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<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN"><HTML><HEAD><TITLE>80386 Programmer's Reference Manual -- Opcode MUL</TITLE></HEAD><BODY><B>up:</B> <A HREF="c17.htm">Chapter 17 -- 80386 Instruction Set</A><BR><B>prev:</B><A HREF="MOVZX.htm"> MOVZX Move with Zero-Extend</A><BR><B>next:</B><A HREF="NEG.htm"> NEG Two's Complement Negation</A><P><HR><P><H1>MUL -- Unsigned Multiplication of AL or AX</H1><PRE>Opcode Instruction Clocks DescriptionF6 /4 MUL AL,r/m8 9-14/12-17 Unsigned multiply (AX := AL * r/m byte)F7 /4 MUL AX,r/m16 9-22/12-25 Unsigned multiply (DX:AX := AX * r/m word)F7 /4 MUL EAX,r/m32 9-38/12-41 Unsigned multiply (EDX:EAX := EAX * r/m dword)</PRE><EM><H3>Notes</H3> The 80386 uses an early-out multiply algorithm. The actual number of clocks depends on the position of the most significant bit in the optimizing multiplier, shown underlined above. The optimization occurs for positive and negative multiplier values. Because of the early-out algorithm, clock counts given are minimum to maximum. To calculate the actual clocks, use the following formula:<PRE> Actual clock = if <> 0 then max(ceiling(log{2}(m)), 3) + 6 clocks; Actual clock = if = 0 then 9 clocks</PRE> where <TT>m</TT> is the multiplier.</EM><H2>Operation</H2><PRE>IF byte-size operationTHEN AX := AL * r/m8ELSE (* word or doubleword operation *) IF OperandSize = 16 THEN DX:AX := AX * r/m16 ELSE (* OperandSize = 32 *) EDX:EAX := EAX * r/m32 FI;FI;</PRE><H2>Description</H2>MUL performs unsigned multiplication. Its actions depend on the sizeof its operand, as follows:<UL> <LI> A byte operand is multiplied by AL; the result is left in AX. The carry and overflow flags are set to 0 if AH is 0; otherwise, they are set to 1. <LI> A word operand is multiplied by AX; the result is left in DX:AX. DX contains the high-order 16 bits of the product. The carry and overflow flags are set to 0 if DX is 0; otherwise, they are set to 1. <LI> A doubleword operand is multiplied by EAX and the result is left in EDX:EAX. EDX contains the high-order 32 bits of the product. The carry and overflow flags are set to 0 if EDX is 0; otherwise, they are set to 1.</UL><H2>Flags Affected</H2>OF and CF as described above; SF, ZF, AF, PF, and CF are undefined<H2>Protected Mode Exceptions</H2>#GP(0) for an illegal memory operand effective address in the CS, DS,ES, FS, or GS segments; #SS(0) for an illegal address in the SS segment;#PF(fault-code) for a page fault<H2>Real Address Mode Exceptions</H2>Interrupt 13 if any part of the operand would lie outside of the effectiveaddress space from 0 to 0FFFFH<H2>Virtual 8086 Mode Exceptions</H2>Same exceptions as in Real Address Mode; #PF(fault-code) for a pagefault<P><HR><P><B>up:</B> <A HREF="c17.htm">Chapter 17 -- 80386 Instruction Set</A><BR><B>prev:</B><A HREF="MOVZX.htm"> MOVZX Move with Zero-Extend</A><BR><B>next:</B><A HREF="NEG.htm"> NEG Two's Complement Negation</A></BODY>⌨️ 快捷键说明
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