GDB Internals

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## Target Architecture Definition

GDB's target architecture defines what sort of machine-language programs GDB can work with, and how it works with them.

The target architecture object is implemented as the C structure `struct gdbarch *`. The structure, and its methods, are generated using the Bourne shell script `gdbarch.sh`.

### Registers and Memory

GDB's model of the target machine is rather simple. GDB assumes the machine includes a bank of registers and a block of memory. Each register may have a different size. GDB does not have a magical way to match up with the compiler's idea of which registers are which; however, it is critical that they do match up accurately. The only way to make this work is to get accurate information about the order that the compiler uses, and to reflect that in the `REGISTER_NAME` and related macros. GDB can handle big-endian, little-endian, and bi-endian architectures.

### Pointers Are Not Always Addresses

On almost all 32-bit architectures, the representation of a pointer is indistinguishable from the representation of some fixed-length number whose value is the byte address of the object pointed to. On such machines, the words "pointer" and "address" can be used interchangeably. However, architectures with smaller word sizes are often cramped for address space, so they may choose a pointer representation that breaks this identity, and allows a larger code address space.

For example, the Mitsubishi D10V is a 16-bit VLIW processor whose instructions are 32 bits long1. If the D10V used ordinary byte addresses to refer to code locations, then the processor would only be able to address 64kb of instructions. However, since instructions must be aligned on four-byte boundaries, the low two bits of any valid instruction's byte address are always zero--byte addresses waste two bits. So instead of byte addresses, the D10V uses word addresses--byte addresses shifted right two bits--to refer to code. Thus, the D10V can use 16-bit words to address 256kb of code space.

However, this means that code pointers and data pointers have different forms on the D10V. The 16-bit word `0xC020` refers to byte address `0xC020` when used as a data address, but refers to byte address `0x30080` when used as a code address.

(The D10V also uses separate code and data address spaces, which also affects the correspondence between pointers and addresses, but we're going to ignore that here; this example is already too long.)

To cope with architectures like this--the D10V is not the only one!--GDB tries to distinguish between addresses, which are byte numbers, and pointers, which are the target's representation of an address of a particular type of data. In the example above, `0xC020` is the pointer, which refers to one of the addresses `0xC020` or `0x30080`, depending on the type imposed upon it. GDB provides functions for turning a pointer into an address and vice versa, in the appropriate way for the current architecture.

Unfortunately, since addresses and pointers are identical on almost all processors, this distinction tends to bit-rot pretty quickly. Thus, each time you port GDB to an architecture which does distinguish between pointers and addresses, you'll probably need to clean up some architecture-independent code.

Here are functions which convert between pointers and addresses:

CORE_ADDR extract_typed_address (void *buf, struct type *type) | Function

Treat the bytes at buf as a pointer or reference of type type, and return the address it represents, in a manner appropriate for the current architecture. This yields an address GDB can use to read target memory, disassemble, etc. Note that buf refers to a buffer in GDB's memory, not the inferior's.

For example, if the current architecture is the Intel x86, this function extracts a little-endian integer of the appropriate length from buf and returns it. However, if the current architecture is the D10V, this function will return a 16-bit integer extracted from buf, multiplied by four if type is a pointer to a function.

If type is not a pointer or reference type, then this function will signal an internal error.

CORE_ADDR store_typed_address (void *buf, struct type *type, CORE_ADDR addr) | Function

Store the address addr in buf, in the proper format for a pointer of type type in the current architecture. Note that buf refers to a buffer in GDB's memory, not the inferior's.

For example, if the current architecture is the Intel x86, this function stores addr unmodified as a little-endian integer of the appropriate length in buf. However, if the current architecture is the D10V, this function divides addr by four if type is a pointer to a function, and then stores it in buf.

If type is not a pointer or reference type, then this function will signal an internal error.

CORE_ADDR value_as_address (struct value *val) | Function

Assuming that val is a pointer, return the address it represents, as appropriate for the current architecture. This function actually works on integral values, as well as pointers. For pointers, it performs architecture-specific conversions as described above for `extract_typed_address`.

CORE_ADDR value_from_pointer (struct type *type, CORE_ADDR addr) | Function

Create and return a value representing a pointer of type type to the address addr, as appropriate for the current architecture. This function performs architecture-specific conversions as described above for `store_typed_address`.

GDB also provides functions that do the same tasks, but assume that pointers are simply byte addresses; they aren't sensitive to the current architecture, beyond knowing the appropriate endianness.

CORE_ADDR extract_address (void *addr, int len) | Function

Extract a len-byte number from addr in the appropriate endianness for the current architecture, and return it. Note that addr refers to GDB's memory, not the inferior's. This function should only be used in architecture-specific code; it doesn't have enough information to turn bits into a true address in the appropriate way for the current architecture. If you can, use `extract_typed_address` instead.

void store_address (void *addr, int len, LONGEST val) | Function

Store val at addr as a len-byte integer, in the appropriate endianness for the current architecture. Note that addr refers to a buffer in GDB's memory, not the inferior's. This function should only be used in architecture-specific code; it doesn't have enough information to turn a true address into bits in the appropriate way for the current architecture. If you can, use `store_typed_address` instead.

Here are some macros which architectures can define to indicate the relationship between pointers and addresses. These have default definitions, appropriate for architectures on which all pointers are simple unsigned byte addresses.

