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250 lines
12 KiB
HTML
4 years ago
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<!-- Copyright (C) 1988-2019 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation; with the
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Invariant Sections being "Free Software" and "Free Software Needs
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Free Documentation", with the Front-Cover Texts being "A GNU Manual,"
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and with the Back-Cover Texts as in (a) below.
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(a) The FSF's Back-Cover Text is: "You are free to copy and modify
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this GNU Manual. Buying copies from GNU Press supports the FSF in
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developing GNU and promoting software freedom." -->
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<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>Registers (Debugging with GDB)</title>
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<meta name="description" content="Registers (Debugging with GDB)">
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<meta name="keywords" content="Registers (Debugging with GDB)">
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<meta name="Generator" content="makeinfo">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="Concept-Index.html#Concept-Index" rel="index" title="Concept Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Data.html#Data" rel="up" title="Data">
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<link href="Floating-Point-Hardware.html#Floating-Point-Hardware" rel="next" title="Floating Point Hardware">
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<link href="Convenience-Funs.html#Convenience-Funs" rel="prev" title="Convenience Funs">
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</head>
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<body lang="en">
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<a name="Registers"></a>
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<div class="header">
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<p>
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Next: <a href="Floating-Point-Hardware.html#Floating-Point-Hardware" accesskey="n" rel="next">Floating Point Hardware</a>, Previous: <a href="Convenience-Funs.html#Convenience-Funs" accesskey="p" rel="prev">Convenience Funs</a>, Up: <a href="Data.html#Data" accesskey="u" rel="up">Data</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="Registers-1"></a>
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<h3 class="section">10.13 Registers</h3>
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<a name="index-registers"></a>
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<p>You can refer to machine register contents, in expressions, as variables
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with names starting with ‘<samp>$</samp>’. The names of registers are different
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for each machine; use <code>info registers</code> to see the names used on
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your machine.
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</p>
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<dl compact="compact">
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<dd><a name="index-info-registers"></a>
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</dd>
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<dt><code>info registers</code></dt>
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<dd><p>Print the names and values of all registers except floating-point
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and vector registers (in the selected stack frame).
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</p>
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<a name="index-info-all_002dregisters"></a>
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<a name="index-floating-point-registers"></a>
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</dd>
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<dt><code>info all-registers</code></dt>
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<dd><p>Print the names and values of all registers, including floating-point
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and vector registers (in the selected stack frame).
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</p>
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</dd>
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<dt><code>info registers <var>reggroup</var> …</code></dt>
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<dd><p>Print the name and value of the registers in each of the specified
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<var>reggroup</var>s. The <var>reggoup</var> can be any of those returned by
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<code>maint print reggroups</code> (see <a href="Maintenance-Commands.html#Maintenance-Commands">Maintenance Commands</a>).
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</p>
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</dd>
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<dt><code>info registers <var>regname</var> …</code></dt>
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<dd><p>Print the <em>relativized</em> value of each specified register <var>regname</var>.
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As discussed in detail below, register values are normally relative to
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the selected stack frame. The <var>regname</var> may be any register name valid on
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the machine you are using, with or without the initial ‘<samp>$</samp>’.
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</p></dd>
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</dl>
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<a name="standard-registers"></a><a name="index-stack-pointer-register"></a>
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<a name="index-program-counter-register"></a>
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<a name="index-process-status-register"></a>
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<a name="index-frame-pointer-register"></a>
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<a name="index-standard-registers"></a>
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<p><small>GDB</small> has four “standard” register names that are available (in
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expressions) on most machines—whenever they do not conflict with an
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architecture’s canonical mnemonics for registers. The register names
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<code>$pc</code> and <code>$sp</code> are used for the program counter register and
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the stack pointer. <code>$fp</code> is used for a register that contains a
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pointer to the current stack frame, and <code>$ps</code> is used for a
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register that contains the processor status. For example,
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you could print the program counter in hex with
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</p>
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<div class="smallexample">
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<pre class="smallexample">p/x $pc
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</pre></div>
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<p>or print the instruction to be executed next with
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</p>
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<div class="smallexample">
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<pre class="smallexample">x/i $pc
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</pre></div>
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<p>or add four to the stack pointer<a name="DOCF11" href="#FOOT11"><sup>11</sup></a> with
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</p>
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<div class="smallexample">
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<pre class="smallexample">set $sp += 4
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</pre></div>
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<p>Whenever possible, these four standard register names are available on
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your machine even though the machine has different canonical mnemonics,
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so long as there is no conflict. The <code>info registers</code> command
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shows the canonical names. For example, on the SPARC, <code>info
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registers</code> displays the processor status register as <code>$psr</code> but you
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can also refer to it as <code>$ps</code>; and on x86-based machines <code>$ps</code>
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is an alias for the <small>EFLAGS</small> register.
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</p>
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<p><small>GDB</small> always considers the contents of an ordinary register as an
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integer when the register is examined in this way. Some machines have
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special registers which can hold nothing but floating point; these
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registers are considered to have floating point values. There is no way
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to refer to the contents of an ordinary register as floating point value
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(although you can <em>print</em> it as a floating point value with
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‘<samp>print/f $<var>regname</var></samp>’).
