The GDB Remote Serial Protocol
The GDB Remote Serial Protocol (RSP) is the lingua franca between gdb and a remote target. It defines messages for reading and
writing data, controlling the application being debugged, and reporting application status. The host side is implemented in gdb's remote.c source file.
The following are short descriptions of the RSP's most important commands. A complete summary of each command is available on the ESP Web site, www.embedded.com/code.htm.
Data exchanged between gdb and a remote target with the GDB Remote Serial Protocol uses plain ASCII characters. Messages begin with a dollar sign ($) and end with a
gridlet (#) and eight-bit checksum. In other words, each message looks like this:
$ <data> # CKSUM_MSN CKSUM_LSN
where <data> is typically a string of ASCII hex [0-9,a-f,A-F] characters.
CKSUM_MSN and CKSUM_LSN are ASCII hex representations of an eight-bit checksum of <data>. Only the hex digits 0 to 9 and a to f are allowed.
When a message is sent, the receiver responds with either:
+ if the received checksum was correct, and the receiver is ready for the
next packet
- the received checksum was incorrect, and the message needs to be retransmitted
A target can respond to a message from gdb with either data or an OK (depending on the message), or a target-defined error code. When gdb receives an error code, it reports the number to the user via the gdb console.
Definitions for <data> are shown below.
| Table A: Register-related commands
|
| Command name
|
<data> definition
|
Description
|
| read registers
|
g
|
Return the values of all registers
|
| write registers
|
GXX..XX
|
Set registers to XX..XX
|
| write register nn
|
Pnn=XX..XX
|
Set the value of register NN
|
| Table B: Memory-related commands
|
| Command name
|
<data>
definition
|
Description
|
| read memory
|
mAA..AA,LL..LL
|
Read values from memory
|
| write memory
|
MAA..AA,LL..LL:XX..XX
|
Write values to memory
|
| Table C: Target infromation commands
|
| Command name
|
<data> definition
|
Description
|
| query section offsets
|
qOffsets
|
Return section offset information
|
| Table D: Program control commands
|
| Command name
|
<data> definition
|
Description
|
| set thread
|
Hc
|
Set current program
thread
|
| step
|
sAA..AA
|
Execute one assembly instruction
|
| continue
|
cAA..AA
|
Resume
application execution
|
| last signal
|
?
|
Report last signal
|
| kill
|
k
|
Terminate
application
|
| Table E: Target Status messages (responses from the target)
|
| Message name
|
<data> definition
|
Description
|
| last signal response
|
Snn
|
Minimal reply to the last signal command
|
| expected response
|
Tnnr...:v...r...:v...;
|
The last
signal reported, plus key register values
|
| console output
|
Ovvvvvvvv...
|
sends text from the target to gdb’s console
|
