Gdb/Bootstrapping
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20.5.2 What You Must Do for the Stub
The debugging stubs that come with GDB are set up for a particular chip architecture, but they have no information about the rest of your debugging target machine.
First of all you need to tell the stub how to communicate with the serial port.
int getDebugChar()
Write this subroutine to read a single character from the serial port. It may be identical to
getchar
for your target system; a different name is used to allow you to distinguish the two if you wish.void putDebugChar(int)
Write this subroutine to write a single character to the serial port. It may be identical to
putchar
for your target system; a different name is used to allow you to distinguish the two if you wish.
If you want GDB to be able to stop your program while it is
running, you need to use an interrupt-driven serial driver, and arrange
for it to stop when it receives a ^C
(‘\003
’, the control-C
character). That is the character which GDB uses to tell the
remote system to stop.
Getting the debugging target to return the proper status to GDB
probably requires changes to the standard stub; one quick and dirty way
is to just execute a breakpoint instruction (the “dirty” part is that
GDB reports a SIGTRAP
instead of a SIGINT
).
Other routines you need to supply are:
void exceptionHandler (int exception_number, void *exception_address)
Write this function to install
exception_address
in the exception handling tables. You need to do this because the stub does not have any way of knowing what the exception handling tables on your target system are like (for example, the processor’s table might be in ROM, containing entries which point to a table in RAM). Theexception_number
specifies the exception which should be changed; its meaning is architecture-dependent (for example, different numbers might represent divide by zero, misaligned access, etc). When this exception occurs, control should be transferred directly toexception_address
, and the processor state (stack, registers, and so on) should be just as it is when a processor exception occurs. So if you want to use a jump instruction to reachexception_address
, it should be a simple jump, not a jump to subroutine.For the 386,
exception_address
should be installed as an interrupt gate so that interrupts are masked while the handler runs. The gate should be at privilege level 0 (the most privileged level). The SPARC and 68k stubs are able to mask interrupts themselves without help fromexceptionHandler
.void flush_i_cache()
On SPARC and SPARCLITE only, write this subroutine to flush the instruction cache, if any, on your target machine. If there is no instruction cache, this subroutine may be a no-op.
On target machines that have instruction caches, GDB requires this function to make certain that the state of your program is stable.
You must also make sure this library routine is available:
void *memset(void *, int, int)
This is the standard library function
memset
that sets an area of memory to a known value. If you have one of the free versions oflibc.a
,memset
can be found there; otherwise, you must either obtain it from your hardware manufacturer, or write your own.
If you do not use the GNU C compiler, you may need other standard
library subroutines as well; this varies from one stub to another,
but in general the stubs are likely to use any of the common library
subroutines which GCC
generates as inline code.
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