Bootstrapping (Debugging with GDB)
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.
Write this subroutine to read a single character from the serial port. It may be identical to
getcharfor your target system; a different name is used to allow you to distinguish the two if you wish.
Write this subroutine to write a single character to the serial port. It may be identical to
putcharfor 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
\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
Other routines you need to supply are:
void exceptionHandler (int exception_number, void *exception_address)
Write this function to install
exception_addressin 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). The
exception_numberspecifies 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 to
exception_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 reach
exception_address, it should be a simple jump, not a jump to subroutine.
For the 386,
exception_addressshould 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 from
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
memsetthat sets an area of memory to a known value. If you have one of the free versions of
memsetcan 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.