"D" Instructions

AC - Instruções

DAA - Decimal Adjust after Addition

See Also: DAS, AAA, AAS, AAM, AAD, Flags

O D I T S Z A P C
?       * * * * *

DAA

Logic:
	If (AL & 0Fh) > 9 or (AF = 1) then
		AL = AL + 6
		AF = 1
	else
		AF = 0

	If (AL > 9Fh) or (CF = 1) then
		AL = AL + 60h
		CF = 1
	else
		CF = 0

DAA corrects the result of a previous addition of two valid packed decimal operands (note that this result must be in AL). This instruction changes the content of AL so that it will contain a pair of valid packed decimal digits.

Operands Clocks Transfers Bytes Example
no operands 4 - 1 DAA

Note: Packed BCD stores one digit per nibble (4 bits); the least significant digit is in the lower nibble. It is not possible to apply an adjustment after division or multiplication of packed BCD numbers. If you need to use multiplication or division, it is better to use unpacked BCD numbers.See, for example, the description of AAM (ASCII Adjust after Multiply).

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DAS - Decimal Adjust after Subtraction

See Also: AAS, DAA, SUB, SBB, DEC, NEG, Flags

O D I T S Z A P C
?       * * * * *

DAS

Logic:
If(AL & 0Fh) > 9 or (AF = 1) then
	AL = AL - 6
	AF = 1
else
	AF = 0
If (AL > 9Fh) or (CF = 1) then
	AL = AL - 60h
	CF = 1
else
	CF = 0

DAS corrects the result of a previous subtraction of two valid packed decimal operands (note that this result must be in AL). This instruction changes the content of AL so that it will contain a pair of valid packed decimal digits.

Operands Clocks Transfers Bytes Example
no operands 4 - 1 DAS

Note: Packed BCD stores one digit per nibble (4 bits); the least significant digit is in the lower nibble.
It is not possible to apply an adjustment after division or multiplication of packed BCD numbers. If you need to use multiplication and division, it is better to use unpacked BCD numbers. See, for example, the description of AAM (ASCII Adjust after Multiply).

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DEC - Decrement

See Also: SUB, SBB, AAS, DAS, EA, Flags

O D I T S Z A P C
*       * * * *  

DEC destination

Logic:
destination = destination - 1

This instruction decrements the destination by one. The destination operand, which may be either a word or a byte, is treated as an unsigned binary number.

Operands Clocks Transfers Bytes Example
reg16 2 - 1 DEC BX
reg8 3 - 2 DEC BL
memory 15(23)+EA 2 2-4 DEC MATRIX[SI]

Note: This instruction does not set the carry, so if you need to decrement a multi-word number, it is better to use the SUB and SBB instructions.

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DIV - Divide (Unsigned)

See Also: IDIV, SHR, AAD, CBW, CWD, INT 00h, EA, Flags

O D I T S Z A P C
?       ? ? ? ? ?

DIV source

Logic:
AL = AX / source	;Source is byte
AH = remainder
or
AX = DX:AX / source	;Source is word
DX = remainder

This instruction performs unsigned division. If the source is a byte, DIV divides the word value in AX by source, returning the quotient in AL and the remainder in AH. If the source is a word, DIV divides the double-word value in DX:AX by the source, returning the quotient in AX and the remainder in DX.

Operands Clocks Transfers Bytes Example
reg8 0-90 - 2 DIV BL
reg16 4-162 - 2 DIV BX
mem8 (86-96)+EA 1 2-4 DIV VYUP
mem16 (154-172)+EA 1 2-4 DIV NCONQUER[SI]

Note: If the result is too large to fit in the destination (AL or AX), an INT 0 (Divide by Zero) is generated, and the quotient and remainder are undefined.
When an Interrupt 0 (Divide by Zero) is generated, the saved CS:IP value on the 80286 and 80386 points to the instruction that failed (the DIV instruction).On the 8088/8086, however, CS:IP points to the instruction following the failed DIV instruction.

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Última actualização: 02 Maio 2005

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