Neso, Microblaze And Linux: How To Boot Linux On Neso Artix 7 FPGA Module From SPI Flash

May 18, 2016

In the previous article we saw how to build Linux Kernel and run it on Neso Artix 7 FPGA Module using Xilinx Platform Cable USB and XMD. Though this is the easier method, there are a couple of drawbacks associated to it. You will need a Xilinx Platform Cable USB which is slightly expensive and the images are programmed to SRAM/DDR instead of SPI flash. It is possible to program the onboard SPI flash with the images which would retain contents even after power cycle and this can be done without buying expensive JTAG cables.

To program the onboard SPI flash with, we will need to make a single binary image that will contain the following individual images.

  • FPGA bit file for Microblaze
  • Linux kernel
  • A bootloader

A bootloader is necessary here to copy the Linux kernel from SPI flash to DDR and execute it. The primary requirements for the bootloader is that it should be capable of reading data from the SPI flash and should fit within the BRAM cache size.

To build a bootloader, launch SDK from Vivado and create a new application project. Let’s call this project “neso_linux_bootloader”. When selecting the application template, select “Hello World”. Once the project is created, open the “neso_linux_bootloader” project in the “Project Explorer” on the left. Double click on the linker script lscript.ld and make sure that all sections are mapped to BRAM (should look something like microblaze_0_local_memory_ilmb_bram_if_cntlr_microblaze_0_local_memory_dlmb_bram_if_cntlr). If not, map all sections to BRAM manually and save the linker script. After saving linker script, build the project and make sure that no errors are reported.

Open helloworld.c in the project and replace the entire code with the code below. Please note, the code below is a bare-bones bootloader designed to be simpler than being robust.

Rebuild the project with newly added code and make sure that no errors reported. If any errors pop up, please fix them before continuing.

The above code will load the kernel image from SPI flash memory and write to DDR and jump to kernel entry point. This code assumes that the kernel image is sitting at a specific address in the SPI flash (0x500000 in this case) and has a specific size (0x700000) in this case. 0x700000 is little bigger than the actual kernel image but that shouldn’t cause any problems. The area starting from 0x000000 to 0x4FFFFF is reserved for FPGA configuration data (enough to hold configuration data for XC7A100T devices whose bitstreams’ size is 3825788 bytes, or 0x3A607C bytes in hex). So in essence, we are targeting the following memory map for the SPI flash binary image we are going to create.

  1. 0x000000 – 0x4FFFFF -> FPGA configuration data
  2. 0x500000 – 0xBFFFFF -> Kernel image
  3. 0xC00000 upwards -> User data (not used in our case)

At this point, we have the bit file for FPGA configuration, Linux kernel image and bootloader image we just built. Before we can proceed with packing these images in to a single binary file, there is one crucial step we need to do. You may remember that fact the the kernel image we built (simpleImage.nesoartix7) is in ELF format. This is helpful when downloading kernel using XMD but the metadata (in addition to executable code) is going to be a problem since our bootloader does not understand ELF format. One option is to add ELF loader code to bootloader but it is going to be time consuming and simply an overkill for such a simple implementation like this. Another option would be to strip the metadata and generate a pure executable binary file.  This can be easily done with objcopy tool (objcopy is part of gnu binutils). But the tricky thing here is that you can not use any objcopy (eg: objcopy built for host machine). You will need to use objcopy that is built for Microblaze. Fortunately, buildroot already built cross objcopy that runs on host machine but can work on Microblaze executables. This happened when we built the Linux kernel earlier (buildroot builds all cross tools needed for building the kernel). All that we need to do is to find where these cross compiled tools are placed by buildroot. With the specific buildroot version we used, the objcopy executable we need was placed in the directory “output/host/usr/microblazeel-buildroot-linux-gnu/bin” under the buildroot root directory.

Switch to output/images directory under buildroot root directory where Linux kernel image is available in ELF format and execute the following command.

If the command was successful, you will see a new file created with the name neso_linux.bin. Copy this file over to the Windows machine.

On the Windows machine, place the following files in the same directory.

