processor configuration with SPI flash

[mahdi259]

Hi @stnolting
Does a processor with SPI flash need IMEM and DMEM?

[stnolting]

The SPI module is just another interface. You can use it to read/write data from/to an external flash, but you still need some program memory where the code is stored that actually generates these SPI accesses. In setups with the bootloader enabled, the bootloader code can transfer an entire program image from the flash into IMEM using the SPI module. Hence, an internal memory (RAM) is required.

In contrast, the XIP module can be used to transform CPU accesses into flash accesses transparently. No additional code memory is needed in this case. But these accesses are read-only.

In both cases you still need some RAM (for the stack etc) which is implemented by the DMEM.

Furthermore, DMEM is just RAM that can also be used to store/execute instruction code. So the processor can also run with DMEM alone (no dedicated IMEM), but this requires some makefile adjustments.

[mahdi259]

Thanks a lot. I will look at it. One more question, how are the base addresses specified in imem and dmem vhdl files of processor?

[mahdi259]

Uh. That's in neorv32_package.vhd
You're great!

[mahdi259]

Hi
I think it's a good practice to store program binaries (program compiled for XIP execution) with FPGA bitstream at the same time. I did that with ISE with neorv32_exe.bin and this is the output when I sent x in terminal:

My question is about raw files like (neorv32_raw_exe.mem). Why do we call it raw?

(It doesn't work with raw files.)

[stnolting]

The "raw" files only contain the plain executable without any neorv32-specific bootloader header.

See 📚 4.4.4. Executable Image Generator:

In your screenshot it looks like your are using a file with this bootloader header - the header data gets interpreted as instructions causing illegal instruction exception ("ERR_EXC 0x00000002").

[mahdi259]

Thanks a lot.

[mahdi259]

After some modifications in common.mk and image_gen.c, I generated neorv32_exe.mem with the header files you mentioned and neorv32_xip_exe.mem without headers.
The picture I sent was for storing XIP program with header files stored in flash directly without using demo_xip and then executing it with x option. It seems that demo_xip removes headers and then stores it to flash.

Modifications.zip

This way I didn't enable IMEM at all!

This is where I wrote program binaries to flash.

[stnolting]

Oh, good old ISE! How I miss that 😅

I did not even know that the programmer there allows to add custom memory files to the bitstream. But that's really cool! 👍

After some modifications in common.mk and image_gen.c, I generated neorv32_exe.mem with the header files you mentioned and neorv32_xip_exe.mem without headers.

That should not be necessary I think. The makefile offers different options for generating "raw" (= no header) memory files: 📚 4.8. Executable Image Formats

[mahdi259]

Thanks