i386/i386.reference/IMUL.htm

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   4 <TITLE>80386 Programmer's Reference Manual -- Opcode IMUL</TITLE>
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   8 Chapter 17 -- 80386 Instruction Set</A><BR>
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  11 <P>
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  13 <P>
  14 <H1>IMUL -- Signed Multiply</H1>
  15
  16
  17 <PRE>
  18 Opcode      Instruction            Clocks      Description
  19
  20 F6  /5      IMUL r/m8              9-14/12-17  AX= AL * r/m byte
  21 F7  /5      IMUL r/m16             9-22/12-25  DX:AX := AX * r/m word
  22 F7  /5      IMUL r/m32             9-38/12-41  EDX:EAX := EAX * r/m dword
  23 0F  AF /r   IMUL r16,r/m16         9-22/12-25  word register := word
  24                                                register * r/m word
  25 0F  AF /r   IMUL r32,r/m32         9-38/12-41  dword register := dword
  26                                                register * r/m dword
  27 6B  /r ib   IMUL r16,r/m16,imm8    9-14/12-17  word register := r/m16 *
  28                                                sign-extended immediate byte
  29 6B  /r ib   IMUL r32,r/m32,imm8    9-14/12-17  dword register := r/m32 *
  30                                                sign-extended immediate byte
  31 6B  /r ib   IMUL r16,imm8          9-14/12-17  word register := word
  32                                                register * sign-extended
  33                                                immediate byte
  34 6B  /r ib   IMUL r32,imm8          9-14/12-17  dword register := dword
  35                                                register * sign-extended
  36                                                immediate byte
  37 69  /r iw   IMUL r16,r/m16,imm16   9-22/12-25  word register := r/m16 *
  38                                                immediate word
  39 69  /r id   IMUL r32,r/m32,imm32   9-38/12-41  dword register := r/m32 *
  40                                                immediate dword
  41 69  /r iw   IMUL r16,imm16         9-22/12-25  word register := r/m16 *
  42                                                immediate word
  43 69  /r id   IMUL r32,imm32         9-38/12-41  dword register := r/m32 *
  44                                                immediate dword
  45 </PRE>
  46
  47 <EM>
  48 <H3>Notes</H3>
  49   The 80386 uses an early-out multiply algorithm. The actual number of
  50   clocks depends on the position of the most significant bit in the
  51   optimizing multiplier, shown underlined above. The optimization occurs for
  52   positive and negative values. Because of the early-out algorithm, clock
  53   counts given are minimum to maximum. To calculate the actual clocks, use
  54   the following formula:
  55
  56 <PRE>
  57   Actual clock = if m <> 0 then max(ceiling(log{2}(m)), 3) + 6 clocks
  58   Actual clock = if m = 0 then 9 clocks
  59   (where m is the multiplier)
  60 </PRE>
  61
  62 Add three clocks if the multiplier is a memory operand.
  63 </EM>
  64
  65 <H2>Operation</H2>
  66
  67 <PRE>
  68 result := multiplicand * multiplier;
  69 </PRE>
  70
  71 <H2>Description</H2>
  72
  73 IMUL performs signed multiplication. Some forms of the instruction
  74 use implicit register operands. The operand combinations for all forms
  75 of the instruction are shown in the "<H2>Description</H2>" column above.
  76 <P>
  77 IMUL clears the overflow and carry flags under the following conditions:
  78 <PRE>
  79    Instruction Form    Condition for Clearing CF and OF
  80    r/m8                AL := sign-extend of AL to 16 bits
  81    r/m16               AX := sign-extend of AX to 32 bits
  82    r/m32               EDX:EAX := sign-extend of EAX to 32 bits
  83    r16,r/m16           Result exactly fits within r16
  84    r/32,r/m32          Result exactly fits within r32
  85    r16,r/m16,imm16     Result exactly fits within r16
  86    r32,r/m32,imm32     Result exactly fits within r32
  87 </PRE>
  88
  89 <H2>Flags Affected</H2>
  90
  91 OF and CF as described above; SF, ZF, AF, and PF are undefined
  92
  93 <H2>Protected Mode Exceptions</H2>
  94
  95 #GP(0) for an illegal memory operand effective address in the CS, DS,
  96 ES, FS, or GS segments; #SS(0) for an illegal address in the SS segment;
  97 #PF(fault-code) for a page fault
  98
  99 <H2>Real Address Mode Exceptions</H2>
 100
 101 Interrupt 13 if any part of the operand would lie outside of the effective
 102 address space from 0 to 0FFFFH
 103
 104 <H2>Virtual 8086 Mode Exceptions</H2>
 105
 106 Same exeptions as in Real Address Mode; #PF(fault-code) for a page
 107 fault
 108
 109 <H2>Notes</H2>
 110
 111 When using the accumulator forms (IMUL r/m8, IMUL r/m16, or IMUL
 112 r/m32), the result of the multiplication is available even if the overflow
 113 flag is set because the result is two times the size of the multiplicand
 114 and multiplier. This is large enough to handle any possible result.
 115
 116
 117 <P>
 118 <HR>
 119 <P>
 120 <B>up:</B> <A HREF="c17.htm">
 121 Chapter 17 -- 80386 Instruction Set</A><BR>
 122 <B>prev:</B><A HREF="IDIV.htm"> IDIV Signed Divide</A><BR>
 123 <B>next:</B><A HREF="IN.htm"> IN Input from Port</A>
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