i386/i386.reference/s03_06.htm

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   4 <TITLE>80386 Programmer's Reference Manual -- Section 3.6</TITLE>
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   7 <B>up:</B> <A HREF="c03.htm">
   8 Chapter 3 -- Applications Instruction Set</A><BR>
   9 <B>prev:</B> <A HREF="s03_05.htm">3.5  Control Transfer Instructions</A><BR>
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  11 <P>
  12 <HR>
  13 <P>
  14 <H1>3.6  String and Character Translation Instructions</H1>
  15 The instructions in this category operate on strings rather than on logical
  16 or numeric values. Refer also to the section on I/O for information about
  17 the string I/O instructions (also known as block I/O).
  18 <P>
  19 The power of 80386 string operations derives from the following features of
  20 the architecture:
  21 <OL>
  22 <LI> A set of primitive string operations
  23 <UL>
  24 <LI><A HREF="MOVS.htm">MOVS</A> -- Move String
  25 <LI><A HREF="CMPS.htm">CMPS</A> -- Compare string
  26 <LI><A HREF="SCAS.htm">SCAS</A> -- Scan string
  27 <LI><A HREF="LODS.htm">LODS</A> -- Load string
  28 <LI><A HREF="STOS.htm">STOS</A> -- Store string
  29 </UL>
  30 <P>
  31 <LI> Indirect, indexed addressing, with automatic incrementing or
  32 decrementing of the indexes.
  33 <DL>
  34 <DT>
  35 Indexes:
  36 <DD>
  37 ESI -- Source index register<BR>
  38 EDI -- Destination index register
  39 <DT>
  40 Control flag:
  41 <DD>
  42 DF -- Direction flag
  43 <DT>
  44 Control flag instructions:
  45 <DD>
  46 <A HREF="CLD.htm">CLD</A> -- Clear direction flag instruction<BR>
  47 <A HREF="STD.htm">STD</A> -- Set direction flag instruction
  48 </DL>
  49 <P>
  50 <LI> Repeat prefixes
  51 <UL>
  52 <LI><A HREF="REP.htm">REP</A> -- Repeat while ECX not xero
  53 <LI><A HREF="REP.htm">REPE/REPZ</A> -- Repeat while equal or zero
  54 <LI><A HREF="REP.htm">REPNE/REPNZ</A> -- Repeat while not equal or not zero
  55 </UL>
  56 </OL>
  57 <P>
  58 The primitive string operations operate on one element of a string. A
  59 string element may be a byte, a word, or a doubleword. The string elements
  60 are addressed by the registers ESI and EDI. After every primitive operation
  61 ESI and/or EDI are automatically updated to point to the next element of the
  62 string. If the direction flag is zero, the index registers are incremented;
  63 if one, they are decremented. The amount of the increment or decrement is
  64 1, 2, or 4 depending on the size of the string element.
  65
  66 <H2>3.6.1  Repeat Prefixes</H2>
  67 The repeat prefixes
  68 <A HREF="REP.htm">REP</A> (Repeat While ECX Not Zero),
  69 <A HREF="REP.htm">REPE/REPZ</A> (Repeat
  70 While Equal/Zero), and
  71 <A HREF="REP.htm">REPNE/REPNZ</A> (Repeat While Not Equal/Not Zero) specify
  72 repeated operation of a string primitive. This form of iteration allows the
  73 CPU to process strings much faster than would be possible with a regular
  74 software loop.
  75 <P>
  76 When a primitive string operation has a repeat prefix, the operation is
  77 executed repeatedly, each time using a different element of the string. The
  78 repetition terminates when one of the conditions specified by the prefix is
  79 satisfied.
  80 <P>
  81 At each repetition of the primitive instruction, the string operation may
  82 be suspended temporarily in order to handle an exception or external
  83 interrupt. After the interruption, the string operation can be restarted
  84 again where it left off. This method of handling strings allows operations
  85 on strings of arbitrary length, without affecting interrupt response.
  86 <P>
  87 All three prefixes causes the hardware to automatically repeat the
  88 associated string primitive until ECX=0. The differences among the repeat
  89 prefixes have to do with the second termination condition.
  90 <A HREF="REP.htm">REPE/REPZ</A> and
  91 <A HREF="REP.htm">REPNE/REPNZ</A> are used exclusively with the
  92 <A HREF="SCAS.htm">SCAS</A> (Scan String) and
  93 <A HREF="CMPS.htm">CMPS</A>
  94 (Compare String) primitives. When these prefixes are used, repetition of the
  95 next instruction depends on the zero flag (ZF) as well as the ECX register.
  96 ZF does not require initialization before execution of a repeated string
  97 instruction, because both
  98 <A HREF="SCAS.htm">SCAS</A> and
  99 <A HREF="CMPS.htm">CMPS</A> set ZF according to the results of
 100 the comparisons they make. The differences are summarized in the
 101 accompanying table.
 102 <PRE>
 103 Prefix                      Termination         Termination
 104 Condition 1         Condition 2
 105 REP                           ECX = 0             (none)
 106 REPE/REPZ                     ECX = 0             ZF = 0
 107 REPNE/REPNZ                   ECX = 0             ZF = 1
 108 </PRE>
 109
 110 <H2>3.6.2  Indexing and Direction Flag Control</H2>
 111 The addresses of the operands of string primitives are determined by the
 112 ESI and EDI registers. ESI points to source operands. By default, ESI refers
 113 to a location in the segment indicated by the DS segment register. A
 114 segment-override prefix may be used, however, to cause ESI to refer to CS,
 115 SS, ES, FS, or GS. EDI points to destination operands in the segment
 116 indicated by ES; no segment override is possible. The use of two different
 117 segment registers in one instruction allows movement of strings between
 118 different segments.
