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<title>Finding the Stack</title>
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<div class="Head2">
<h2>Finding the Stack</h2>
</div><div>
<p>To trace back through a thread’s kernel or user stack, you first
need to find the stack pointer value. On the ARM, R13 always
points to the stack, but there are different R13 registers for
each processor mode:</p>
<ul>
<li><p>In thread context:</p>
<ul>
<li>
<p>R13usr points to the thread’s user
stack,</p>
</li>
<li>
<p>R13svc points to the thread’s kernel
stack.</p>
</li>
</ul>
</li>
<li>
<p>When handling interrupts, dedicated stacks are used:</p>
<ul>
<li>
<p>R13Fiq points to the stack used when
processing fast interrupts (FIQ).</p>
</li>
<li>
<p>R13Irq points to the stack used when
processing general purpose interrupts (IRQ)</p>
</li>
</ul>
</li>
</ul>
<p>To find out which stack to inspect, you need to know what mode the
CPU was in when the fault occurred. The
<a href="CrashDebuggerARMexceptionsProcessorModes.guide.html" title="ARM Exception types, fault status register values, processor modes / ARM processor modes (CPSR register)">processor mode</a> is identified by the five least-significant bits of the CPSR
register. To get the value of the CPSR register:</p>
<ul>
<li>
<p>use the <a href="CrashDebugger_cmd_f.guide.html" title="The debug monitor and command syntax / f - display kernel fault information">f</a> command when the
debug monitor is triggered by a hardware exception.</p>
</li>
<li>
<p>use the <a href="CrashDebugger_cmd_r.guide.html" title="The debug monitor and command syntax / r - dump register contents">r</a> command when the
debug monitor is triggered by a panic.</p>
</li>
</ul>
<p>The following examples show how to find the stack(s):</p>
<ul>
<li>
<p><b><a href="CrashDebuggerCallStack.guide03.html#call01" title="Examining the call stack / Finding the stack / Kernel & user stacks of the current thread after a hardware exception">Kernel & user stacks of the current thread after a hardware exception</a></b></p>
</li>
<li class="style6">
<p><a href="CrashDebuggerCallStack.guide03.html#call02" title="Examining the call stack / Finding the stack / Kernel & user stacks of the current thread after a panic">Kernel & user stacks of the current thread after a panic</a></p>
</li>
<li class="style6">
<p><a href="CrashDebuggerCallStack.guide03.html#call03" title="Examining the call stack / Finding the stack / Interrupt stacks">Interrupt stacks</a></p>
</li>
<li>
<p><b><a href="CrashDebuggerCallStack.guide03.html#call04" title="Examining the call stack / Finding the stack / Kernel & user stacks of a non-current thread">Kernel & user stacks of a non-current thread</a></b></p>
</li>
</ul>
</div>
<div class="Head3">
<h3><a name="call01"></a>Kernel & user stacks of the current thread after a
hardware exception</h3>
</div><div>
<p>Use the <a href="CrashDebugger_cmd_f.guide.html" title="The debug monitor and command syntax / f - display kernel fault information">f</a> command.</p>
<p class="CodeBlock">Fault Category: Exception Fault Reason: 10000000<br>ExcId 00000001 CodeAddr f816c908 DataAddr 80000001 Extra c0007003<br>Exc 1 Cpsr=60000010 FAR=80000001 FSR=c0007003<br> R0=00000000 R1=00000000 R2=30000000 R3=80000001<br> R4=00000001 R5=00403d88 R6=00002000 R7=f816c768<br> R8=00000012 R9=00000040 R10=00000000 R11=00403fa4<br>R12=00403d5c R13=00403d70 R14=f80906f8 R15=f816c908<br>R13Svc=6571e000 R14Svc=f80074bc SpsrSvc=80000010</p>
<p>In this example:</p>
<ul>
<li>
<p>the kernel stack is the value of R13Svc, i.e.
0x6571e00.</p>
</li>
<li>
<p>the user stack is the value of R13, i.e.
