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GDB and remote MIPS boards

GDB can use the MIPS remote debugging protocol to talk to a MIPS board attached to a serial line.

This is available when you configure GDB with --target=mips-idt-ecoff.

Use the following GDB commands to specify the connection to your target board.

target mips port
To run a program on the board, start up gdb with the name of your program as the argument. To connect to the board, use the command ‘target mips port’, where port is the name of the serial port connected to the board. If the program has not already been downloaded to the board, you may use the load command to download it. You can then use all the usual GDB commands.

For example, the following sequence connects to the target board through a serial port, and loads and runs a program called prog through the debugger.

host$ gdb prog
GDB is free software and ...
(gdb) target mips /dev/ttyb
(gdb) load prog
(gdb) run

target mips hostname:portnumber
On some GDB host configurations, you can specify a TCP connection (for instance, to a serial line managed by a terminal concentrator) instead of a serial port, using the syntax ‘hostname:portnumber’.

GDB also supports the following special commands for MIPS targets.

set processor args

show processor
Use the set processor command to set the type of MIPS processor when you want to access processor-type-specific registers. For example, set processor r3041 tells GDB to use the CPO registers appropriate for the 3041 chip. Use the show processor command to see what MIPS processor GDB is using. Use the info reg command to see what registers GDB is using.

set mipsfpu double

set mipsfpu single

set mipsfpu none

show mipsfpu
If your target board does not support the MIPS floating point coprocessor, you should use the command ‘set mipsfpu none’ (if you need this, you may wish to put the command in your .gdbinit file). This tells GDB how to find the return value of functions which return floating point values. It also allows GDB to avoid saving the floating point registers when calling functions on the board. If you are using a floating point coprocessor with only single precision floating point support, as on the R4650 processor, use the command ‘set mipsfpu single’. The default double precision floating point coprocessor may be selected using ‘set mipsfpu double’.

In previous versions the only choices were double precision or no floating point, so ‘set mipsfpu on’ will select double precision and ‘set mipsfpu off’ will select no floating point. As usual, you can inquire about the mipsfpu variable with ‘show mipsfpu’.

set remotedebug n

show remotedebug
You can see some debugging information about communications with the board by setting the remotedebug variable. If you set it to 1 using ‘set remotedebug 1’, every packet is displayed. If you set it to 2, every character is displayed. You can check the current value at any time with the command, show remotedebug.

set timeout seconds

set retransmit-timeout seconds

show timeout

show retransmit-timeout
You can control the timeout used while waiting for a packet, in the MIPS remote protocol, with the set timeout seconds command. The default is 5 seconds. Similarly, you can control the timeout used while waiting for an acknowledgement of a packet with the set retransmit-timeout seconds command. The default is 3 seconds. You can inspect both values with show timeout and show retransmit-timeout.

Note: These commands are available only when GDB is configured for ‘--target=mips-idt-ecoff’.

The timeout set by set timeout does not apply when GDB is waiting for your program to stop. In that case, GDB waits forever because it has no way of knowing how long the program is going to run before stopping.