CORE_ADDR POINTER_TO_ADDRESS (struct type *type, char *buf) | Target Macro

Assume that buf holds a pointer of type type, in the appropriate format for the current architecture. Return the byte address the pointer refers to. This function may safely assume that type is either a pointer or a C`++` reference type.

void ADDRESS_TO_POINTER (struct type *type, char *buf, CORE_ADDR addr) | Target Macro

Store in buf a pointer of type type representing the address addr, in the appropriate format for the current architecture. This function may safely assume that type is either a pointer or a C`++` reference type.

### Using Different Register and Memory Data Representations

Maintainer's note: The way GDB manipulates registers is undergoing significant change. Many of the macros and functions refered to in the sections below are likely to be made obsolete. See the file `TODO` for more up-to-date information.

Some architectures use one representation for a value when it lives in a register, but use a different representation when it lives in memory. In GDB's terminology, the raw representation is the one used in the target registers, and the virtual representation is the one used in memory, and within GDB `struct value` objects.

For almost all data types on almost all architectures, the virtual and raw representations are identical, and no special handling is needed. However, they do occasionally differ. For example:

- The x86 architecture supports an 80-bit `long double` type. However, when we store those values in memory, they occupy twelve bytes: the floating-point number occupies the first ten, and the final two bytes are unused. This keeps the values aligned on four-byte boundaries, allowing more efficient access. Thus, the x86 80-bit floating-point type is the raw representation, and the twelve-byte loosely-packed arrangement is the virtual representation.
- Some 64-bit MIPS targets present 32-bit registers to GDB as 64-bit registers, with garbage in their upper bits. GDB ignores the top 32 bits. Thus, the 64-bit form, with garbage in the upper 32 bits, is the raw representation, and the trimmed 32-bit representation is the virtual representation.

In general, the raw representation is determined by the architecture, or GDB's interface to the architecture, while the virtual representation can be chosen for GDB's convenience. GDB's register file, `registers`, holds the register contents in raw format, and the GDB remote protocol transmits register values in raw format.

Your architecture may define the following macros to request conversions between the raw and virtual format:

int REGISTER_CONVERTIBLE (int reg) | Target Macro

Return non-zero if register number reg's value needs different raw and virtual formats. You should not use `REGISTER_CONVERT_TO_VIRTUAL` for a register unless this macro returns a non-zero value for that register.

int REGISTER_RAW_SIZE (int reg) | Target Macro

The size of register number reg's raw value. This is the number of bytes the register will occupy in `registers`, or in a GDB remote protocol packet.

int REGISTER_VIRTUAL_SIZE (int reg) | Target Macro

The size of register number reg's value, in its virtual format. This is the size a `struct value`'s buffer will have, holding that register's value.

struct type *REGISTER_VIRTUAL_TYPE (int reg) | Target Macro

This is the type of the virtual representation of register number reg. Note that there is no need for a macro giving a type for the register's raw form; once the register's value has been obtained, GDB always uses the virtual form.

void REGISTER_CONVERT_TO_VIRTUAL (int reg, struct type *type, char *from, char *to) | Target Macro

Convert the value of register number reg to type, which should always be `REGISTER_VIRTUAL_TYPE (`reg`)`. The buffer at from holds the register's value in raw format; the macro should convert the value to virtual format, and place it at to.

Note that `REGISTER_CONVERT_TO_VIRTUAL` and `REGISTER_CONVERT_TO_RAW` take their reg and type arguments in different orders.

You should only use `REGISTER_CONVERT_TO_VIRTUAL` with registers for which the `REGISTER_CONVERTIBLE` macro returns a non-zero value.

void REGISTER_CONVERT_TO_RAW (struct type *type, int reg, char *from, char *to) | Target Macro

Convert the value of register number reg to type, which should always be `REGISTER_VIRTUAL_TYPE (`reg`)`. The buffer at from holds the register's value in raw format; the macro should convert the value to virtual format, and place it at to. Note that REGISTER_CONVERT_TO_VIRTUAL and REGISTER_CONVERT_TO_RAW take their reg and type arguments in different orders.

### Frame Interpretation

### Inferior Call Setup

### Compiler Characteristics

### Target Conditionals

This section describes the macros that you can use to define the target machine.

`ADDITIONAL_OPTIONS`
`ADDITIONAL_OPTION_CASES`
`ADDITIONAL_OPTION_HANDLER`
`ADDITIONAL_OPTION_HELP`
  These are a set of macros that allow the addition of additional command line options to GDB. They are currently used only for the unsupported i960 Nindy target, and should not be used in any other configuration.
`ADDR_BITS_REMOVE (addr)`
  If a raw machine instruction address includes any bits that are not really part of the address, then define this macro to expand into an expression that zeroes those bits in addr. This is only used for addresses of instructions, and even then not in all contexts.

  For example, the two low-order bits of the PC on the Hewlett-Packard PA 2.0 architecture contain the privilege level of the corresponding instruction. Since instructions must always be aligned on four-byte boundaries, the processor masks out these bits to generate the actual address of the instruction. ADDR_BITS_REMOVE should filter out these bits with an expression such as `((addr) & ~3)`.

