hw/i386.reference/STOS.htm

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   4 <TITLE>80386 Programmer's Reference Manual -- Opcode STOS</TITLE>
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   7 <B>up:</B> <A HREF="c17.htm">
   8 Chapter 17 -- 80386 Instruction Set</A><BR>
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  10 <B>next:</B><A HREF="STR.htm"> STR Store Task Register</A>
  11 <P>
  12 <HR>
  13 <P>
  14 <H1>STOS/STOSB/STOSW/STOSD -- Store String Data</H1>
  15
  16 <PRE>
  17 Opcode  Instruction  Clocks   Description
  18
  19 AA      STOS m8      4        Store AL in byte ES:[(E)DI], update (E)DI
  20 AB      STOS m16     4        Store AX in word ES:[(E)DI], update (E)DI
  21 AB      STOS m32     4        Store EAX in dword ES:[(E)DI], update (E)DI
  22 AA      STOSB        4        Store AL in byte ES:[(E)DI], update (E)DI
  23 AB      STOSW        4        Store AX in word ES:[(E)DI], update (E)DI
  24 AB      STOSD        4        Store EAX in dword ES:[(E)DI], update (E)DI
  25 </PRE>
  26
  27
  28 <H2>Operation</H2>
  29
  30 <PRE>
  31 IF AddressSize = 16
  32 THEN use ES:DI for DestReg
  33 ELSE (* AddressSize = 32 *) use ES:EDI for DestReg;
  34 FI;
  35 IF byte type of instruction
  36 THEN
  37    (ES:DestReg) := AL;
  38    IF DF = 0
  39    THEN DestReg := DestReg + 1;
  40    ELSE DestReg := DestReg - 1;
  41    FI;
  42 ELSE IF OperandSize = 16
  43    THEN
  44       (ES:DestReg) := AX;
  45       IF DF = 0
  46       THEN DestReg := DestReg + 2;
  47       ELSE DestReg := DestReg - 2;
  48       FI;
  49    ELSE (* OperandSize = 32 *)
  50       (ES:DestReg) := EAX;
  51       IF DF = 0
  52       THEN DestReg := DestReg + 4;
  53       ELSE DestReg := DestReg - 4;
  54       FI;
  55    FI;
  56 FI;
  57 </PRE>
  58
  59 <H2>Description</H2>
  60
  61 STOS transfers the contents of all AL, AX, or EAX register to the memory
  62 byte or word given by the destination register relative to the ES segment.
  63 The destination register is DI for an address-size attribute of 16 bits or
  64 EDI for an address-size attribute of 32 bits.
  65 <P>
  66 The destination operand must be addressable from the ES register. A segment
  67 override is not possible.
  68 <P>
  69 The address of the destination is determined by the contents of the
  70 destination register, not by the explicit operand of STOS. This operand is
  71 used only to validate ES segment addressability and to determine the data
  72 type. Load the correct index value into the destination register before
  73 executing STOS.
  74 <P>
  75 After the transfer is made, DI is automatically updated. If the direction
  76 flag is 0
  77 (<A HREF="CLD.htm">CLD</A> was executed), DI is incremented;
  78 if the direction flag is
  79 1
  80 (<A HREF="STD.htm">STD</A> was executed),
  81 DI is decremented. DI is incremented or decremented by
  82 1 if a byte is stored, by 2 if a word is stored, or by 4 if a doubleword is
  83 stored.
  84 <P>
  85 STOSB, STOSW, and STOSD are synonyms for the byte, word, and doubleword STOS
  86 instructions, that do not require an operand. They are simpler to use, but
  87 provide no type or segment checking.
  88 <P>
  89 STOS can be preceded by the
  90 <A HREF="REP.htm">REP</A> prefix for a block fill of CX or ECX bytes,
  91 words, or doublewords. Refer to the
  92 <A HREF="REP.htm">REP</A> instruction for further details.
  93
  94 <H2>Flags Affected</H2>
  95
  96 None
  97
  98 <H2>Protected Mode Exceptions</H2>
  99
 100 #GP(0) if the result is in a nonwritable segment; #GP(0) for an illegal
 101 memory operand effective address in the CS, DS, ES, FS, or GS segments;
 102 #SS(0) for an illegal address in the SS segment; #PF(fault-code) for a page
 103 fault
 104
 105 <H2>Real Address Mode Exceptions</H2>
 106
 107 Interrupt 13 if any part of the operand would lie outside of the effective
 108 address space from 0 to 0FFFFH
 109
 110 <H2>Virtual 8086 Mode Exceptions</H2>
 111
 112 Same exceptions as in Real Address Mode; #PF(fault-code) for a page fault
 113
 114
 115 <P>
 116 <HR>
 117 <P>
 118 <B>up:</B> <A HREF="c17.htm">
 119 Chapter 17 -- 80386 Instruction Set</A><BR>
 120 <B>prev:</B><A HREF="STI.htm"> STI Set Interrupt Flag</A><BR>
 121 <B>next:</B><A HREF="STR.htm"> STR Store Task Register</A>
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