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193 lines
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<title>DJGPP Native - Debugging with GDB</title>
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<meta name="description" content="Debugging with GDB">
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<link rel="up" href="Native.html#Native" title="Native">
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<div class="node">
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<a name="DJGPP-Native"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Cygwin-Native.html#Cygwin-Native">Cygwin Native</a>,
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Previous: <a rel="previous" accesskey="p" href="Process-Information.html#Process-Information">Process Information</a>,
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Up: <a rel="up" accesskey="u" href="Native.html#Native">Native</a>
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<hr>
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</div>
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<h4 class="subsection">21.1.3 Features for Debugging <span class="sc">djgpp</span> Programs</h4>
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<p><a name="index-g_t_0040sc_007bdjgpp_007d-debugging-1467"></a><a name="index-native-_0040sc_007bdjgpp_007d-debugging-1468"></a><a name="index-MS_002dDOS_002dspecific-commands-1469"></a>
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<a name="index-DPMI-1470"></a><span class="sc">djgpp</span> is a port of the <span class="sc">gnu</span> development tools to MS-DOS and
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MS-Windows. <span class="sc">djgpp</span> programs are 32-bit protected-mode programs
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that use the <dfn>DPMI</dfn> (DOS Protected-Mode Interface) API to run on
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top of real-mode DOS systems and their emulations.
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<p><span class="sc">gdb</span> supports native debugging of <span class="sc">djgpp</span> programs, and
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defines a few commands specific to the <span class="sc">djgpp</span> port. This
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subsection describes those commands.
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<a name="index-info-dos-1471"></a>
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<dl><dt><code>info dos</code><dd>This is a prefix of <span class="sc">djgpp</span>-specific commands which print
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information about the target system and important OS structures.
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<p><a name="index-sysinfo-1472"></a><a name="index-MS_002dDOS-system-info-1473"></a><a name="index-free-memory-information-_0028MS_002dDOS_0029-1474"></a><br><dt><code>info dos sysinfo</code><dd>This command displays assorted information about the underlying
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platform: the CPU type and features, the OS version and flavor, the
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DPMI version, and the available conventional and DPMI memory.
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<p><a name="index-GDT-1475"></a><a name="index-LDT-1476"></a><a name="index-IDT-1477"></a><a name="index-segment-descriptor-tables-1478"></a><a name="index-descriptor-tables-display-1479"></a><br><dt><code>info dos gdt</code><dt><code>info dos ldt</code><dt><code>info dos idt</code><dd>These 3 commands display entries from, respectively, Global, Local,
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and Interrupt Descriptor Tables (GDT, LDT, and IDT). The descriptor
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tables are data structures which store a descriptor for each segment
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that is currently in use. The segment's selector is an index into a
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descriptor table; the table entry for that index holds the
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descriptor's base address and limit, and its attributes and access
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rights.
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<p>A typical <span class="sc">djgpp</span> program uses 3 segments: a code segment, a data
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segment (used for both data and the stack), and a DOS segment (which
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allows access to DOS/BIOS data structures and absolute addresses in
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conventional memory). However, the DPMI host will usually define
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additional segments in order to support the DPMI environment.
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<p><a name="index-garbled-pointers-1480"></a>These commands allow to display entries from the descriptor tables.
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Without an argument, all entries from the specified table are
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displayed. An argument, which should be an integer expression, means
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display a single entry whose index is given by the argument. For
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example, here's a convenient way to display information about the
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debugged program's data segment:
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<pre class="smallexample"> <br><code>(gdb) info dos ldt $ds</code><br>
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<br><code>0x13f: base=0x11970000 limit=0x0009ffff 32-Bit Data (Read/Write, Exp-up)</code><br>
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</pre>
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<p class="noindent">This comes in handy when you want to see whether a pointer is outside
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the data segment's limit (i.e. <dfn>garbled</dfn>).
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<p><a name="index-page-tables-display-_0028MS_002dDOS_0029-1481"></a><br><dt><code>info dos pde</code><dt><code>info dos pte</code><dd>These two commands display entries from, respectively, the Page
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Directory and the Page Tables. Page Directories and Page Tables are
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data structures which control how virtual memory addresses are mapped
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into physical addresses. A Page Table includes an entry for every
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page of memory that is mapped into the program's address space; there
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may be several Page Tables, each one holding up to 4096 entries. A
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Page Directory has up to 4096 entries, one each for every Page Table
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that is currently in use.
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<p>Without an argument, <kbd>info dos pde</kbd> displays the entire Page
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Directory, and <kbd>info dos pte</kbd> displays all the entries in all of
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the Page Tables. An argument, an integer expression, given to the
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<kbd>info dos pde</kbd> command means display only that entry from the Page
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Directory table. An argument given to the <kbd>info dos pte</kbd> command
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means display entries from a single Page Table, the one pointed to by
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the specified entry in the Page Directory.