`ADDRESS_TO_POINTER (`type`,` buf`,` addr`)`
  Store in buf a pointer of type type representing the address addr, in the appropriate format for the current architecture. This macro may safely assume that type is either a pointer or a C`++` reference type. See Pointers Are Not Always Addresses.
`BEFORE_MAIN_LOOP_HOOK`
  Define this to expand into any code that you want to execute before the main loop starts. Although this is not, strictly speaking, a target conditional, that is how it is currently being used. Note that if a configuration were to define it one way for a host and a different way for the target, GDB will probably not compile, let alone run correctly. This macro is currently used only for the unsupported i960 Nindy target, and should not be used in any other configuration.
`BELIEVE_PCC_PROMOTION`
  Define if the compiler promotes a `short` or `char` parameter to an `int`, but still reports the parameter as its original type, rather than the promoted type.
`BELIEVE_PCC_PROMOTION_TYPE`
  Define this if GDB should believe the type of a `short` argument when compiled by `pcc`, but look within a full int space to get its value. Only defined for Sun-3 at present.
`BITS_BIG_ENDIAN`
  Define this if the numbering of bits in the targets does not match the endianness of the target byte order. A value of 1 means that the bits are numbered in a big-endian bit order, 0 means little-endian.
`BREAKPOINT`
  This is the character array initializer for the bit pattern to put into memory where a breakpoint is set. Although it's common to use a trap instruction for a breakpoint, it's not required; for instance, the bit pattern could be an invalid instruction. The breakpoint must be no longer than the shortest instruction of the architecture.

  `BREAKPOINT` has been deprecated in favor of `BREAKPOINT_FROM_PC`.

`BIG_BREAKPOINT`
`LITTLE_BREAKPOINT`
  Similar to BREAKPOINT, but used for bi-endian targets.

  `BIG_BREAKPOINT` and `LITTLE_BREAKPOINT` have been deprecated in favor of `BREAKPOINT_FROM_PC`.

`REMOTE_BREAKPOINT`
`LITTLE_REMOTE_BREAKPOINT`
`BIG_REMOTE_BREAKPOINT`
  Similar to BREAKPOINT, but used for remote targets.

  `BIG_REMOTE_BREAKPOINT` and `LITTLE_REMOTE_BREAKPOINT` have been deprecated in favor of `BREAKPOINT_FROM_PC`.

`BREAKPOINT_FROM_PC (`pcptr`,` lenptr`)`
  Use the program counter to determine the contents and size of a breakpoint instruction. It returns a pointer to a string of bytes that encode a breakpoint instruction, stores the length of the string to *lenptr, and adjusts pc (if necessary) to point to the actual memory location where the breakpoint should be inserted.

  Although it is common to use a trap instruction for a breakpoint, it's not required; for instance, the bit pattern could be an invalid instruction. The breakpoint must be no longer than the shortest instruction of the architecture.

  Replaces all the other BREAKPOINT macros.

`MEMORY_INSERT_BREAKPOINT (`addr`,` contents_cache`)`
`MEMORY_REMOVE_BREAKPOINT (`addr`,` contents_cache`)`
  Insert or remove memory based breakpoints. Reasonable defaults (`default_memory_insert_breakpoint` and `default_memory_remove_breakpoint` respectively) have been provided so that it is not necessary to define these for most architectures. Architectures which may want to define `MEMORY_INSERT_BREAKPOINT` and `MEMORY_REMOVE_BREAKPOINT` will likely have instructions that are oddly sized or are not stored in a conventional manner.

  It may also be desirable (from an efficiency standpoint) to define custom breakpoint insertion and removal routines if `BREAKPOINT_FROM_PC` needs to read the target's memory for some reason.

`CALL_DUMMY_P`
  A C expresson that is non-zero when the target suports inferior function calls.
`CALL_DUMMY_WORDS`
  Pointer to an array of `LONGEST` words of data containing host-byte-ordered `REGISTER_BYTES` sized values that partially specify the sequence of instructions needed for an inferior function call.

  Should be deprecated in favor of a macro that uses target-byte-ordered data.

`SIZEOF_CALL_DUMMY_WORDS`
  The size of `CALL_DUMMY_WORDS`. When `CALL_DUMMY_P` this must return a positive value. See also `CALL_DUMMY_LENGTH`.
`CALL_DUMMY`
  A static initializer for `CALL_DUMMY_WORDS`. Deprecated.
`CALL_DUMMY_LOCATION`
  See the file `inferior.h`.
`CALL_DUMMY_STACK_ADJUST`
  Stack adjustment needed when performing an inferior function call.

  Should be deprecated in favor of something like `STACK_ALIGN`.

`CALL_DUMMY_STACK_ADJUST_P`
  Predicate for use of `CALL_DUMMY_STACK_ADJUST`.

  Should be deprecated in favor of something like `STACK_ALIGN`.

`CANNOT_FETCH_REGISTER (`regno`)`
  A C expression that should be nonzero if regno cannot be fetched from an inferior process. This is only relevant if `FETCH_INFERIOR_REGISTERS` is not defined.
`CANNOT_STORE_REGISTER (`regno`)`
  A C expression that should be nonzero if regno should not be written to the target. This is often the case for program counters, status words, and other special registers. If this is not defined, GDB will assume that all registers may be written.
`DO_DEFERRED_STORES`
`CLEAR_DEFERRED_STORES`
  Define this to execute any deferred stores of registers into the inferior, and to cancel any deferred stores.

  Currently only implemented correctly for native Sparc configurations?

`COERCE_FLOAT_TO_DOUBLE (`formal`,` actual`)`
  Return non-zero if GDB should promote `float` values to `double` when calling a non-prototyped function. The argument actual is the type of the value we want to pass to the function. The argument formal is the type of this argument, as it appears in the function's definition. Note that formal may be zero if we have no debugging information for the function, or if we're passing more arguments than are officially declared (for example, varargs). This macro is never invoked if the function definitely has a prototype.

  How you should pass arguments to a function depends on whether it was defined in K&R style or prototype style. If you define a function using the K&R syntax that takes a `float` argument, then callers must pass that argument as a `double`. If you define the function using the prototype syntax, then you must pass the argument as a `float`, with no promotion.

