operands because,
whether the operands are signed or unsigned, the low-order half of the
product is the same.


3.2.4  Division Instructions

The 80386 has separate division instructions for unsigned and signed
operands. DIV operates on unsigned numbers, while IDIV operates on signed
integers as well as unsigned. In either case, an exception (interrupt zero)
occurs if the divisor is zero or if the quotient is too large for AL, AX, or
EAX.

DIV (Unsigned Integer Divide) performs an unsigned division of the
accumulator by the source operand. The dividend (the accumulator) is twice
the size of the divisor (the source operand); the quotient and remainder
have the same size as the divisor, as the following table shows.

Size of Source Operand
      (divisor)             Dividend       Quotient      Remainder

Byte                        AX             AL            AH
Word                        DX:AX          AX            DX
Doubleword                  EDX:EAX        EAX           EDX

Non-integral quotients are truncated to integers toward 0. The remainder is
always less than the divisor. For unsigned byte division, the largest
quotient is 255. For unsigned word division, the largest quotient is 65,535.
For unsigned doubleword division the largest quotient is 2^(32) -1.

IDIV (Signed Integer Divide) performs a signed division of the accumulator
by the source operand. IDIV uses the same registers as the DIV instruction.

For signed byte division, the maximum positive quotient is +127, and the
minimum negative quotient is -128. For signed word division, the maximum
positive quotient is +32,767, and the minimum negative quotient is -32,768.
For signed doubleword division the maximum positive quotient is 2^(31) -1,
the minimum negative quotient is -2^(31). Non-integral results are truncated
towards 0. The remainder always has the same sign as the dividend and is
less than the divisor in magnitude.