### G.2 Target Description Format ¶

A target description annex is an XML document which complies with the Document Type Definition provided in the GDB sources in `gdb/features/gdb-target.dtd`. This means you can use generally available tools like `xmllint` to check that your feature descriptions are well-formed and valid. However, to help people unfamiliar with XML write descriptions for their targets, we also describe the grammar here.

Target descriptions can identify the architecture of the remote target and (for some architectures) provide information about custom register sets. They can also identify the OS ABI of the remote target. GDB can use this information to autoconfigure for your target, or to warn you if you connect to an unsupported target.

Here is a simple target description:

    <target version="1.0">
      <architecture>i386:x86-64</architecture>
    </target>

This minimal description only says that the target uses the x86-64 architecture.

A target description has the following overall form, with [ ] marking optional elements and … marking repeatable elements. The elements are explained further below.

    <?xml version="1.0"?>
    <!DOCTYPE target SYSTEM "gdb-target.dtd">
    <target version="1.0">
      [architecture]
      [osabi]
      [compatible]
      [feature...]
    </target>

The description is generally insensitive to whitespace and line breaks, under the usual common-sense rules. The XML version declaration and document type declaration can generally be omitted (GDB does not require them), but specifying them may be useful for XML validation tools. The ‘`version`’ attribute for ‘`<target>`’ may also be omitted, but we recommend including it; if future versions of GDB use an incompatible revision of `gdb-target.dtd`, they will detect and report the version mismatch.

#### G.2.1 Inclusion ¶

It can sometimes be valuable to split a target description up into several different annexes, either for organizational purposes, or to share files between different possible target descriptions. You can divide a description into multiple files by replacing any element of the target description with an inclusion directive of the form:

    <xi:include href="document"/>

When GDB encounters an element of this form, it will retrieve the named XML document, and replace the inclusion directive with the contents of that document. If the current description was read using ‘`qXfer`’, then so will be the included document; document will be interpreted as the name of an annex. If the current description was read from a file, GDB will look for document as a file in the same directory where it found the original description.

#### G.2.2 Architecture ¶

An ‘`<architecture>`’ element has this form:

      <architecture>arch</architecture>

arch is one of the architectures from the set accepted by `set architecture` (see Specifying a Debugging Target).

#### G.2.3 OS ABI ¶

This optional field was introduced in GDB version 7.0. Previous versions of GDB ignore it.

An ‘`<osabi>`’ element has this form:

      <osabi>abi-name</osabi>

abi-name is an OS ABI name from the same selection accepted by `set osabi` (see Configuring the Current ABI).

#### G.2.4 Compatible Architecture ¶

This optional field was introduced in GDB version 7.0. Previous versions of GDB ignore it.

A ‘`<compatible>`’ element has this form:

      <compatible>arch</compatible>

arch is one of the architectures from the set accepted by `set architecture` (see Specifying a Debugging Target).

A ‘`<compatible>`’ element is used to specify that the target is able to run binaries in some other than the main target architecture given by the ‘`<architecture>`’ element. For example, on the Cell Broadband Engine, the main architecture is `powerpc:common` or `powerpc:common64`, but the system is able to run binaries in the `spu` architecture as well. The way to describe this capability with ‘`<compatible>`’ is as follows:

      <architecture>powerpc:common</architecture>
      <compatible>spu</compatible>

#### G.2.5 Features ¶

Each ‘`<feature>`’ describes some logical portion of the target system. Features are currently used to describe available CPU registers and the types of their contents. A ‘`<feature>`’ element has this form:

    <feature name="name">
      [type...]
      reg...
    </feature>

Each feature’s name should be unique within the description. The name of a feature does not matter unless GDB has some special knowledge of the contents of that feature; if it does, the feature should have its standard name. See Standard Target Features.

#### G.2.6 Types ¶

Any register’s value is a collection of bits which GDB must interpret. The default interpretation is a two’s complement integer, but other types can be requested by name in the register description. Some predefined types are provided by GDB (see Predefined Target Types), and the description can define additional composite and enum types.

Each type element must have an ‘`id`’ attribute, which gives a unique (within the containing ‘`<feature>`’) name to the type. Types must be defined before they are used.

Some targets offer vector registers, which can be treated as arrays of scalar elements. These types are written as ‘`<vector>`’ elements, specifying the array element type, type, and the number of elements, count:

    <vector id="id" type="type" count="count"/>

If a register’s value is usefully viewed in multiple ways, define it with a union type containing the useful representations. The ‘`<union>`’ element contains one or more ‘`<field>`’ elements, each of which has a name and a type:

    <union id="id">
      <field name="name" type="type"/>
      ...
    </union>

If a register’s value is composed from several separate values, define it with either a structure type or a flags type. A flags type may only contain bitfields. A structure type may either contain only bitfields or contain no bitfields. If the value contains only bitfields, its total size in bytes must be specified.

Non-bitfield values have a name and type.

    <struct id="id">
      <field name="name" type="type"/>
      ...
    </struct>

Both name and type values are required. No implicit padding is added.

Bitfield values have a name, start, end and type.

    <struct id="id" size="size">
      <field name="name" start="start" end="end" type="type"/>
      ...
    </struct>

    <flags id="id" size="size">
      <field name="name" start="start" end="end" type="type"/>
      ...
    </flags>

The name value is required. Bitfield values may be named with the empty string, ‘`""`’, in which case the field is “filler” and its value is not printed. Not all bits need to be specified, so “filler” fields are optional.

The start and end values are required, and type is optional. The field’s start must be less than or equal to its end, and zero represents the least significant bit.

The default value of type is `bool` for single bit fields, and an unsigned integer otherwise.

Which to choose? Structures or flags?

Registers defined with ‘`flags`’ have these advantages over defining them with ‘`struct`’:

- Arithmetic may be performed on them as if they were integers.
- They are printed in a more readable fashion.

Registers defined with ‘`struct`’ have one advantage over defining them with ‘`flags`’:

- One can fetch individual fields like in ‘`C`’.

      (gdb) print $my_struct_reg.field3
      $1 = 42

#### G.2.7 Registers ¶

Each register is represented as an element with this form:

    <reg name="name"
         bitsize="size"
         [regnum="num"]
         [save-restore="save-restore"]
         [type="type"]
         [group="group"]/>

The components are as follows:

name

  The register’s name; it must be unique within the target description.

bitsize

  The register’s size, in bits.

regnum

  The register’s number. If omitted, a register’s number is one greater than that of the previous register (either in the current feature or in a preceding feature); the first register in the target description defaults to zero. This register number is used to read or write the register; e.g. it is used in the remote `p` and `P` packets, and registers appear in the `g` and `G` packets in order of increasing register number.

save-restore

  Whether the register should be preserved across inferior function calls; this must be either `yes` or `no`. The default is `yes`, which is appropriate for most registers except for some system control registers; this is not related to the target’s ABI.

type

  The type of the register. It may be a predefined type, a type defined in the current feature, or one of the special types `int` and `float`. `int` is an integer type of the correct size for bitsize, and `float` is a floating point type (in the architecture’s normal floating point format) of the correct size for bitsize. The default is `int`.

group

  The register group to which this register belongs. It can be one of the standard register groups `general`, `float`, `vector` or an arbitrary string. Group names should be limited to alphanumeric characters. If a group name is made up of multiple words the words may be separated by hyphens; e.g. `special-group` or `ultra-special-group`. If no group is specified, GDB will not display the register in `info registers`.