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AAA Adjust AL after BCD Addition
Flags affected:
O D I T S Z A P C OF: Overflow flag TF: Trap flag AF: Aux carry
F F F F F F F F F DF: Direction flag SF: Sign flag PF: Parity flag
* * IF: Interrupt flag ZF: Zero flag CF: Carry flag
Legal forms:
AAA
Examples:
AAA
Notes:
AAA makes an addition come out right in AL when what you're adding are BCD values rather than ordinary binary values. Note well that AAA does not perform the arithmetic itself, but is a postprocessor after ADD or ADC. The AL register is an implied operand and may not be explicitly stated-so make sure that the preceding ADD or ADC instruction leaves its results in AL!A BCD digit is a byte with the high 4 bits set to 0, and the low 4 bits containing a digit from 0 to 9. AAA will yield garbage results if the preceding ADD or ADC acted upon one or both operands with values greater than 09.After the addition of two legal BCD values, AAA will adjust a non-BCD result (that is, a result greater than 09 in AL) to a value between 0 and 9. This is called a decimal carry, since it is the carry of a BCD digit and not simply the carry of a binary bit.For example, if ADD added 08 and 04 (both legal BCD values) to produce 0C in AL, AAA will take the 0C and adjust it to 02. The decimal carry goes to AH, not to the upper 4 bits of AL, which are always cleared to 0 by AAA.If the preceding ADD or ADC resulted in a decimal carry (as in the preceding example), both CF and AF are set to 1 and AH is incremented by 1. Otherwise, AH is not incremented and CF and AF are cleared to 0.This instruction is subtle. See the detailed discussion in Chapter 11.
r8 = AL AH BL BH CL CH DL DH r16 = AX BX CX DX BP SP SI DI
sr = CS DS SS ES FS GS r32 = EAX EBX ECX EDX EBP ESP ESI EDI
m8 = 8-bit memory data m16 = 16-bit memory data
m32 = 32-bit memory data i8 = 8-bit immediate data
i16 = 16-bit immediate data i32 = 32-bit immediate data
d8 = 8-bit signed displacement d16 = 16-bit signed displacement
d32 = 32-bit unsigned displacement
