ia-32_volume1_basic-arch

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Unformatted text preview: e keeping its magnitude. 7.3.2.5 Multiplication and Divide Instructions The processor provides two multiply instructions, MUL (unsigned multiply) and IMUL signed multiply), and two divide instructions, DIV (unsigned divide) and IDIV (signed divide). The MUL instruction multiplies two unsigned integer operands. The result is computed to twice the size of the source operands (for example, if word operands are being multiplied, the result is a doubleword). The IMUL instruction multiplies two signed integer operands. The result is computed to twice the size of the source operands; however, in some cases the result is truncated to the size of the source operands (see "IMUL--Signed Multiply" in Chapter 3, "Instruction Set Reference, A-M," of the Intel 64 and IA-32 Architectures Software Developer's Manual, Volume 3A). The DIV instruction divides one unsigned operand by another unsigned operand and returns a quotient and a remainder. The IDIV instruction is identical to the DIV instruction, except that IDIV performs a signed division. 7.3.3 Decimal Arithmetic Instructions Decimal arithmetic can be performed by combining the binary arithmetic instructions ADD, SUB, MUL, and DIV (discussed in Section 7.3.2, "Binary Arithmetic Instructions") with the decimal arithmetic instructions. The decimal arithmetic instructions are provided to carry out the following operations: To adjust the results of a previous binary arithmetic operation to produce a valid BCD result. To adjust the operands of a subsequent binary arithmetic operation so that the operation will produce a valid BCD result. These instructions operate on both packed and unpacked BCD values. For the purpose of this discussion, the decimal arithmetic instructions are divided subordinate subgroups of instructions that provide: Packed BCD adjustments Unpacked BCD adjustments 7.3.3.1 Packed BCD Adjustment Instructions The DAA (decimal adjust after addition) and DAS (decimal adjust after subtraction) instructions adjust the results of operations performed on packed BCD integers 7-12 Vol. 1 PROGRAMMING WITH GENERAL-PURPOSE INSTRUCTIONS (see Section 4.7, "BCD and Packed BCD Integers"). Adding two packed BCD values requires two instructions: an ADD instruction followed by a DAA instruction. The ADD instruction adds (binary addition) the two values and stores the result in the AL register. The DAA instruction then adjusts the value in the AL register to obtain a valid, 2-digit, packed BCD value and sets the CF flag if a decimal carry occurred as the result of the addition. Likewise, subtracting one packed BCD value from another requires a SUB instruction followed by a DAS instruction. The SUB instruction subtracts (binary subtraction) one BCD value from another and stores the result in the AL register. The DAS instruction then adjusts the value in the AL register to obtain a valid, 2-digit, packed BCD value and sets the CF flag if a decimal borrow occurred as the result...
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This note was uploaded on 10/01/2013 for the course CPE 103 taught by Professor Watlins during the Winter '11 term at Mississippi State.

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