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<p><a name="index-direct-memory-access-_0028DMA_0029-on-MS_002dDOS-1482"></a>These commands are useful when your program uses <dfn>DMA</dfn> (Direct
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Memory Access), which needs physical addresses to program the DMA
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controller.
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<p>These commands are supported only with some DPMI servers.
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<p><a name="index-physical-address-from-linear-address-1483"></a><br><dt><code>info dos address-pte </code><var>addr</var><dd>This command displays the Page Table entry for a specified linear
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address. The argument <var>addr</var> is a linear address which should
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already have the appropriate segment's base address added to it,
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because this command accepts addresses which may belong to <em>any</em>
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segment. For example, here's how to display the Page Table entry for
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the page where a variable <code>i</code> is stored:
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<pre class="smallexample"> <br><code>(gdb) info dos address-pte __djgpp_base_address + (char *)&i</code><br>
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<br><code>Page Table entry for address 0x11a00d30:</code><br>
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<br><code>Base=0x02698000 Dirty Acc. Not-Cached Write-Back Usr Read-Write +0xd30</code><br>
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</pre>
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<p class="noindent">This says that <code>i</code> is stored at offset <code>0xd30</code> from the page
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whose physical base address is <code>0x02698000</code>, and shows all the
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attributes of that page.
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<p>Note that you must cast the addresses of variables to a <code>char *</code>,
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since otherwise the value of <code>__djgpp_base_address</code>, the base
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address of all variables and functions in a <span class="sc">djgpp</span> program, will
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be added using the rules of C pointer arithmetics: if <code>i</code> is
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declared an <code>int</code>, <span class="sc">gdb</span> will add 4 times the value of
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<code>__djgpp_base_address</code> to the address of <code>i</code>.
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<p>Here's another example, it displays the Page Table entry for the
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transfer buffer:
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<pre class="smallexample"> <br><code>(gdb) info dos address-pte *((unsigned *)&_go32_info_block + 3)</code><br>
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<br><code>Page Table entry for address 0x29110:</code><br>
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<br><code>Base=0x00029000 Dirty Acc. Not-Cached Write-Back Usr Read-Write +0x110</code><br>
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</pre>
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<p class="noindent">(The <code>+ 3</code> offset is because the transfer buffer's address is the
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3rd member of the <code>_go32_info_block</code> structure.) The output
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clearly shows that this DPMI server maps the addresses in conventional
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memory 1:1, i.e. the physical (<code>0x00029000</code> + <code>0x110</code>) and
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linear (<code>0x29110</code>) addresses are identical.
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<p>This command is supported only with some DPMI servers.
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</dl>
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<p><a name="index-DOS-serial-data-link_002c-remote-debugging-1484"></a>In addition to native debugging, the DJGPP port supports remote
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debugging via a serial data link. The following commands are specific
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to remote serial debugging in the DJGPP port of <span class="sc">gdb</span>.
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<a name="index-set-com1base-1485"></a>
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<a name="index-set-com1irq-1486"></a>
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<a name="index-set-com2base-1487"></a>
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<a name="index-set-com2irq-1488"></a>
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<a name="index-set-com3base-1489"></a>
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<a name="index-set-com3irq-1490"></a>
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<a name="index-set-com4base-1491"></a>
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<a name="index-set-com4irq-1492"></a>
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<dl><dt><code>set com1base </code><var>addr</var><dd>This command sets the base I/O port address of the <samp><span class="file">COM1</span></samp> serial
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port.
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<br><dt><code>set com1irq </code><var>irq</var><dd>This command sets the <dfn>Interrupt Request</dfn> (<code>IRQ</code>) line to use
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for the <samp><span class="file">COM1</span></samp> serial port.
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<p>There are similar commands ‘<samp><span class="samp">set com2base</span></samp>’, ‘<samp><span class="samp">set com3irq</span></samp>’,
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etc. for setting the port address and the <code>IRQ</code> lines for the
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other 3 COM ports.
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<p><a name="index-show-com1base-1493"></a><a name="index-show-com1irq-1494"></a><a name="index-show-com2base-1495"></a><a name="index-show-com2irq-1496"></a><a name="index-show-com3base-1497"></a><a name="index-show-com3irq-1498"></a><a name="index-show-com4base-1499"></a><a name="index-show-com4irq-1500"></a>The related commands ‘<samp><span class="samp">show com1base</span></samp>’, ‘<samp><span class="samp">show com1irq</span></samp>’ etc.
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display the current settings of the base address and the <code>IRQ</code>
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lines used by the COM ports.
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<br><dt><code>info serial</code><dd><a name="index-info-serial-1501"></a><a name="index-DOS-serial-port-status-1502"></a>This command prints the status of the 4 DOS serial ports. For each
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port, it prints whether it's active or not, its I/O base address and
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IRQ number, whether it uses a 16550-style FIFO, its baudrate, and the
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counts of various errors encountered so far.
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</dl>
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</body></html>
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