hw/i386.reference/REP.htm

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   4 <TITLE>80386 Programmer's Reference Manual -- Opcode REP</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>REP/REPE/REPZ/REPNE/REPNZ -- Repeat Following String Operation</H1>
  15
  16
  17 <PRE>
  18 Opcode    Instruction         Clocks          Description
  19
  20 F3  6C    REP INS r/m8, DX    13+6*(E)CX,
  21                               pm=7+6*(E)CX
  22 If CPL <= IOPL/
  23                               27+6*(E)CX
  24 If CPL > IOPL or if in virtual 8086 mode      Input (E)CX bytes from port
  25                                                DX into ES:[(E)DI]
  26 F3  6D    REP INS r/m16,DX    13+6*(E)CX,
  27                               pm=7+6*(E)CX
  28 If CPL <= IOPL/
  29                               27+6*(E)CX
  30 If CPL > IOPL or if in virtual 8086 mode      Input (E)CX words from port
  31                                                DX into ES:[(E)DI]
  32 F3  6D    REP INS r/m32,DX    13+6*(E)CX,
  33                               pm=7+6*(E)CX
  34 If CPL <= IOPL/
  35                               27+6*(E)CX
  36 If CPL > IOPL or if in virtual 8086 mode      Input (E)CX dwords from port
  37                                                DX into ES:[(E)DI]
  38 F3  A4    REP MOVS m8,m8      5+4*(E)CX        Move (E)CX bytes from
  39                                                [(E)SI] to ES:[(E)DI]
  40 F3  A5    REP MOVS m16,m16    5+4*(E)CX        Move (E)CX words from
  41                                                [(E)SI] to ES:[(E)DI]
  42 F3  A5    REP MOVS m32,m32    5+4*(E)CX        Move (E)CX dwords from
  43                                                [(E)SI] to ES:[(E)DI]
  44 F3  6E    REP OUTS DX,r/m8    5+12*(E)CX,
  45                               pm=6+5*(E)CX
  46 If CPL <= IOPL/
  47                               26+5*(E)CX
  48 If CPL > IOPL or if in virtual 8086 mode      Output (E)CX bytes from
  49                                                [(E)SI] to port DX
  50 F3  6F    REP OUTS DX,r/m16   5+12*(E)CX,
  51                               pm=6+5*(E)CX
  52 If CPL <= IOPL/
  53                               26+5*(E)CX
  54 If CPL > IOPL or if in virtual 8086 mode      Output (E)CX words from
  55                                                [(E)SI] to port DX
  56 F3  6F    REP OUTS DX,r/m32   5+12*(E)CX,
  57                               pm=6+5*(E)CX
  58 If CPL <= IOPL/
  59                               26+5*(E)CX
  60 If CPL > IOPL or if in virtual 8086 mode      Output (E)CX dwords from
  61                                                [(E)SI] to port DX
  62 F3  AA    REP STOS m8         5+5*(E)CX        Fill (E)CX bytes at
  63                                                ES:[(E)DI] with AL
  64 F3  AB    REP STOS m16        5+5*(E)CX        Fill (E)CX words at
  65                                                ES:[(E)DI] with AX
  66 F3  AB    REP STOS m32        5+5*(E)CX        Fill (E)CX dwords at
  67                                                ES:[(E)DI] with EAX
  68 F3  A6    REPE CMPS m8,m8     5+9*N            Find nonmatching bytes in
  69                                                ES:[(E)DI] and [(E)SI]
  70 F3  A7    REPE CMPS m16,m16   5+9*N            Find nonmatching words in
  71                                                ES:[(E)DI] and [(E)SI]
  72 F3  A7    REPE CMPS m32,m32   5+9*N            Find nonmatching dwords in
  73                                                ES:[(E)DI] and [(E)SI]
  74 F3  AE    REPE SCAS m8        5+8*N            Find non-AL byte starting
  75                                                at ES:[(E)DI]
  76 F3  AF    REPE SCAS m16       5+8*N            Find non-AX word starting
  77                                                at ES:[(E)DI]
  78 F3  AF    REPE SCAS m32       5+8*N            Find non-EAX dword starting
  79                                                at ES:[(E)DI]
  80 F2  A6    REPNE CMPS m8,m8    5+9*N            Find matching bytes in
  81                                                ES:[(E)DI] and [(E)SI]
  82 F2  A7    REPNE CMPS m16,m16  5+9*N            Find matching words in
  83                                                ES:[(E)DI] and [(E)SI]
  84 F2  A7    REPNE CMPS m32,m32  5+9*N            Find matching dwords in
  85                                                ES:[(E)DI] and [(E)SI]
  86 F2  AE    REPNE SCAS m8       5+8*N            Find AL, starting at
  87                                                ES:[(E)DI]
  88 F2  AF    REPNE SCAS m16      5+8*N            Find AX, starting at
  89                                                ES:[(E)DI]
  90 F2  AF    REPNE SCAS m32      5+8*N            Find EAX, starting at
  91                                                ES:[(E)DI]
  92 </PRE>
  93
  94 <H2>Operation</H2>
  95
  96 <PRE>
  97 IF AddressSize = 16
  98 THEN use CX for CountReg;
  99 ELSE (* AddressSize = 32 *) use ECX for CountReg;
 100 FI;
 101 WHILE CountReg <> 0
 102 DO
 103    service pending interrupts (if any);
 104    perform primitive string instruction;
 105    CountReg := CountReg - 1;
 106    IF primitive operation is CMPB, CMPW, SCAB, or SCAW
 107    THEN
 108       IF (instruction is REP/REPE/REPZ) AND (ZF=1)
 109       THEN exit WHILE loop
 110       ELSE
 111          IF (instruction is REPNZ or REPNE) AND (ZF=0)
 112          THEN exit WHILE loop;
 113          FI;
 114       FI;
 115    FI;
 116 OD;
 117 </PRE>
 118
 119 <H2>Description</H2>
 120
 121 REP, REPE (repeat while equal), and REPNE (repeat while not equal)
 122 are prefix that are applied to string operation. Each prefix cause the
 123 string instruction that follows to be repeated the number of times
 124 indicated in the count register or (for REPE and REPNE) until the
 125 indicated condition in the zero flag is no longer met.
