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Unformatted text preview: of the subtraction. 7.3.3.2 Unpacked BCD Adjustment Instructions The AAA (ASCII adjust after addition), AAS (ASCII adjust after subtraction), AAM (ASCII adjust after multiplication), and AAD (ASCII adjust before division) instructions adjust the results of arithmetic operations performed in unpacked BCD values (see Section 4.7, "BCD and Packed BCD Integers"). All these instructions assume that the value to be adjusted is stored in the AL register or, in one instance, the AL and AH registers. The AAA instruction adjusts the contents of the AL register following the addition of two unpacked BCD values. It converts the binary value in the AL register into a decimal value and stores the result in the AL register in unpacked BCD format (the decimal number is stored in the lower 4 bits of the register and the upper 4 bits are cleared). If a decimal carry occurred as a result of the addition, the CF flag is set and the contents of the AH register are incremented by 1. The AAS instruction adjusts the contents of the AL register following the subtraction of two unpacked BCD values. Here again, a binary value is converted into an unpacked BCD value. If a borrow was required to complete the decimal subtract, the CF flag is set and the contents of the AH register are decremented by 1. The AAM instruction adjusts the contents of the AL register following a multiplication of two unpacked BCD values. It converts the binary value in the AL register into a decimal value and stores the least significant digit of the result in the AL register (in unpacked BCD format) and the most significant digit, if there is one, in the AH register (also in unpacked BCD format). The AAD instruction adjusts a twodigit BCD value so that when the value is divided with the DIV instruction, a valid unpacked BCD result is obtained. The instruction converts the BCD value in registers AH (most significant digit) and AL (least significant digit) into a binary value and stores the result in register AL. When the value in AL is divided by an unpacked BCD value, the quotient and remainder will be automatically encoded in unpacked BCD format. Vol. 1 713 PROGRAMMING WITH GENERALPURPOSE INSTRUCTIONS 7.3.4 Decimal Arithmetic Instructions in 64Bit Mode Decimal arithmetic instructions are not supported in 64bit mode, They are either invalid or not encodable. 7.3.5 Logical Instructions The logical instructions AND, OR, XOR (exclusive or), and NOT perform the standard Boolean operations for which they are named. The AND, OR, and XOR instructions require two operands; the NOT instruction operates on a single operand. 7.3.6 Shift and Rotate Instructions The shift and rotate instructions rearrange the bits within an operand. For the purpose of this discussion, these instructions are further divided subordinate subgroups of instructions that: Shift bits Doubleshift bits (move them between operands) Rotate bits 7.3.6.1 Shift Instructions The SAL (shift arithmetic left), SHL (shift logical left)...
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 Winter '11
 Watlins

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