  1. The Linux image in binary format (neso_linux.bin we just created)
  2. The bit file(.bit) for the Microblaze system and the Memory Map file (.mmi)  (Both files can be found in your SDK’s Project Explorer under project named “neso_linux_mb_wrapper_hw_platform_0” or similar. Please see the image below)Neso_Bootloader_1_mmi_bit_files
  3. The bootloader executable (This file can be found in the directory neso_linux_bootloader\Debug under the SDK bootloader project folder and has .elf extension)


In my case, I have the following files in my directory as shown in the image. Your file names may be different depending on the project names etc…


With all necessary images available now, let us move forward with creating the final binary image. We will do the following steps to create a single binary image.

  1. Merge the FPGA configuration (.bit file) and the Bootloader executable image (.elf). This is because the contents of the executable needs to go in to the FPGA BRAM when FPGA initialize after power up.
  2. Concatenate the combined FPGA configuration file (generated in previous step) with Linux kernel image.

To merge Bootloader executable image with bit file, run the following command.


If the command ran successfully, you will see a new file created in the directory with name “download.bit”. This file now has both the FPGA configuration stream and Bootloader code.

Now we can concatenate the FPGA configuration file we just generated (download.bit) with Linux kernel binary image (neso_linux.bin) to create a single binary image that can be downloaded to the SPI flash. Run the following commands to generate single binary image.

First command exits the Vivado tcl shell. Second command loads the ISE command line environment. And finally the third one actually does the work of merging two files into a final download.bin file.


If this command succeeded, you will see a few new files in the directory and one of them would be “download.bin”. This is the final binary image that we need to download to Neso’s SPI flash. The following image shows all files in my directory after executing all above mentioned steps.


The final image is approximately 11.10MB in size and this leaves with approximately 4.9MB of available space in the SPI flash for custom user data.

Now download the latest  Neso Artix 7 FPGA Module configuration tool from the product page and use that to download the final binary image “download.bin” to the SPI flash (see image below).


It may take a couple of minutes for the download to complete. While download is in progress, go ahead and start your favorite serial terminal emulation program (Hyperterminal, PUTTY, TeraTerm etc..) and open the COM port corresponding to Neso’s FT2232 channel B and set baud rate to 115200 and set handshaking to off/none (You should have configured channel B of Neso’s FT2232H as serial port. If not, please follow this tutorial to do so).

Once the binary image is completed downloading, the tool will reboot the FPGA and let it load the configuration data from newly downloaded image. If everything went well, the bootloader will start copying Linux kernel image to DDR.


It may take a few seconds for the bootloader to copy the Linux kernel. Once copying is completed, bootloader will start the kernel and you will see the kernel booting as shown below.


You can login to Linux as root with password as “neso”. Image below shows root directory structure, cpuinfo and “uname -a” outputs:


Was this helpful?

  • Tony says:

    I followed this procedure to the best of my ability and i have a few things that i’d like to ask.
    1) The code that you have above where we are supposed to c&p into the helloworld.c file. I noticed that I get the following 3 errors when it came to build
    a) ‘XPAR_MIG_7SERIES_0_BASEADDR’ undeclared (first use in this function)
    b) conflicting types for ‘print’
    c) make: *** [src/helloworld.o] Error 1
    What i did to overcome this was to make the following adjustments to the code
    void print(char *str);
    //void print(char *str);

    once I made edits to “IS” i then re-built and this seemed to solve the problems. However, I dont know if there are other issues that now have been invoked by doing so.
    2) When I first used the updatemem command I got another that stated the following
    ERROR: [Updatemem 57-85] Invalid processor specification of: msys_i/sys_microblaze. The known processors are: msys_i/sys_microblaze

    To solve this issue I went into the *.mmi file and did the following edit:


    Again like in 1) I dont know if this causes other issues down the road (I dont think so) but it seemed to have to solve that issue for now

    3) Now when I seemed to overcame 2) I got another error that stated the following (this one has stumped for the time being)
    CRITICAL WARNING: [Updatemem 57-154] Matching address space for code segment 1 n
    ot found. Code segment occupies [0x80000000:0x800033AB]
    ERROR: [Updatemem 57-153] Failed to update the BRAM INIT strings for neso_linux_bootloader.elf and msys_wrapper.mmi.
    ERROR: [Common 17-39] ‘update_mem’ failed due to earlier errors.

    Any thoughts? I am thinking that perhaps it has something to do with the *.ld file but I could be wrong.