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</p>
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<p>Some registers have distinct “raw” and “virtual” data formats. This
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means that the data format in which the register contents are saved by
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the operating system is not the same one that your program normally
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sees. For example, the registers of the 68881 floating point
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coprocessor are always saved in “extended” (raw) format, but all C
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programs expect to work with “double” (virtual) format. In such
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cases, <small>GDB</small> normally works with the virtual format only (the format
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that makes sense for your program), but the <code>info registers</code> command
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prints the data in both formats.
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</p>
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<a name="index-SSE-registers-_0028x86_0029"></a>
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<a name="index-MMX-registers-_0028x86_0029"></a>
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<p>Some machines have special registers whose contents can be interpreted
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in several different ways. For example, modern x86-based machines
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have SSE and MMX registers that can hold several values packed
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together in several different formats. <small>GDB</small> refers to such
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registers in <code>struct</code> notation:
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</p>
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<div class="smallexample">
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<pre class="smallexample">(gdb) print $xmm1
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$1 = {
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v4_float = {0, 3.43859137e-038, 1.54142831e-044, 1.821688e-044},
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v2_double = {9.92129282474342e-303, 2.7585945287983262e-313},
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v16_int8 = "\000\000\000\000\3706;\001\v\000\000\000\r\000\000",
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v8_int16 = {0, 0, 14072, 315, 11, 0, 13, 0},
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v4_int32 = {0, 20657912, 11, 13},
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v2_int64 = {88725056443645952, 55834574859},
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uint128 = 0x0000000d0000000b013b36f800000000
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}
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</pre></div>
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<p>To set values of such registers, you need to tell <small>GDB</small> which
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view of the register you wish to change, as if you were assigning
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value to a <code>struct</code> member:
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</p>
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<div class="smallexample">
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<pre class="smallexample"> (gdb) set $xmm1.uint128 = 0x000000000000000000000000FFFFFFFF
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</pre></div>
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<p>Normally, register values are relative to the selected stack frame
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(see <a href="Selection.html#Selection">Selecting a Frame</a>). This means that you get the
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value that the register would contain if all stack frames farther in
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were exited and their saved registers restored. In order to see the
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true contents of hardware registers, you must select the innermost
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frame (with ‘<samp>frame 0</samp>’).
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</p>
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<a name="index-caller_002dsaved-registers"></a>
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<a name="index-call_002dclobbered-registers"></a>
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<a name="index-volatile-registers"></a>
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<a name="index-_003cnot-saved_003e-values"></a>
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<p>Usually ABIs reserve some registers as not needed to be saved by the
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callee (a.k.a.: “caller-saved”, “call-clobbered” or “volatile”
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registers). It may therefore not be possible for <small>GDB</small> to know
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the value a register had before the call (in other words, in the outer
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frame), if the register value has since been changed by the callee.
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<small>GDB</small> tries to deduce where the inner frame saved
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(“callee-saved”) registers, from the debug info, unwind info, or the
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machine code generated by your compiler. If some register is not
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saved, and <small>GDB</small> knows the register is “caller-saved” (via
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its own knowledge of the ABI, or because the debug/unwind info
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explicitly says the register’s value is undefined), <small>GDB</small>
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displays ‘<samp><not saved></samp>’<!-- /@w --> as the register’s value. With targets
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that <small>GDB</small> has no knowledge of the register saving convention,
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if a register was not saved by the callee, then its value and location
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in the outer frame are assumed to be the same of the inner frame.
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This is usually harmless, because if the register is call-clobbered,
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the caller either does not care what is in the register after the
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call, or has code to restore the value that it does care about. Note,
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however, that if you change such a register in the outer frame, you
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may also be affecting the inner frame. Also, the more “outer” the
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frame is you’re looking at, the more likely a call-clobbered
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register’s value is to be wrong, in the sense that it doesn’t actually
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represent the value the register had just before the call.
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</p>
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<div class="footnote">
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<hr>
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<h4 class="footnotes-heading">Footnotes</h4>
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<h3><a name="FOOT11" href="#DOCF11">(11)</a></h3>
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<p>This is a way of removing
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one word from the stack, on machines where stacks grow downward in
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memory (most machines, nowadays). This assumes that the innermost
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stack frame is selected; setting <code>$sp</code> is not allowed when other
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stack frames are selected. To pop entire frames off the stack,
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regardless of machine architecture, use <code>return</code>;
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see <a href="Returning.html#Returning">Returning from a Function</a>.</p>
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</div>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Floating-Point-Hardware.html#Floating-Point-Hardware" accesskey="n" rel="next">Floating Point Hardware</a>, Previous: <a href="Convenience-Funs.html#Convenience-Funs" accesskey="p" rel="prev">Convenience Funs</a>, Up: <a href="Data.html#Data" accesskey="u" rel="up">Data</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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</body>
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</html>
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