 119 <P>
 120 This use of ESI and DSI has led to the descriptive names source index and
 121 destination index for the ESI and EDI registers, respectively. In all
 122 cases other than string instructions, however, the ESI and EDI registers may
 123 be used as general-purpose registers.
 124 <P>
 125 When ESI and EDI are used in string primitives, they are automatically
 126 incremented or decremented after to operation. The direction flag determines
 127 whether they are incremented or decremented. The instruction
 128 <A HREF="CLD.htm">CLD</A> puts zero
 129 in DF, causing the index registers to be incremented; the instruction
 130 <A HREF="STD.htm">STD</A>
 131 puts one in DF, causing the index registers to be decremented. Programmers
 132 should always put a known value in DF before using string instructions in a
 133 procedure.
 134
 135 <H2>3.6.3  String Instructions</H2>
 136 <A HREF="MOVS.htm">MOVS</A> (Move String)
 137 moves the string element pointed to by ESI to the
 138 location pointed to by EDI.
 139 <A HREF="MOVS.htm">MOVSB</A> operates on byte elements,
 140 <A HREF="MOVS.htm">MOVSW</A> operates on word elements, and
 141 <A HREF="MOVS.htm">MOVSD</A> operates on doublewords. The destination segment
 142 register cannot be overridden by a segment override prefix, but the source
 143 segment register can be overridden.
 144 <P>
 145 The <A HREF="MOVS.htm">MOVS</A> instruction, when accompanied by the
 146 <A HREF="REP.htm">REP</A> prefix, operates as a
 147 memory-to-memory block transfer. To set up for this operation, the program
 148 must initialize ECX and the register pairs ESI and EDI. ECX specifies the
 149 number of bytes, words, or doublewords in the block.
 150 <P>
 151 If DF=0, the program must point ESI to the first element of the source
 152 string and point EDI to the destination address for the first element. If
 153 DF=1, the program must point these two registers to the last element of the
 154 source string and to the destination address for the last element,
 155 respectively.
 156 <P>
 157 <A HREF="CMPS.htm">CMPS</A> (Compare Strings)
 158 subtracts the destination string element (at ES:EDI)
 159 from the source string element (at ESI) and updates the flags AF, SF, PF, CF
 160 and OF. If the string elements are equal, ZF=1; otherwise, ZF=0. If DF=0,
 161 the processor increments the memory pointers (ESI and EDI) for the two
 162 strings.
 163 <A HREF="CMPS.htm">CMPSB</A> compares bytes,
 164 <A HREF="MOVS.htm">MOVSW</A> compares words, and
 165 <A HREF="MOVS.htm">MOVSD</A> compares doublewords.
 166 The segment register used for the source address can be changed
 167 with a segment override prefix while the destination segment register
 168 cannot be overridden.
 169 <P>
 170 <A HREF="SCAS.htm">SCAS</A> (Scan String)
 171 subtracts the destination string element at ES:EDI from
 172 EAX, AX, or AL and updates the flags AF, SF, ZF, PF, CF and OF. If the
 173 values are equal, ZF=1; otherwise, ZF=0. If DF=0, the processor increments
 174 the memory pointer (EDI) for the string.
 175 <A HREF="SCAS.htm">SCASB</A> scans bytes;
 176 <A HREF="SCAS.htm">SCASW</A> scans words;
 177 <A HREF="SCAS.htm">SCASD</A> scans doublewords.
 178 The destination segment register (ES) cannot
 179 be overridden.
 180 <P>
 181 When either the
 182 <A HREF="REP.htm">REPE</A> or
 183 <A HREF="REP.htm">REPNE</A> prefix modifies either the
 184 <A HREF="SCAS.htm">SCAS</A> or
 185 <A HREF="CMPS.htm">CMPS</A>
 186 primitives, the processor compares the value of the current string element
 187 with the value in EAX for doubleword elements, in AX for word elements, or
 188 in AL for byte elements. Termination of the repeated operation depends on
 189 the resulting state of ZF as well as on the value in ECX.
 190 <P>
 191 <A HREF="LODS.htm">LODS</A> (Load String)
 192 places the source string element at ESI into EAX for
 193 doubleword strings, into AX for word strings, or into AL for byte strings.
 194 <A HREF="LODS.htm">LODS</A> increments or decrements ESI according to DF.
 195 <P>
 196 <A HREF="STOS.htm">STOS</A> (Store String)
 197 places the source string element from EAX, AX, or AL
 198 into the string at ES:DSI.
 199 <A HREF="STOS.htm">STOS</A> increments or decrements EDI according to
 200 DF.
 201 <P>
 202 <HR>
 203 <P>
 204 <B>up:</B> <A HREF="c03.htm">
 205 Chapter 3 -- Applications Instruction Set</A><BR>
 206 <B>prev:</B> <A HREF="s03_05.htm">3.5  Control Transfer Instructions</A><BR>
 207 <B>next:</B> <A HREF="s03_07.htm">3.7  Instructions for Block-Structured Languages</A>
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