ASCII Arithmetic | Microprocessor | Computer and IT

ASCII Arithmetic

    The ASCII arithmetic instruction function with ASCII-coded numbers. These numbers range in value from 30H to 39H for the numbers 0 to 9. There are four instructions used with ASCII arithmetic operations:

1. AAA (ASCII Adjust after Addition)
2. AAD (ASCII Adjust before Division)
3. AAM (ASCII Adjust after Multiplication)
4. AAS (ASCII Adjust after Subtraction)

    These instructions use register AX as the source and as destination.

     1. AAA Instruction

• Adjust the sum of two unpacked BCD values to create an unpacked BCD result.
• The AL register is the implied source and destination operand for this instruction.
• The AAA instruction is only useful when its follow an ADD instruction that adds (binary addition) two unpacked BCD values.
• The AAA instruction then adjusts the contents of AL register to contain the correct 1-digit unpacked BCD result.
• If the addition produces a decimal carry, the AH register is incremented by 1, and the CF and AF flags are set.

     2. AAD Instruction

• Unlike all other adjustment instructions, the AAD instruction appears before a division.
• The AAD instruction requires that the AX register contain a two-digit unpacked BCD number (not ASCII) before executing.
• After adjusting the AX register with AAD, it is divided by an unpacked BCD number to generate a single-digit result in AL with any remainder in AH.
• Example:  

          MOV BL, 9H 

          MOV AX, 702H

          AAD

          DIV BL

• The above example show how 72 is unpacked BCD is divided by 9 to produce a quotient of 8. The 0702H loaded into AX register is adjusted by the AAD instruction to 0048H.

     3. AAM Instruction

• Adjust the result of the multiplication of two unpacked BCD values to create a pair of unpacked BCD values.
• The AX register is the implied source and destination operand for this instruction.
• The AAM instruction is only useful when it follows an MUL instruction that multiplies (binary multiplication) two unpacked BCD values and stores a word result in AX registers.

     4. AAS Instruction

• Adjust the result of the subtraction of two unpacked BCD values to create a unpacked BCD result.
• The AL register is the implied source and destination for this instructions.
• The AAS instruction is only useful when it follows a SUB instruction that subtracts (binary subtraction) one unpacked BCD valued from another and stores a byte result in the AL register.
• If the subtraction produced a decimal carry, the AH register is decremented by 1, and the CF and AF flags are set.
• If no decimal carry occurred, the CF and AF flags are cleared, and the AH register is unchanged.