How do I write a GDB function to make a comparison to the program counter

Question

I am attempting to write a gdb function that loops until the passed parameter equals the program counter.

I am working with a primitive CPU, 68332. No hardware breakpoints. No OS that supports GDB software breakpoints, just a single instruction step. GDB provides the 'software' emulation of 'nexti count'. The JTAG provides a run to address.

However, the JTAG, for some reason, overwhelms the CPU when used to run to address, and I get bus errors. I can only seem to reliably use 'step' single instruction.

If I use GDB to 'step' to the address, I don't get bus errors.

Below is my attempt at such a GDB function.

define mtia
    if $argc == 1 then
    set $address = *(unsigned char*)$arg0
    while($address != $pc)
      nexti
    end
end

I just can't seem to get the syntax correct to be able to get GDB to accept and run the function.

What is the correct syntax?

score 1 · Accepted Answer · answered Jul 18 '13 at 18:35

The correct syntax for the GDB command function above is

define mtia
    if $argc == 1 then
    set $address = (unsigned char*)$arg0
    while($address != $pc)
      nexti
    end
end

I had an asterisk in front of (unsigned char*). This removed the pointer type. This caused a syntax error with $address != $pc as $pc is considered a pointer.

This function performs a single assembly instruction step, until the address passed into the function is reached. Slow, but on the old M68332 CPU, this is all I have.

How do I write a GDB function to make a comparison to the program counter

1 Answers1