mingw/info/gdb/Memory.html

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  12 Node:<a name="Memory">Memory</a>,
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  18
  19 <h3 class="section">Examining memory</h3>
  20
  21    <p>You can use the command <code>x</code> (for "examine") to examine memory in
  22 any of several formats, independently of your program's data types.
  23
  24      <dl>
  25 <dt><code>x/</code><var>nfu</var><code> </code><var>addr</var><code></code>
  26      <dd><dt><code>x </code><var>addr</var><code></code>
  27      <dd><dt><code>x</code>
  28      <dd>Use the <code>x</code> command to examine memory.
  29 </dl>
  30
  31    <p><var>n</var>, <var>f</var>, and <var>u</var> are all optional parameters that specify how
  32 much memory to display and how to format it; <var>addr</var> is an
  33 expression giving the address where you want to start displaying memory.
  34 If you use defaults for <var>nfu</var>, you need not type the slash <code>/</code>.
  35 Several commands set convenient defaults for <var>addr</var>.
  36
  37      <dl>
  38 <dt><var>n</var>, the repeat count
  39      <dd>The repeat count is a decimal integer; the default is 1.  It specifies
  40 how much memory (counting by units <var>u</var>) to display.
  41
  42      <br><dt><var>f</var>, the display format
  43      <dd>The display format is one of the formats used by <code>print</code>,
  44 <code>s</code> (null-terminated string), or <code>i</code> (machine instruction).
  45 The default is <code>x</code> (hexadecimal) initially.
  46 The default changes each time you use either <code>x</code> or <code>print</code>.
  47
  48      <br><dt><var>u</var>, the unit size
  49      <dd>The unit size is any of
  50
  51           <dl>
  52 <dt><code>b</code>
  53           <dd>Bytes.
  54 <br><dt><code>h</code>
  55           <dd>Halfwords (two bytes).
  56 <br><dt><code>w</code>
  57           <dd>Words (four bytes).  This is the initial default.
  58 <br><dt><code>g</code>
  59           <dd>Giant words (eight bytes).
  60 </dl>
  61
  62      <p>Each time you specify a unit size with <code>x</code>, that size becomes the
  63 default unit the next time you use <code>x</code>.  (For the <code>s</code> and
  64 <code>i</code> formats, the unit size is ignored and is normally not written.)
  65
  66      <br><dt><var>addr</var>, starting display address
  67      <dd><var>addr</var> is the address where you want GDB to begin displaying
  68 memory.  The expression need not have a pointer value (though it may);
  69 it is always interpreted as an integer address of a byte of memory.
  70 See <a href="Expressions.html#Expressions">Expressions</a>, for more information on expressions.  The default for
  71 <var>addr</var> is usually just after the last address examined--but several
  72 other commands also set the default address: <code>info breakpoints</code> (to
  73 the address of the last breakpoint listed), <code>info line</code> (to the
  74 starting address of a line), and <code>print</code> (if you use it to display
  75 a value from memory).
  76 </dl>
  77
  78    <p>For example, <code>x/3uh 0x54320</code> is a request to display three halfwords
  79 (<code>h</code>) of memory, formatted as unsigned decimal integers (<code>u</code>),
  80 starting at address <code>0x54320</code>.  <code>x/4xw $sp</code> prints the four
  81 words (<code>w</code>) of memory above the stack pointer (here, <code>$sp</code>;
  82 see <a href="Registers.html#Registers">Registers</a>) in hexadecimal (<code>x</code>).
  83
  84    <p>Since the letters indicating unit sizes are all distinct from the
  85 letters specifying output formats, you do not have to remember whether
  86 unit size or format comes first; either order works.  The output
  87 specifications <code>4xw</code> and <code>4wx</code> mean exactly the same thing.
  88 (However, the count <var>n</var> must come first; <code>wx4</code> does not work.)
  89
  90    <p>Even though the unit size <var>u</var> is ignored for the formats <code>s</code>
  91 and <code>i</code>, you might still want to use a count <var>n</var>; for example,
  92 <code>3i</code> specifies that you want to see three machine instructions,
  93 including any operands.  The command <code>disassemble</code> gives an
  94 alternative way of inspecting machine instructions; see <a href="Machine-Code.html#Machine%20Code">Source and machine code</a>.
  95
  96    <p>All the defaults for the arguments to <code>x</code> are designed to make it
  97 easy to continue scanning memory with minimal specifications each time
  98 you use <code>x</code>.  For example, after you have inspected three machine
  99 instructions with <code>x/3i </code><var>addr</var><code></code>, you can inspect the next seven
 100 with just <code>x/7</code>.  If you use &lt;RET&gt; to repeat the <code>x</code> command,
 101 the repeat count <var>n</var> is used again; the other arguments default as
 102 for successive uses of <code>x</code>.
 103
 104    <p>The addresses and contents printed by the <code>x</code> command are not saved
 105 in the value history because there is often too much of them and they
 106 would get in the way.  Instead, GDB makes these values available for
 107 subsequent use in expressions as values of the convenience variables
 108 <code>$_</code> and <code>$__</code>.  After an <code>x</code> command, the last address
 109 examined is available for use in expressions in the convenience variable
 110 <code>$_</code>.  The contents of that address, as examined, are available in
 111 the convenience variable <code>$__</code>.
 112
 113    <p>If the <code>x</code> command has a repeat count, the address and contents saved
 114 are from the last memory unit printed; this is not the same as the last
 115 address printed if several units were printed on the last line of output.
 116
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