Capture variable value in simulation

[mahdi259]

Hi This is actually a question. I use simple testbench for simulation. What is the best way to capture the value of a variable of C code without using UART to print it?

Describe the solution you'd like I want to know the result of an operation in simulation mode without needing to print it using UART.

Thanks.

[stnolting]

Hey @mahdi259!

Hmm that's an interesting question! Are we talking about a variable in main memory?

Let's assume you have something like this:

volatile uint32_t some_variable;

This variable will be placed in main memory at a fixed address as it is marked as volatile. You can use make elf_info to find out the actual memory address of this variable:

In this example, some_variable is placed at 0x800000a4. Then, you could connect some VHDL report block to the CPU's data interface (in neorv32_top.vhd) to track write accesses to this address:

process(clk_i)
begin
  if rising_edge(clk_i) then
    if (cpu_d_req.stb = '1') and (cpu_d_req.rw = '1') and (cpu_d_req.addr = x"800000a4") then
      report "MEM 0x" & to_hstring32_f(cpu_d_req.data) & " -> [0x" & to_hstring32_f(cpu_d_req.addr) & "]" severity note;
    end if;
  end if;
end process;

Now, when running some C code like this

some_variable = 3;
some_variable = 5;

you'll get the according simulator reports in the console:

[mahdi259]

Thanks for your response. Unfortunately make elf_info doesn't show some_variable. 1.txt code.txt

[mahdi259]

Ok I should define it as global variable outside main...

[stnolting]

Ok I should define it as global variable outside main...

Right, your variable is just local so it will be put on the stack. Thus, it might not have a static address.

[mahdi259]

You really solved my question. I wish great success for you in your career and life.

[mahdi259]

Hi @stnolting . Sorry can you mention how to use this code in testbench with xbus signals? It seems that it changes address on xbus.

[stnolting]

Sorry can you mention how to use this code in testbench with xbus signals? It seems that it changes address on xbus.

Just put that somewhere into neorv32_top.vhd.

I'm not really sure what you want to monitor? What does your processor configuration look like? Do you use internal memories (IMEM / DMEM) or do you use some external memory coupled via XBUS?

[mahdi259]

Hi. As mentioned above, I monitor value change of volatile int temp_i variable with address specified in make elf_info that is 0x80000000. I use neorv32_tb.simple.vhd that utilizes external memories and I disable instruction and data caches.

I wonder why the same writing isn't seen on xbus signals.
codes.zip

[stnolting]

For the default testbench setup, address 0x80000000 is mapped to the internal DMEM. Hence, when accessing this variable / address there will be NO activity on the external bus interface (XBUS).

[mahdi259]

Sorry, you're right.