  Unfortunately, on certain older platforms, the debug info doesn't indicate reliably how each function was defined. A function type's `TYPE_FLAG_PROTOTYPED` flag may be unset, even if the function was defined in prototype style. When calling a function whose `TYPE_FLAG_PROTOTYPED` flag is unset, GDB consults the `COERCE_FLOAT_TO_DOUBLE` macro to decide what to do.

  For modern targets, it is proper to assume that, if the prototype flag is unset, that can be trusted: `float` arguments should be promoted to `double`. You should use the function `standard_coerce_float_to_double` to get this behavior.

  For some older targets, if the prototype flag is unset, that doesn't tell us anything. So we guess that, if we don't have a type for the formal parameter (i.e., the first argument to `COERCE_FLOAT_TO_DOUBLE` is null), then we should promote it; otherwise, we should leave it alone. The function `default_coerce_float_to_double` provides this behavior; it is the default value, for compatibility with older configurations.

`CPLUS_MARKER`
  Define this to expand into the character that G`++` uses to distinguish compiler-generated identifiers from programmer-specified identifiers. By default, this expands into `'$'`. Most System V targets should define this to `'.'`.
`DBX_PARM_SYMBOL_CLASS`
  Hook for the `SYMBOL_CLASS` of a parameter when decoding DBX symbol information. In the i960, parameters can be stored as locals or as args, depending on the type of the debug record.
`DECR_PC_AFTER_BREAK`
  Define this to be the amount by which to decrement the PC after the program encounters a breakpoint. This is often the number of bytes in `BREAKPOINT`, though not always. For most targets this value will be 0.
`DECR_PC_AFTER_HW_BREAK`
  Similarly, for hardware breakpoints.
`DISABLE_UNSETTABLE_BREAK (`addr`)`
  If defined, this should evaluate to 1 if addr is in a shared library in which breakpoints cannot be set and so should be disabled.
`DO_REGISTERS_INFO`
  If defined, use this to print the value of a register or all registers.
`PRINT_FLOAT_INFO()`
  #findex PRINT_FLOAT_INFO If defined, then the `info float` command will print information about the processor's floating point unit.
`DWARF_REG_TO_REGNUM`
  Convert DWARF register number into GDB regnum. If not defined, no conversion will be performed.
`DWARF2_REG_TO_REGNUM`
  Convert DWARF2 register number into GDB regnum. If not defined, no conversion will be performed.
`ECOFF_REG_TO_REGNUM`
  Convert ECOFF register number into GDB regnum. If not defined, no conversion will be performed.
`END_OF_TEXT_DEFAULT`
  This is an expression that should designate the end of the text section.
`EXTRACT_RETURN_VALUE(`type`,` regbuf`,` valbuf`)`
  Define this to extract a function's return value of type type from the raw register state regbuf and copy that, in virtual format, into valbuf.
`EXTRACT_STRUCT_VALUE_ADDRESS(`regbuf`)`
  When defined, extract from the array regbuf (containing the raw register state) the `CORE_ADDR` at which a function should return its structure value.

  If not defined, `EXTRACT_RETURN_VALUE` is used.

`EXTRACT_STRUCT_VALUE_ADDRESS_P()`
  Predicate for `EXTRACT_STRUCT_VALUE_ADDRESS`.
`FLOAT_INFO`
  Deprecated in favor of `PRINT_FLOAT_INFO`.
`FP_REGNUM`
  If the virtual frame pointer is kept in a register, then define this macro to be the number (greater than or equal to zero) of that register.

  This should only need to be defined if `TARGET_READ_FP` and `TARGET_WRITE_FP` are not defined.

`FRAMELESS_FUNCTION_INVOCATION(`fi`)`
  Define this to an expression that returns 1 if the function invocation represented by fi does not have a stack frame associated with it. Otherwise return 0.
`FRAME_ARGS_ADDRESS_CORRECT`
  See `stack.c`.
`FRAME_CHAIN(`frame`)`
  Given frame, return a pointer to the calling frame.
`FRAME_CHAIN_COMBINE(`chain`,` frame`)`
  Define this to take the frame chain pointer and the frame's nominal address and produce the nominal address of the caller's frame. Presently only defined for HP PA.
`FRAME_CHAIN_VALID(`chain`,` thisframe`)`
  Define this to be an expression that returns zero if the given frame is an outermost frame, with no caller, and nonzero otherwise. Several common definitions are available:
  - `file_frame_chain_valid` is nonzero if the chain pointer is nonzero and given frame's PC is not inside the startup file (such as `crt0.o`).
  - `func_frame_chain_valid` is nonzero if the chain pointer is nonzero and the given frame's PC is not in `main` or a known entry point function (such as `_start`).
  - `generic_file_frame_chain_valid` and `generic_func_frame_chain_valid` are equivalent implementations for targets using generic dummy frames.
`FRAME_INIT_SAVED_REGS(`frame`)`
  See `frame.h`. Determines the address of all registers in the current stack frame storing each in `frame->saved_regs`. Space for `frame->saved_regs` shall be allocated by `FRAME_INIT_SAVED_REGS` using either `frame_saved_regs_zalloc` or `frame_obstack_alloc`.

  `FRAME_FIND_SAVED_REGS` and `EXTRA_FRAME_INFO` are deprecated.

