10. Target Descriptions

The target architecture definition (see section 9. Target Architecture Definition) contains GDB's hard-coded knowledge about an architecture. For some platforms, it is handy to have more flexible knowledge about a specific instance of the architecture--for instance, a processor or development board. Target descriptions provide a mechanism for the user to tell GDB more about what their target supports, or for the target to tell GDB directly.

For details on writing, automatically supplying, and manually selecting target descriptions, see section `Target Descriptions' in Debugging with GDB. This section will cover some related topics about the GDB internals.

10.1 Target Descriptions Implementation
10.2 Adding Target Described Register Support

10.1 Target Descriptions Implementation

Before GDB connects to a new target, or runs a new program on an existing target, it discards any existing target description and reverts to a default gdbarch. Then, after connecting, it looks for a new target description by calling target_find_description.

A description may come from a user specified file (XML), the remote `qXfer:features:read' packet (also XML), or from any custom to_read_description routine in the target vector. For instance, the remote target supports guessing whether a MIPS target is 32-bit or 64-bit based on the size of the `g' packet.

If any target description is found, GDB creates a new gdbarch incorporating the description by calling gdbarch_update_p. Any `<architecture>' element is handled first, to determine which architecture's gdbarch initialization routine is called to create the new architecture. Then the initialization routine is called, and has a chance to adjust the constructed architecture based on the contents of the target description. For instance, it can recognize any properties set by a to_read_description routine. Also see 10.2 Adding Target Described Register Support.

10.2 Adding Target Described Register Support

Target descriptions can report additional registers specific to an instance of the target. But it takes a little work in the architecture specific routines to support this.

A target description must either have no registers or a complete set--this avoids complexity in trying to merge standard registers with the target defined registers. It is the architecture's responsibility to validate that a description with registers has everything it needs. To keep architecture code simple, the same mechanism is used to assign fixed internal register numbers to standard registers.

If tdesc_has_registers returns 1, the description contains registers. The architecture's gdbarch_init routine should:

• Call tdesc_data_alloc to allocate storage, early, before searching for a matching gdbarch or allocating a new one.

• Use tdesc_find_feature to locate standard features by name.

• Use tdesc_numbered_register and tdesc_numbered_register_choices to locate the expected registers in the standard features.

• Return NULL if a required feature is missing, or if any standard feature is missing expected registers. This will produce a warning that the description was incomplete.

• Free the allocated data before returning, unless tdesc_use_registers is called.

• Call set_gdbarch_num_regs as usual, with a number higher than any fixed number passed to tdesc_numbered_register.

• Call tdesc_use_registers after creating a new gdbarch, before returning it.

After tdesc_use_registers has been called, the architecture's register_name, register_type, and register_reggroup_p routines will not be called; that information will be taken from the target description. num_regs may be increased to account for any additional registers in the description.

Pseudo-registers require some extra care:

• Using tdesc_numbered_register allows the architecture to give constant register numbers to standard architectural registers, e.g. as an enum in `arch-tdep.h'. But because pseudo-registers are always numbered above num_regs, which may be increased by the description, constant numbers can not be used for pseudos. They must be numbered relative to num_regs instead.

• The description will not describe pseudo-registers, so the architecture must call set_tdesc_pseudo_register_name, set_tdesc_pseudo_register_type, and set_tdesc_pseudo_register_reggroup_p to supply routines describing pseudo registers. These routines will be passed internal register numbers, so the same routines used for the gdbarch equivalents are usually suitable.