hw/i386.reference/SCAS.htm

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   4 <TITLE>80386 Programmer's Reference Manual -- Opcode SCAS</TITLE>
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  13 <P>
  14 <H1>SCAS/SCASB/SCASW/SCASD -- Compare String Data</H1>
  15
  16 <PRE>
  17 Opcode  Instruction  Clocks  Description
  18
  19 AE      SCAS m8      7       Compare bytes AL-ES:[DI], update (E)DI
  20 AF      SCAS m16     7       Compare words AX-ES:[DI], update (E)DI
  21 AF      SCAS m32     7       Compare dwords EAX-ES:[DI], update (E)DI
  22 AE      SCASB        7       Compare bytes AL-ES:[DI], update (E)DI
  23 AF      SCASW        7       Compare words AX-ES:[DI], update (E)DI
  24 AF      SCASD        7       Compare dwords EAX-ES:[DI], update (E)DI
  25 </PRE>
  26
  27
  28 <H2>Operation</H2>
  29
  30 <PRE>
  31 IF AddressSize = 16
  32 THEN use DI for dest-index;
  33 ELSE (* AddressSize = 32 *) use EDI for dest-index;
  34 FI;
  35 IF byte type of instruction
  36 THEN
  37    AL - [dest-index]; (* Compare byte in AL and dest *)
  38    IF DF = 0 THEN IndDec := 1 ELSE IncDec := -1; FI;
  39 ELSE
  40    IF OperandSize = 16
  41    THEN
  42       AX - [dest-index]; (* compare word in AL and dest *)
  43       IF DF = 0 THEN IncDec := 2 ELSE IncDec := -2; FI;
  44    ELSE (* OperandSize = 32 *)
  45       EAX - [dest-index];(* compare dword in EAX & dest *)
  46       IF DF = 0 THEN IncDec := 4 ELSE IncDec := -4; FI;
  47    FI;
  48 FI;
  49 dest-index := dest-index + IncDec
  50 </PRE>
  51
  52 <H2>Description</H2>
  53
  54 SCAS subtracts the memory byte or word at the destination register from
  55 the AL, AX or EAX register. The result is discarded; only the flags are set.
  56 The operand must be addressable from the ES segment; no segment override is
  57 possible.
  58 <P>
  59 If the address-size attribute for this instruction is 16 bits, DI is used
  60 as the destination register; otherwise, the address-size attribute is 32
  61 bits and EDI is used.
  62 <P>
  63 The address of the memory data being compared is determined solely by the
  64 contents of the destination register, not by the operand to SCAS. The
  65 operand validates ES segment addressability and determines the data type.
  66 Load the correct index value into DI or EDI before executing SCAS.
  67 <P>
  68 After the comparison is made, the destination register is automatically
  69 updated. If the direction flag is 0
  70 (<A HREF="CLD.htm">CLD</A> was executed), the destination
  71 register is incremented; if the direction flag is 1
  72 (<A HREF="STD.htm">STD</A> was executed), it
  73 is decremented. The increments or decrements are by 1 if bytes are compared,
  74 by 2 if words are compared, or by 4 if doublewords are compared.
  75 <P>
  76 SCASB, SCASW, and SCASD are synonyms for the byte, word and
  77 doubleword SCAS instructions that don't require operands. They are
  78 simpler to code, but provide no type or segment checking.
  79 <P>
  80 SCAS can be preceded by the
  81 <A HREF="REP.htm">REPE</A> or
  82 <A HREF="REP.htm">REPNE</A> prefix for a block search
  83 of CX or ECX bytes or words. Refer to the
  84 <A HREF="REP.htm">REP</A> instruction for further
  85 details.
  86
  87 <H2>Flags Affected</H2>
  88
  89 OF, SF, ZF, AF, PF, and CF as described in <A HREF="appc.htm">Appendix C</A>
  90
  91 <H2>Protected Mode Exceptions</H2>
  92
  93 #GP(0) for an illegal memory operand effective address in the CS, DS,
  94 ES, FS, or GS segments; #SS(0) for an illegal address in the SS segment;
  95 #PF(fault-code) for a page fault
  96
  97 <H2>Real Address Mode Exceptions</H2>
  98
  99 Interrupt 13 if any part of the operand would lie outside of the effective
 100 address space from 0 to 0FFFFH
 101
 102 <H2>Virtual 8086 Mode Exceptions</H2>
 103
 104 Same exceptions as in Real Address Mode; #PF(fault-code) for a page
 105 fault
 106
 107
 108 <P>
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 110 <P>
 111 <B>up:</B> <A HREF="c17.htm">
 112 Chapter 17 -- 80386 Instruction Set</A><BR>
 113 <B>prev:</B><A HREF="SBB.htm"> SBB Integer Subtraction with Borrow</A><BR>
 114 <B>next:</B><A HREF="SETcc.htm"> SETcc Byte Set on Condition</A>
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