Aaa-arithmetic instruction-microprocessor, Assembly Language

AAA: ASCII Adjust after Addition operation the AAA instruction is executed after an ADD instruction that adds 2 ASCII coded operands to give a byte of outcome in the AL. The AAA instruction converts the resulting contents of AL to unpack the decimal digits. After the addition operation, the AAA instruction checked the lower 4 bits of AL to examine whether it have a valid BCD number in the range 0 to 9. If it is between range 0 to 9 and AF is set to zero, AAA sets the 4 high order bits of AL to 0. The AH might be cleared before addition operation. If the lower digit of the AL is between the range 0 to 9 and AF is set, 06 are added to the AL. The upper 4 bits of the AL are cleared and the AH is incremented by the value one. If the value in the lower nibble of the AL is greater than 9 then the AL is incremented by the value 06, AH is incremented by value I, the AF and the CF flags are set to I, and the higher 4 bits of the AL are cleared to the value 0. The remaining flags are not affected. The AH is modified as sum of earlier contents (typically 00) and the carry from the adjustment, as shown in given figure. This instruction does not give precise ASCII codes of the sum, but they may be obtained by adding 3030H to  the AX.

425_ascii add.jpg

               Figure: ASCII Adjust After Addition Instruction

Posted Date: 10/12/2012 1:38:00 AM | Location : United States

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