`FRAME_NUM_ARGS (`fi`)`
  For the frame described by fi return the number of arguments that are being passed. If the number of arguments is not known, return `-1`.
`FRAME_SAVED_PC(`frame`)`
  Given frame, return the pc saved there. This is the return address.
`FUNCTION_EPILOGUE_SIZE`
  For some COFF targets, the `x_sym.x_misc.x_fsize` field of the function end symbol is 0. For such targets, you must define `FUNCTION_EPILOGUE_SIZE` to expand into the standard size of a function's epilogue.
`FUNCTION_START_OFFSET`
  An integer, giving the offset in bytes from a function's address (as used in the values of symbols, function pointers, etc.), and the function's first genuine instruction.

  This is zero on almost all machines: the function's address is usually the address of its first instruction. However, on the VAX, for example, each function starts with two bytes containing a bitmask indicating which registers to save upon entry to the function. The VAX `call` instructions check this value, and save the appropriate registers automatically. Thus, since the offset from the function's address to its first instruction is two bytes, `FUNCTION_START_OFFSET` would be 2 on the VAX.

`GCC_COMPILED_FLAG_SYMBOL`
`GCC2_COMPILED_FLAG_SYMBOL`
  If defined, these are the names of the symbols that GDB will look for to detect that GCC compiled the file. The default symbols are `gcc_compiled.` and `gcc2_compiled.`, respectively. (Currently only defined for the Delta 68.)
`GDB_MULTI_ARCH`
  If defined and non-zero, enables suport for multiple architectures within GDB.

  This support can be enabled at two levels. At level one, only definitions for previously undefined macros are provided; at level two, a multi-arch definition of all architecture dependant macros will be defined.

`GDB_TARGET_IS_HPPA`
  This determines whether horrible kludge code in `dbxread.c` and `partial-stab.h` is used to mangle multiple-symbol-table files from HPPA's. This should all be ripped out, and a scheme like `elfread.c` used instead.
`GET_LONGJMP_TARGET`
  For most machines, this is a target-dependent parameter. On the DECstation and the Iris, this is a native-dependent parameter, since trhe header file `setjmp.h` is needed to define it.

  This macro determines the target PC address that `longjmp` will jump to, assuming that we have just stopped at a `longjmp` breakpoint. It takes a `CORE_ADDR *` as argument, and stores the target PC value through this pointer. It examines the current state of the machine as needed.

`GET_SAVED_REGISTER`
  Define this if you need to supply your own definition for the function `get_saved_register`.
`HAVE_REGISTER_WINDOWS`
  Define this if the target has register windows.
`REGISTER_IN_WINDOW_P (`regnum`)`
  Define this to be an expression that is 1 if the given register is in the window.
`IBM6000_TARGET`
  Shows that we are configured for an IBM RS/6000 target. This conditional should be eliminated (FIXME) and replaced by feature-specific macros. It was introduced in a haste and we are repenting at leisure.
`I386_USE_GENERIC_WATCHPOINTS`
  An x86-based target can define this to use the generic x86 watchpoint support; see I386_USE_GENERIC_WATCHPOINTS.
`SYMBOLS_CAN_START_WITH_DOLLAR`
  Some systems have routines whose names start with `$`. Giving this macro a non-zero value tells GDB's expression parser to check for such routines when parsing tokens that begin with `$`.

  On HP-UX, certain system routines (millicode) have names beginning with `$` or `$$`. For example, `$$dyncall` is a millicode routine that handles inter-space procedure calls on PA-RISC.

`INIT_EXTRA_FRAME_INFO (`fromleaf`,` frame`)`
  If additional information about the frame is required this should be stored in `frame->extra_info`. Space for `frame->extra_info` is allocated using `frame_obstack_alloc`.
`INIT_FRAME_PC (`fromleaf`,` prev`)`
  This is a C statement that sets the pc of the frame pointed to by prev. [By default...]
`INNER_THAN (`lhs`,` rhs`)`
  Returns non-zero if stack address lhs is inner than (nearer to the stack top) stack address rhs. Define this as `lhs < rhs` if the target's stack grows downward in memory, or `lhs > rsh` if the stack grows upward.
`gdbarch_in_function_epilogue_p (`gdbarch`,` pc`)`
  Returns non-zero if the given pc is in the epilogue of a function. The epilogue of a function is defined as the part of a function where the stack frame of the function already has been destroyed up to the final `return from function call' instruction.
`IN_SIGTRAMP (`pc`,` name`)`
  Define this to return non-zero if the given pc and/or name indicates that the current function is a `sigtramp`.
`SIGTRAMP_START (`pc`)`
`SIGTRAMP_END (`pc`)`
  Define these to be the start and end address of the `sigtramp` for the given pc. On machines where the address is just a compile time constant, the macro expansion will typically just ignore the supplied pc.
`IN_SOLIB_CALL_TRAMPOLINE (`pc`,` name`)`
  Define this to evaluate to nonzero if the program is stopped in the trampoline that connects to a shared library.
`IN_SOLIB_RETURN_TRAMPOLINE (`pc`,` name`)`
  Define this to evaluate to nonzero if the program is stopped in the trampoline that returns from a shared library.
`IN_SOLIB_DYNSYM_RESOLVE_CODE (`pc`)`
  Define this to evaluate to nonzero if the program is stopped in the dynamic linker.
`SKIP_SOLIB_RESOLVER (`pc`)`
  Define this to evaluate to the (nonzero) address at which execution should continue to get past the dynamic linker's symbol resolution function. A zero value indicates that it is not important or necessary to set a breakpoint to get through the dynamic linker and that single stepping will suffice.
`INTEGER_TO_ADDRESS (`type`,` buf`)`
  Define this when the architecture needs to handle non-pointer to address conversions specially. Converts that value to an address according to the current architectures conventions.

