gdb: DJGPP Native
21.1.3 Features for Debugging DJGPP Programs
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DJGPP is a port of the GNU development tools to MS-DOS and MS-Windows.
DJGPP programs are 32-bit protected-mode programs that use the "DPMI"
(DOS Protected-Mode Interface) API to run on top of real-mode DOS
systems and their emulations.
GDB supports native debugging of DJGPP programs, and defines a few
commands specific to the DJGPP port. This subsection describes those
commands.
'info dos'
This is a prefix of DJGPP-specific commands which print information
about the target system and important OS structures.
'info dos sysinfo'
This command displays assorted information about the underlying
platform: the CPU type and features, the OS version and flavor, the
DPMI version, and the available conventional and DPMI memory.
'info dos gdt'
'info dos ldt'
'info dos idt'
These 3 commands display entries from, respectively, Global, Local,
and Interrupt Descriptor Tables (GDT, LDT, and IDT). The descriptor
tables are data structures which store a descriptor for each
segment that is currently in use. The segment's selector is an
index into a descriptor table; the table entry for that index holds
the descriptor's base address and limit, and its attributes and
access rights.
A typical DJGPP program uses 3 segments: a code segment, a data
segment (used for both data and the stack), and a DOS segment
(which allows access to DOS/BIOS data structures and absolute
addresses in conventional memory). However, the DPMI host will
usually define additional segments in order to support the DPMI
environment.
These commands allow to display entries from the descriptor tables.
Without an argument, all entries from the specified table are
displayed. An argument, which should be an integer expression,
means display a single entry whose index is given by the argument.
For example, here's a convenient way to display information about
the debugged program's data segment:
(gdb) info dos ldt $ds
0x13f: base=0x11970000 limit=0x0009ffff 32-Bit Data (Read/Write, Exp-up)
This comes in handy when you want to see whether a pointer is
outside the data segment's limit (i.e. "garbled").
'info dos pde'
'info dos pte'
These two commands display entries from, respectively, the Page
Directory and the Page Tables. Page Directories and Page Tables
are data structures which control how virtual memory addresses are
mapped into physical addresses. A Page Table includes an entry for
every page of memory that is mapped into the program's address
space; there may be several Page Tables, each one holding up to
4096 entries. A Page Directory has up to 4096 entries, one each
for every Page Table that is currently in use.
Without an argument, 'info dos pde' displays the entire Page
Directory, and 'info dos pte' displays all the entries in all of
the Page Tables. An argument, an integer expression, given to the
'info dos pde' command means display only that entry from the Page
Directory table. An argument given to the 'info dos pte' command
means display entries from a single Page Table, the one pointed to
by the specified entry in the Page Directory.
These commands are useful when your program uses "DMA" (Direct
Memory Access), which needs physical addresses to program the DMA
controller.
These commands are supported only with some DPMI servers.
'info dos address-pte ADDR'
This command displays the Page Table entry for a specified linear
address. The argument ADDR is a linear address which should
already have the appropriate segment's base address added to it,
because this command accepts addresses which may belong to _any_
segment. For example, here's how to display the Page Table entry
for the page where a variable 'i' is stored:
(gdb) info dos address-pte __djgpp_base_address + (char *)&i
Page Table entry for address 0x11a00d30:
Base=0x02698000 Dirty Acc. Not-Cached Write-Back Usr Read-Write +0xd30
This says that 'i' is stored at offset '0xd30' from the page whose
physical base address is '0x02698000', and shows all the attributes
of that page.
Note that you must cast the addresses of variables to a 'char *',
since otherwise the value of '__djgpp_base_address', the base
address of all variables and functions in a DJGPP program, will be
added using the rules of C pointer arithmetics: if 'i' is declared
an 'int', GDB will add 4 times the value of '__djgpp_base_address'
to the address of 'i'.
Here's another example, it displays the Page Table entry for the
transfer buffer:
(gdb) info dos address-pte *((unsigned *)&_go32_info_block + 3)
Page Table entry for address 0x29110:
Base=0x00029000 Dirty Acc. Not-Cached Write-Back Usr Read-Write +0x110
(The '+ 3' offset is because the transfer buffer's address is the
3rd member of the '_go32_info_block' structure.) The output
clearly shows that this DPMI server maps the addresses in
conventional memory 1:1, i.e. the physical ('0x00029000' + '0x110')
and linear ('0x29110') addresses are identical.
This command is supported only with some DPMI servers.
In addition to native debugging, the DJGPP port supports remote
debugging via a serial data link. The following commands are specific
to remote serial debugging in the DJGPP port of GDB.
'set com1base ADDR'
This command sets the base I/O port address of the 'COM1' serial
port.
'set com1irq IRQ'
This command sets the "Interrupt Request" ('IRQ') line to use for
the 'COM1' serial port.
There are similar commands 'set com2base', 'set com3irq', etc. for
setting the port address and the 'IRQ' lines for the other 3 COM
ports.
The related commands 'show com1base', 'show com1irq' etc. display
the current settings of the base address and the 'IRQ' lines used
by the COM ports.
'info serial'
This command prints the status of the 4 DOS serial ports. For each
port, it prints whether it's active or not, its I/O base address
and IRQ number, whether it uses a 16550-style FIFO, its baudrate,
and the counts of various errors encountered so far.
Info Catalog
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