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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.

At present, the target architecture definition consists of a number of C macros.

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.

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 address includes any bits that are not really part of the address, then define this macro to expand into an expression that zeros those bits in addr. For example, the two low-order bits of a Motorola 88K address may be used by some kernels for their own purposes, since addresses must always be 4-byte aligned, and so are of no use for addressing. Those bits should be filtered out with an expression such as ((addr) & ~3).
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 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 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 favour 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 favour 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 favour 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.
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 favour 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
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?
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.
END_OF_TEXT_DEFAULT
This is an expression that should designate the end of the text section (? FIXME ?)
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 EXTRACT_STRUCT_VALUE_ADDRESS_P this is used to to extract from an array regbuf (containing the raw register state) the address in which a function should return its structure value, as a CORE_ADDR (or an expression that can be used as one).
EXTRACT_STRUCT_VALUE_ADDRESS_P
Predicate for EXTRACT_STRUCT_VALUE_ADDRESS.
FLOAT_INFO
If defined, then the `info float' command will print information about the processor's floating point unit.
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
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. Three common definitions are available. default_frame_chain_valid (the default) is nonzero if the chain pointer is nonzero and given frame's PC is not inside the startup file (such as `crt0.o'). alternate_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()).
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. That 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.
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. The 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.
GDB_TARGET_IS_MACH386
GDB_TARGET_IS_SUN3
GDB_TARGET_IS_SUN386
Kludges that should go away.
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 <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 haste and we are repenting at leisure.
IEEE_FLOAT
Define this if the target system uses IEEE-format floating point numbers.
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.
IN_SIGTRAMP (pc, name)
Define this to return true 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.
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.)
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 (dependant on insert_breapoints_p) breakpoints at each possible destinations of the next instruction. See sparc-tdep.c and rs6000-tdep.c for examples.
PCC_SOL_BROKEN
(Used only in the Convex target.)
PC_IN_CALL_DUMMY
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.
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.
PUSH_ARGUMENTS (nargs, args, sp, struct_return, struct_addr)
Define this to push arguments onto the stack for inferior function call. Return 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_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_BYTE_ORDER_DEFAULT
The ordering of bytes in the target. This must be either BIG_ENDIAN or LITTLE_ENDIAN. This macro replaces TARGET_BYTE_ORDER which is deprecated.
TARGET_BYTE_ORDER_SELECTABLE_P
Non-zero if the target has both BIG_ENDIAN and LITTLE_ENDIAN variants. This macro replaces TARGET_BYTE_ORDER_SELECTABLE which is deprecated.
TARGET_CHAR_BIT
Number of bits in a char; defaults to 8.
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.
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 define 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=...'. 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 and may go away in future versions of GDB.
`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.
`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/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.
`gdb/arch-tdep.c'
Similarly, there are often common subroutines that are shared by all target machines that use this particular architecture.

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 #includes `tm-arch.h' and `config/tm-os.h'.


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