i386/i386.reference/VERR.htm

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   4 <TITLE>80386 Programmer's Reference Manual -- Opcode VERR</TITLE>
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  14 <H1>VERR, VERW -- Verify a Segment for Reading or Writing</H1>
  15
  16 <PRE>
  17 Opcode       Instruction   Clocks      Description
  18
  19 0F  00 /4    VERR r/m16    pm=10/11    Set ZF=1 if segment can be read,
  20                                        selector in r/m16
  21 0F  00 /5    VERW r/m16    pm=15/16    Set ZF=1 if segment can be written,
  22                                        selector in r/m16
  23 </PRE>
  24
  25
  26 <H2>Operation</H2>
  27
  28 <PRE>
  29 IF segment with selector at (r/m) is accessible
  30    with current protection level
  31    AND ((segment is readable for VERR) OR
  32       (segment is writable for VERW))
  33 THEN ZF := 0;
  34 ELSE ZF := 1;
  35 FI;
  36 </PRE>
  37
  38 <H2>Description</H2>
  39
  40 The two-byte register or memory operand of VERR and VERW contains
  41 the value of a selector. VERR and VERW determine whether the
  42 segment denoted by the selector is reachable from the current privilege
  43 level and whether the segment is readable (VERR) or writable (VERW).
  44 If the segment is accessible, the zero flag is set to 1; if the segment is
  45 not accessible, the zero flag is set to 0. To set ZF, the following
  46 conditions must be met:
  47 <UL>
  48  <LI> The selector must denote a descriptor within the bounds of the table
  49      (GDT or LDT); the selector must be "defined."
  50
  51  <LI> The selector must denote the descriptor of a code or data segment
  52      (not that of a task state segment, LDT, or a gate).
  53
  54  <LI> For VERR, the segment must be readable. For VERW, the segment
  55      must be a writable data segment.
  56
  57  <LI> If the code segment is readable and conforming, the descriptor
  58      privilege level (DPL) can be any value for VERR. Otherwise, the
  59      DPL must be greater than or equal to (have less or the same
  60      privilege as) both the current privilege level and the selector's RPL.
  61 </UL>
  62
  63 The validation performed is the same as if the segment were loaded into
  64 DS, ES, FS, or GS, and the indicated access (read or write) were
  65 performed. The zero flag receives the result of the validation. The
  66 selector's value cannot result in a protection exception, enabling the
  67 software to anticipate possible segment access problems.
  68
  69 <H2>Flags Affected</H2>
  70
  71 ZF as described above
  72
  73 <H2>Protected Mode Exceptions</H2>
  74
  75 Faults generated by illegal addressing of the memory operand that
  76 contains the selector, the selector is not loaded into any segment
  77 register, and no faults attributable to the selector operand are generated
  78 <P>
  79 #GP(0) for an illegal memory operand effective address in the CS, DS,
  80 ES, FS, or GS segments; #SS(0) for an illegal address in the SS segment;
  81 #PF(fault-code) for a page fault
  82
  83 <H2>Real Address Mode Exceptions</H2>
  84
  85 Interrupt 6; VERR and VERW are not recognized in Real Address Mode
  86
  87 <H2>Virtual 8086 Mode Exceptions</H2>
  88
  89 Same exceptions as in Real Address Mode; #PF(fault-code) for a page
  90 fault
  91
  92
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  96 <B>up:</B> <A HREF="c17.htm">
  97 Chapter 17 -- 80386 Instruction Set</A><BR>
  98 <B>prev:</B><A HREF="TEST.htm"> TEST Logical Compare</A><BR>
  99 <B>next:</B><A HREF="WAIT.htm"> WAIT Wait until BUSY# Pin is Inactive (HIGH)</A>
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