 126 <P>
 127 Synonymous forms of REPE and REPNE are REPZ and REPNZ,
 128 respectively.
 129 <P>
 130 The REP prefixes apply only to one string instruction at a time. To repeat
 131 a block of instructions, use the
 132 <A HREF="LOOP.htm">LOOP</A> instruction or another looping
 133 construct.
 134 <P>
 135 The precise action for each iteration is as follows:
 136
 137 <OL>
 138  <LI> If the address-size attribute is 16 bits, use CX for the count
 139       register; if the address-size attribute is 32 bits, use ECX for the
 140       count register.
 141
 142  <LI> Check CX. If it is zero, exit the iteration, and move to the next
 143       instruction.
 144
 145  <LI> Acknowledge any pending interrupts.
 146
 147  <LI> Perform the string operation once.
 148
 149  <LI> Decrement CX or ECX by one; no flags are modified.
 150
 151  <LI> Check the zero flag if the string operation is
 152 <A HREF="SCAS.htm">SCAS</A> or <A HREF="CMPS.htm">CMPS</A>. If
 153       the repeat condition does not hold, exit the iteration and move to
 154       the next instruction. Exit the iteration if the prefix is REPE and ZF
 155       is 0 (the last comparison was not equal), or if the prefix is REPNE
 156       and ZF is one (the last comparison was equal).
 157
 158  <LI> Return to step 1 for the next iteration.
 159 </OL>
 160
 161 Repeated
 162 <A HREF="CMPS.htm">CMPS</A> and <A HREF="SCAS.htm">SCAS</A>
 163 instructions can be exited if the count is
 164 exhausted or if the zero flag fails the repeat condition. These two cases
 165 can be distinguished by using either the
 166 <A HREF="Jcc.htm">JCXZ</A> instruction, or by using
 167 the conditional jumps that test the zero flag (<A HREF="Jcc.htm">JZ</A>,
 168 <A HREF="Jcc.htm">JNZ</A>, and
 169 <A HREF="Jcc.htm">JNE</A>).
 170
 171 <H2>Flags Affected</H2>
 172
 173 ZF by REP <A HREF="CMPS.htm">CMPS</A> and REP
 174 <A HREF="SCAS.htm">SCAS</A> as described above
 175
 176 <H2>Protected Mode Exceptions</H2>
 177
 178 #UD if a repeat prefix is used before an instruction that is not in the
 179 list above; further exceptions can be generated when the string operation is
 180 executed; refer to the descriptions of the string instructions themselves
 181
 182 <H2>Real Address Mode Exceptions</H2>
 183
 184 Interrupt 6 if a repeat prefix is used before an instruction that is not in
 185 the list above; further exceptions can be generated when the string
 186 operation is executed; refer to the descriptions of the string instructions
 187 themselves
 188
 189 <H2>Virtual 8086 Mode Exceptions</H2>
 190
 191 #UD if a repeat prefix is used before an instruction that is not in the
 192 list above; further exceptions can be generated when the string operation is
 193 executed; refer to the descriptions of the string instructions themselves
 194
 195 <H2>Notes</H2>
 196
 197 Not all input/output ports can handle the rate at which the REP
 198 <A HREF="INS.htm">INS</A>
 199 and REP <A HREF="OUTS.htm">OUTS</A> instructions execute.
 200 <P>
 201 <HR>
 202 <P>
 203 <B>up:</B> <A HREF="c17.htm">
 204 Chapter 17 -- 80386 Instruction Set</A><BR>
 205 <B>prev:</B><A HREF="RCL.htm"> RCL/RCR/ROL/ROR Rotate</A><BR>
 206 <B>next:</B><A HREF="RET.htm"> RET Return from Procedure</A>
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