  Pragmatics: When the user copies a well defined expression from their source code and passes it, as a parameter, to GDB's `print` command, they should get the same value as would have been computed by the target program. Any deviation from this rule can cause major confusion and annoyance, and needs to be justified carefully. In other words, GDB doesn't really have the freedom to do these conversions in clever and useful ways. It has, however, been pointed out that users aren't complaining about how GDB casts integers to pointers; they are complaining that they can't take an address from a disassembly listing and give it to `x/i`. Adding an architecture method like `INTEGER_TO_ADDRESS` certainly makes it possible for GDB to "get it right" in all circumstances.

  See Pointers Are Not Always Addresses.

`IS_TRAPPED_INTERNALVAR (`name`)`
  This is an ugly hook to allow the specification of special actions that should occur as a side-effect of setting the value of a variable internal to GDB. Currently only used by the h8500. Note that this could be either a host or target conditional.
`NEED_TEXT_START_END`
  Define this if GDB should determine the start and end addresses of the text section. (Seems dubious.)
`NO_HIF_SUPPORT`
  (Specific to the a29k.)
`POINTER_TO_ADDRESS (`type`,` buf`)`
  Assume that buf holds a pointer of type type, in the appropriate format for the current architecture. Return the byte address the pointer refers to. See Pointers Are Not Always Addresses.
`REGISTER_CONVERTIBLE (`reg`)`
  Return non-zero if reg uses different raw and virtual formats. See Using Different Register and Memory Data Representations.
`REGISTER_RAW_SIZE (`reg`)`
  Return the raw size of reg. See Using Different Register and Memory Data Representations.
`REGISTER_VIRTUAL_SIZE (`reg`)`
  Return the virtual size of reg. See Using Different Register and Memory Data Representations.
`REGISTER_VIRTUAL_TYPE (`reg`)`
  Return the virtual type of reg. See Using Different Register and Memory Data Representations.
`REGISTER_CONVERT_TO_VIRTUAL(`reg`,` type`,` from`,` to`)`
  Convert the value of register reg from its raw form to its virtual form. See Using Different Register and Memory Data Representations.
`REGISTER_CONVERT_TO_RAW(`type`,` reg`,` from`,` to`)`
  Convert the value of register reg from its virtual form to its raw form. See Using Different Register and Memory Data Representations.
`RETURN_VALUE_ON_STACK(`type`)`

  Return non-zero if values of type TYPE are returned on the stack, using the "struct convention" (i.e., the caller provides a pointer to a buffer in which the callee should store the return value). This controls how the `finish` command finds a function's return value, and whether an inferior function call reserves space on the stack for the return value.

  The full logic GDB uses here is kind of odd.

  - If the type being returned by value is not a structure, union, or array, and `RETURN_VALUE_ON_STACK` returns zero, then GDB concludes the value is not returned using the struct convention.
  - Otherwise, GDB calls `USE_STRUCT_CONVENTION` (see below). If that returns non-zero, GDB assumes the struct convention is in use.

  In other words, to indicate that a given type is returned by value using the struct convention, that type must be either a struct, union, array, or something `RETURN_VALUE_ON_STACK` likes, and something that `USE_STRUCT_CONVENTION` likes.

  Note that, in C and C`++`, arrays are never returned by value. In those languages, these predicates will always see a pointer type, never an array type. All the references above to arrays being returned by value apply only to other languages.

`SOFTWARE_SINGLE_STEP_P()`
  Define this as 1 if the target does not have a hardware single-step mechanism. The macro `SOFTWARE_SINGLE_STEP` must also be defined.
`SOFTWARE_SINGLE_STEP(`signal`,` insert_breapoints_p`)`
  A function that inserts or removes (depending on insert_breapoints_p) breakpoints at each possible destinations of the next instruction. See `sparc-tdep.c` and `rs6000-tdep.c` for examples.
`SOFUN_ADDRESS_MAYBE_MISSING`
  Somebody clever observed that, the more actual addresses you have in the debug information, the more time the linker has to spend relocating them. So whenever there's some other way the debugger could find the address it needs, you should omit it from the debug info, to make linking faster.

  `SOFUN_ADDRESS_MAYBE_MISSING` indicates that a particular set of hacks of this sort are in use, affecting `N_SO` and `N_FUN` entries in stabs-format debugging information. `N_SO` stabs mark the beginning and ending addresses of compilation units in the text segment. `N_FUN` stabs mark the starts and ends of functions.

  `SOFUN_ADDRESS_MAYBE_MISSING` means two things:

  - `N_FUN` stabs have an address of zero. Instead, you should find the addresses where the function starts by taking the function name from the stab, and then looking that up in the minsyms (the linker/assembler symbol table). In other words, the stab has the name, and the linker/assembler symbol table is the only place that carries the address.
  - `N_SO` stabs have an address of zero, too. You just look at the `N_FUN` stabs that appear before and after the `N_SO` stab, and guess the starting and ending addresses of the compilation unit from them.
`PCC_SOL_BROKEN`
  (Used only in the Convex target.)
`PC_IN_CALL_DUMMY`
  See `inferior.h`.
`PC_LOAD_SEGMENT`
  If defined, print information about the load segment for the program counter. (Defined only for the RS/6000.)
`PC_REGNUM`
  If the program counter is kept in a register, then define this macro to be the number (greater than or equal to zero) of that register.

  This should only need to be defined if `TARGET_READ_PC` and `TARGET_WRITE_PC` are not defined.

