Communication protocol
......................

   The stub files provided with GDB implement the target side of the
communication protocol, and the GDB side is implemented in the GDB
source file `remote.c'.  Normally, you can simply allow these
subroutines to communicate, and ignore the details.  (If you're
implementing your own stub file, you can still ignore the details: start
with one of the existing stub files.  `sparc-stub.c' is the best
organized, and therefore the easiest to read.)

   However, there may be occasions when you need to know something about
the protocol--for example, if there is only one serial port to your
target machine, you might want your program to do something special if
it recognizes a packet meant for GDB.

   All GDB commands and responses (other than acknowledgements, which
are single characters) are sent as a packet which includes a checksum.
A packet is introduced with the character `$', and ends with the
character `#' followed by a two-digit checksum:

     $PACKET INFO#CHECKSUM

CHECKSUM is computed as the modulo 256 sum of the PACKET INFO
characters.

   When either the host or the target machine receives a packet, the
first response expected is an acknowledgement: a single character,
either `+' (to indicate the package was received correctly) or `-' (to
request retransmission).

   The host (GDB) sends commands, and the target (the debugging stub
incorporated in your program) sends data in response.  The target also
sends data when your program stops.

   Command packets are distinguished by their first character, which
identifies the kind of command.

   These are some of the commands currently supported (for a complete
list of commands, look in `gdb/remote.c.'):

`g'
     Requests the values of CPU registers.

`G'
     Sets the values of CPU registers.

`mADDR,COUNT'
     Read COUNT bytes at location ADDR.

`MADDR,COUNT:...'
     Write COUNT bytes at location ADDR.

`c'
`cADDR'
     Resume execution at the current address (or at ADDR if supplied).

`s'
`sADDR'
     Step the target program for one instruction, from either the
     current program counter or from ADDR if supplied.

`k'
     Kill the target program.

`?'
     Report the most recent signal.  To allow you to take advantage of
     the GDB signal handling commands, one of the functions of the
     debugging stub is to report CPU traps as the corresponding POSIX
     signal values.

`T'
     Allows the remote stub to send only the registers that GDB needs
     to make a quick decision about single-stepping or conditional
     breakpoints.  This eliminates the need to fetch the entire
     register set for each instruction being stepped through.

     The GDB remote serial protocol now implements a write-through
     cache for registers.  GDB only re-reads the registers if the
     target has run.

   If you have trouble with the serial connection, you can use the
command `set remotedebug'.  This makes GDB report on all packets sent
back and forth across the serial line to the remote machine.  The
packet-debugging information is printed on the GDB standard output
stream.  `set remotedebug off' turns it off, and `show remotedebug'
shows you its current state.