0x00403d70.</p>
</li>
</ul>
</div>
<div class="Head3">
<h3><a name="call02" id="call02"></a>Kernel & user stacks of the current thread after a
panic</h3>
</div><div>
<p>Use the <a href="CrashDebugger_cmd_r.guide.html" title="The debug monitor and command syntax / r - dump register contents">r</a> command.</p>
<p class="CodeBlock">MODE_USR:<br> R0=6571de54 R1=0000002a R2=00000002 R3=ffffffff<br> R4=0000002a R5=f8170414 R6=6571df14 R7=6403cc50<br> R8=00000001 R9=6403c44c R10=640002f8 R11=6571de70<br>R12=00000020 R13=00404e00 R14=f80818c0 R15=f800bfa8<br>CPSR=60000013<br>MODE_FIQ:<br> R8=00000000 R9=ffffffff R10=ffffffff R11=00000000<br>R12=00000000 R13=64000d0c R14=c080079c SPSR=e00000dc<br>MODE_IRQ:<br>R13=6400110c R14=00000013 SPSR=20000013<br>MODE_SVC:<br>R13=6571de54 R14=f80328bc SPSR=60000010<br>MODE_ABT:<br>R13=6400090c R14=ccbfd0e0 SPSR=b00000d9<br>MODE_UND:<br>R13=6400090c R14=b5a39950 SPSR=f000009d</p>
<p>In this example:</p>
<ul>
<li>
<p>the kernel stack is the value of R13 under
MODE_SVC:, i.e. 0x6571de54.</p>
</li>
<li>
<p>the user stack is the value of R13 under
MODE_USR:, i.e. 0x00404e00.</p>
</li>
</ul>
</div>
<div class="Head3">
<h3><a name="call03" id="call03"></a>Interrupt stacks</h3>
</div><div>
<p>Use the <a href="CrashDebugger_cmd_r.guide.html" title="The debug monitor and command syntax / r - dump register contents">r</a> command.</p>
<p class="CodeBlock">MODE_USR:<br> R0=6571de54 R1=0000002a R2=00000002 R3=ffffffff<br> R4=0000002a R5=f8170414 R6=6571df14 R7=6403cc50<br> R8=00000001 R9=6403c44c R10=640002f8 R11=6571de70<br>R12=00000020 R13=00404e00 R14=f80818c0 R15=f800bfa8<br>CPSR=60000013<br>MODE_FIQ:<br> R8=00000000 R9=ffffffff R10=ffffffff R11=00000000<br>R12=00000000 R13=64000d0c R14=c080079c SPSR=e00000dc<br>MODE_IRQ:<br>R13=6400110c R14=00000013 SPSR=20000013<br>MODE_SVC:<br>R13=6571de54 R14=f80328bc SPSR=60000010<br>MODE_ABT:<br>R13=6400090c R14=ccbfd0e0 SPSR=b00000d9<br>MODE_UND:<br>R13=6400090c R14=b5a39950 SPSR=f000009d</p>
<p>In this example:</p>
<ul>
<li>
<p>the IRQ stack is the value of R13 under
MODE_IRQ:, i.e. 0x6400110c.</p>
</li>
<li>
<p>the FRQ stack is the value of R13 under
MODE_FIQ:, i.e. 0x64000d0c.</p>
</li>
</ul>
</div>
<div class="Head3">
<h3><a name="call04" id="call04"></a>Kernel & user stacks of a non-current
thread</h3>
</div><div>
<p>Use the output of the
<a href="CrashDebugger_cmd_i.guide.html" title="The debug monitor and command syntax / i - display information for the current process and thread">i</a>,
<a href="CrashDebugger_cmd_q.guide.html" title="The debug monitor and command syntax / q - display full DObject information">q</a> and
<a href="CrashDebugger_cmd_c.guide.html" title="The debug monitor and command syntax / c - display contents of object container">c</a>
commands.</p>
<p class="CodeBlock">THREAD at 6403c194 VPTR=f8046c18 AccessCount=5 Owner=6403bb4c<br>Full name t_dmasim::Main<br>Thread MState READY<br>Default priority 12 WaitLink Priority 12<br>ExitInfo 3,0,<br>Flags 00000002, Handles 6403b418<br>Supervisor stack base 6571d000 size 1000<br>User stack base 00403000 size 2000<br>Id=25, Alctr=00700000, Created alctr=00700000, Frame=00000000<br>Trap handler=00000000, ActiveScheduler=007000c8, Exception handler=00000000<br>TempObj=00000000 TempAlloc=00000000<br>NThread @ 6403c44c Pri 12 NState READY<br>Next=6403c44c Prev=6403c44c Att=03 iUserContextType=02<br>HeldFM=00000000 WaitFM=00000000 AddrSp=6403bb4c<br>Time=0 Timeslice=20 ReqCount=0<br>SuspendCount=0 CsCount=1 CsFunction=00000000<br>SavedSP=6571df98<br>DACR f800bd2c<br>R13_USR 0d404c38 R14_USR 00000001 SPSR_SVC 00000000<br> R4 f8022d84 R5 6571dfd4 R6 6571dfbc R7 f8022db8<br> R8 f800bddc R9 f800a454 R10 00000000 R11 f801daac<br> PC 60000010</p>
<p>In this example:</p>
<ul>
<li>
<p>the kernel stack is the value of SavedSP, i.e.
0x6571df98.</p>
</li>
<li>
<p>the user stack is the value of R13_USR, i.e.
0x0d404c38.</p>
</li>
</ul>
<h5>Related tasks</h5>
<ul>
<li><a href="CrashDebuggerCallStack.guide02.html">General Points</a></li>
<li><a href="CrashDebuggerCallStack.guide04.html">Tracing through the Call Stack Heuristically</a></li>
<li><a href="CrashDebuggerCallStack.guide05.html">Walking through the Call Stack</a></li>
</ul>
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