`NPC_REGNUM`
  The number of the "next program counter" register, if defined.
`NNPC_REGNUM`
  The number of the "next next program counter" register, if defined. Currently, this is only defined for the Motorola 88K.
`PARM_BOUNDARY`
  If non-zero, round arguments to a boundary of this many bits before pushing them on the stack.
`PRINT_REGISTER_HOOK (`regno`)`
  If defined, this must be a function that prints the contents of the given register to standard output.
`PRINT_TYPELESS_INTEGER`
  This is an obscure substitute for `print_longest` that seems to have been defined for the Convex target.
`PROCESS_LINENUMBER_HOOK`
  A hook defined for XCOFF reading.
`PROLOGUE_FIRSTLINE_OVERLAP`
  (Only used in unsupported Convex configuration.)
`PS_REGNUM`
  If defined, this is the number of the processor status register. (This definition is only used in generic code when parsing "$ps".)
`POP_FRAME`
  Used in `call_function_by_hand` to remove an artificial stack frame and in `return_command` to remove a real stack frame.
`PUSH_ARGUMENTS (`nargs`,` args`,` sp`,` struct_return`,` struct_addr`)`
  Define this to push arguments onto the stack for inferior function call. Returns the updated stack pointer value.
`PUSH_DUMMY_FRAME`
  Used in `call_function_by_hand` to create an artificial stack frame.
`REGISTER_BYTES`
  The total amount of space needed to store GDB's copy of the machine's register state.
`REGISTER_NAME(`i`)`
  Return the name of register i as a string. May return `NULL` or `NUL` to indicate that register i is not valid.
`REGISTER_NAMES`
  Deprecated in favor of `REGISTER_NAME`.
`REG_STRUCT_HAS_ADDR (`gcc_p`,` type`)`
  Define this to return 1 if the given type will be passed by pointer rather than directly.
`SAVE_DUMMY_FRAME_TOS (`sp`)`
  Used in `call_function_by_hand` to notify the target dependent code of the top-of-stack value that will be passed to the the inferior code. This is the value of the `SP` after both the dummy frame and space for parameters/results have been allocated on the stack.
`SDB_REG_TO_REGNUM`
  Define this to convert sdb register numbers into GDB regnums. If not defined, no conversion will be done.
`SHIFT_INST_REGS`
  (Only used for m88k targets.)
`SKIP_PERMANENT_BREAKPOINT`
  Advance the inferior's PC past a permanent breakpoint. GDB normally steps over a breakpoint by removing it, stepping one instruction, and re-inserting the breakpoint. However, permanent breakpoints are hardwired into the inferior, and can't be removed, so this strategy doesn't work. Calling `SKIP_PERMANENT_BREAKPOINT` adjusts the processor's state so that execution will resume just after the breakpoint. This macro does the right thing even when the breakpoint is in the delay slot of a branch or jump.
`SKIP_PROLOGUE (`pc`)`
  A C expression that returns the address of the "real" code beyond the function entry prologue found at pc.
`SKIP_PROLOGUE_FRAMELESS_P`
  A C expression that should behave similarly, but that can stop as soon as the function is known to have a frame. If not defined, `SKIP_PROLOGUE` will be used instead.
`SKIP_TRAMPOLINE_CODE (`pc`)`
  If the target machine has trampoline code that sits between callers and the functions being called, then define this macro to return a new PC that is at the start of the real function.
`SP_REGNUM`
  If the stack-pointer is kept in a register, then define this macro to be the number (greater than or equal to zero) of that register.

  This should only need to be defined if `TARGET_WRITE_SP` and `TARGET_WRITE_SP` are not defined.

`STAB_REG_TO_REGNUM`
  Define this to convert stab register numbers (as gotten from `r' declarations) into GDB regnums. If not defined, no conversion will be done.
`STACK_ALIGN (`addr`)`
  Define this to adjust the address to the alignment required for the processor's stack.
`STEP_SKIPS_DELAY (`addr`)`
  Define this to return true if the address is of an instruction with a delay slot. If a breakpoint has been placed in the instruction's delay slot, GDB will single-step over that instruction before resuming normally. Currently only defined for the Mips.
`STORE_RETURN_VALUE (`type`,` valbuf`)`
  A C expression that stores a function return value of type type, where valbuf is the address of the value to be stored.
`SUN_FIXED_LBRAC_BUG`
  (Used only for Sun-3 and Sun-4 targets.)
`SYMBOL_RELOADING_DEFAULT`
  The default value of the "symbol-reloading" variable. (Never defined in current sources.)
`TARGET_CHAR_BIT`
  Number of bits in a char; defaults to 8.
`TARGET_CHAR_SIGNED`
  Non-zero if `char` is normally signed on this architecture; zero if it should be unsigned.

  The ISO C standard requires the compiler to treat `char` as equivalent to either `signed char` or `unsigned char`; any character in the standard execution set is supposed to be positive. Most compilers treat `char` as signed, but `char` is unsigned on the IBM S/390, RS6000, and PowerPC targets.

`TARGET_COMPLEX_BIT`
  Number of bits in a complex number; defaults to `2 * TARGET_FLOAT_BIT`.

  At present this macro is not used.

`TARGET_DOUBLE_BIT`
  Number of bits in a double float; defaults to `8 * TARGET_CHAR_BIT`.
`TARGET_DOUBLE_COMPLEX_BIT`
  Number of bits in a double complex; defaults to `2 * TARGET_DOUBLE_BIT`.

  At present this macro is not used.