    4) I decided to put this version aside and start a new one using my co workers SPI Boot sources and using your procedure. With this version I have overcame the problem with the updatemem command (I dont know how! but ok) but I have gotten stumped again with the promgen command from the following error
    Release 14.7 – Promgen P.20131013 (nt64)
    Copyright (c) 1995-2013 Xilinx, Inc. All rights reserved.
    0x947a5c (9730652) bytes loaded up from 0x0
    ERROR:Bitstream:32 – 0x62cfb4 bytes loaded up from 0x500000 overlaps load at 0x0

    Any thoughts? If i come across something ill come back and post something for future people following this

    March 16, 2017 at 5:30 pm
    • Tony says:

      Sorry I noticed that my WAS and IS in 2) was deleted for some reason. here they are

      Processor Endianness=”Little” InstPath=”msys_i/sys_microblaze”

      Processor Endianness=”Little” InstPath=”neso_linux_mb_i/microblaze_0″

      March 16, 2017 at 5:33 pm
      • Tony says:

        Ok, so I was able to solve 3). Since the error it produced said that “Code segment occupies [0x80000000:0x800033AB]” I went to the lscript.ld file and opened it up in Xilnx SDK with through the use of the text editor tool. I then made the following edit:

        axi_bram_ctrl_0_S_AXI_BASEADDR : ORIGIN = 0xC0000000, LENGTH = 0x2000

        axi_bram_ctrl_0_S_AXI_BASEADDR : ORIGIN = 0xC0000000, LENGTH = 0x4000

        Since the error said it took 33AB worth of code segment, I increased the size to 4000 and it seems to take it.

        I then rebuilt and was able to produce the download.bit file from the updatemem command

        I now am stuck at 4) on both versions.

        March 16, 2017 at 9:02 pm
    • Rohit Singh says:

      Hi Tony,

      For the Error 4) This is the log you are getting:
      0x947a5c (9730652) bytes loaded up from 0x0
      ERROR:Bitstream:32 – 0x62cfb4 bytes loaded up from 0x500000 overlaps load at 0x0

      ^ There is clearly something wrong going on here. Command is loading 0x947A5C bytes from offset 0x00, which will clearly overlap with 0x500000 offset from where the linux image is going to be loaded.

      Can you check the size of your bitstream file? It should be around 2-3MBs and not more, definitely should not be 0x947A5C bytes.

      For Error 1) Xilinx might have changed the #defines in newer Vivado versions. If that’s the case, please update it. And you need to overwrite the complete helloworld.c file created by Xilinx with the code in this article. Also would also need to update the linker script to make sure entire code is mapped to Block RAM and not DDR.

      For 4) you can also use Vivado’s command with this format: write_cfgmem -force -format BIN -size 16 -interface SPIx1 -loadbit “up 0x00 download.bit” -loaddata “up 0x500000 neso_linux.bin” download.bin

      March 17, 2017 at 5:07 am
  • Tony says:

    Hi Rohit.. ahh.. yes. So I have my own microblaze design using an Artix 7 board but yes my bitstream file is about 9.5MB. I was just reading right now on how to use the promgen command. I tinkered around with it and did the following


    promgen -w -p bin -c FF -o download -s 16384 -u 0 download.bit -data_file up 500000 neso_linux.bin -spi


    promgen -w -p bin -c FF -o download -s 16384 -u 0 download.bit -data_file up 950000 neso_linux.bin -spi

    that produced this output

    C:\Xilinx\14.7\ISE_DS>promgen -w -p bin -c FF -o download -s 16384 -u 0 download.bit -data_file up 950000 neso_linux.bin -spi
    Release 14.7 – Promgen P.20131013 (nt64)
    Copyright (c) 1995-2013 Xilinx, Inc. All rights reserved.
    0x947a5c (9730652) bytes loaded up from 0x0
    0x62cfb4 (6475700) bytes loaded up from 0x950000
    Using user-specified prom size of 16384K
    Writing file “download.bin”.
    Writing file “download.prm”.
    Writing file “download.cfi”.

    but i have a feeling that i dont think this is all i have to do i have to go back and do this whole process again because i have to change perhaps the following as well

    #define FLASH_IMAGE_START_ADDRESS 0x500000


    #define FLASH_IMAGE_START_ADDRESS 0x950000

    in the helloworld.c file correct? i think? it seems like its something i need to do. anyways im going to tinker around some more

    March 17, 2017 at 4:54 pm

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