`TARGET_FLOAT_BIT`
  Number of bits in a float; defaults to `4 * TARGET_CHAR_BIT`.
`TARGET_INT_BIT`
  Number of bits in an integer; defaults to `4 * TARGET_CHAR_BIT`.
`TARGET_LONG_BIT`
  Number of bits in a long integer; defaults to `4 * TARGET_CHAR_BIT`.
`TARGET_LONG_DOUBLE_BIT`
  Number of bits in a long double float; defaults to `2 * TARGET_DOUBLE_BIT`.
`TARGET_LONG_LONG_BIT`
  Number of bits in a long long integer; defaults to `2 * TARGET_LONG_BIT`.
`TARGET_PTR_BIT`
  Number of bits in a pointer; defaults to `TARGET_INT_BIT`.
`TARGET_SHORT_BIT`
  Number of bits in a short integer; defaults to `2 * TARGET_CHAR_BIT`.
`TARGET_READ_PC`
`TARGET_WRITE_PC (`val`,` pid`)`
`TARGET_READ_SP`
`TARGET_WRITE_SP`
`TARGET_READ_FP`
`TARGET_WRITE_FP`
  These change the behavior of `read_pc`, `write_pc`, `read_sp`, `write_sp`, `read_fp` and `write_fp`. For most targets, these may be left undefined. GDB will call the read and write register functions with the relevant `_REGNUM` argument.

  These macros are useful when a target keeps one of these registers in a hard to get at place; for example, part in a segment register and part in an ordinary register.

`TARGET_VIRTUAL_FRAME_POINTER(`pc`,` regp`,` offsetp`)`
  Returns a `(register, offset)` pair representing the virtual frame pointer in use at the code address pc. If virtual frame pointers are not used, a default definition simply returns `FP_REGNUM`, with an offset of zero.
`TARGET_HAS_HARDWARE_WATCHPOINTS`
  If non-zero, the target has support for hardware-assisted watchpoints. See watchpoints, for more details and other related macros.
`TARGET_PRINT_INSN (`addr`,` info`)`
  This is the function used by GDB to print an assembly instruction. It prints the instruction at address addr in debugged memory and returns the length of the instruction, in bytes. If a target doesn't define its own printing routine, it defaults to an accessor function for the global pointer `tm_print_insn`. This usually points to a function in the `opcodes` library (see Opcodes). info is a structure (of type `disassemble_info`) defined in `include/dis-asm.h` used to pass information to the instruction decoding routine.
`USE_STRUCT_CONVENTION (`gcc_p`,` type`)`
  If defined, this must be an expression that is nonzero if a value of the given type being returned from a function must have space allocated for it on the stack. gcc_p is true if the function being considered is known to have been compiled by GCC; this is helpful for systems where GCC is known to use different calling convention than other compilers.
`VARIABLES_INSIDE_BLOCK (`desc`,` gcc_p`)`
  For dbx-style debugging information, if the compiler puts variable declarations inside LBRAC/RBRAC blocks, this should be defined to be nonzero. desc is the value of `n_desc` from the `N_RBRAC` symbol, and gcc_p is true if GDB has noticed the presence of either the `GCC_COMPILED_SYMBOL` or the `GCC2_COMPILED_SYMBOL`. By default, this is 0.
`OS9K_VARIABLES_INSIDE_BLOCK (`desc`,` gcc_p`)`
  Similarly, for OS/9000. Defaults to 1.

Motorola M68K target conditionals.

`BPT_VECTOR`
  Define this to be the 4-bit location of the breakpoint trap vector. If not defined, it will default to `0xf`.
`REMOTE_BPT_VECTOR`
  Defaults to `1`.

### Adding a New Target

The following files add a target to GDB:

`gdb/config/`arch`/`ttt`.mt`
  Contains a Makefile fragment specific to this target. Specifies what object files are needed for target ttt, by defining `TDEPFILES=...` and `TDEPLIBS=...`. Also specifies the header file which describes ttt, by defining `TM_FILE= tm-`ttt`.h`.

  You can also define `TM_CFLAGS`, `TM_CLIBS`, `TM_CDEPS`, but these are now deprecated, replaced by autoconf, and may go away in future versions of GDB.

`gdb/`ttt`-tdep.c`
  Contains any miscellaneous code required for this target machine. On some machines it doesn't exist at all. Sometimes the macros in `tm-`ttt`.h` become very complicated, so they are implemented as functions here instead, and the macro is simply defined to call the function. This is vastly preferable, since it is easier to understand and debug.
`gdb/`arch`-tdep.c`
`gdb/`arch`-tdep.h`
  This often exists to describe the basic layout of the target machine's processor chip (registers, stack, etc.). If used, it is included by ``ttt`-tdep.h`. It can be shared among many targets that use the same processor.
`gdb/config/`arch`/tm-`ttt`.h`
  (`tm.h` is a link to this file, created by `configure`). Contains macro definitions about the target machine's registers, stack frame format and instructions.

  New targets do not need this file and should not create it.

`gdb/config/`arch`/tm-`arch`.h`
  This often exists to describe the basic layout of the target machine's processor chip (registers, stack, etc.). If used, it is included by `tm-`ttt`.h`. It can be shared among many targets that use the same processor.

  New targets do not need this file and should not create it.

If you are adding a new operating system for an existing CPU chip, add a `config/tm-`os`.h` file that describes the operating system facilities that are unusual (extra symbol table info; the breakpoint instruction needed; etc.). Then write a ``arch`/tm-`os`.h` that just `#include`s `tm-`arch`.h` and `config/tm-`os`.h`.

#### Footnotes

1. Some D10V instructions are actually pairs of 16-bit sub-instructions. However, since you can't jump into the middle of such a pair, code addresses can only refer to full 32 bit instructions, which is what matters in this explanation.