Using QEMU for Embedded Systems Development, Part 3

It's time to u-boot

It's time to u-boot

This is the last article of this series on QEMU. In the previous article, we worked on bare-metal programming, and discussed the need for a bootloader. Most GNU/Linux distros use GRUB as their boot-loader (earlier, LILO was the choice). In this article, we will test the famous U-Boot (Universal BootLoader).

In embedded systems, especially in mobile devices, ARM processor-based devices are leading the market. For ARM, U-Boot is the best choice for a bootloader. The good thing about it is that we can use it for different architectures like PPC, MIPS, x86, etc. So let’s get started.

Download and compile U-Boot

U-Boot is released under a GPL licence. Download it from this FTP server, which has every version of U-Boot available. For this article, I got version 1.2.0 (u-boot-1.2.0.tar.bz2). Extract the downloaded tar ball and enter the source code directory:

# tar -jxvf u-boot-1.2.0.tar.bz2
# cd u-boot-1.2.0

To begin, we must configure U-Boot for a particular board. We will use the same ARM Versatile Platform Baseboard (versatilepb) we used in the previous article, so let’s run:

# make versatilepb_config arch=ARM CROSS_COMPILE=arm-none-eabi-
Configuring for versatile board...
Variant:: PB926EJ-S

After configuration is done, compile the source code:

# make all arch=ARM CROSS_COMPILE=arm-none-eabi-
for dir in tools examples post post/cpu ; do make -C $dir _depend ; done
make[1]: Entering directory `/root/qemu/u-boot-1.2.0/tools'
ln -s ../common/environment.c environment.c
G++_Lite/bin/../lib/gcc/arm-none-eabi/4.4.1 -lgcc \
			-Map -o u-boot
arm-none-eabi-objcopy --gap-fill=0xff -O srec u-boot u-boot.srec
arm-none-eabi-objcopy --gap-fill=0xff -O binary u-boot u-boot.bin

Find the size of the compiled U-Boot binary file (around 72 KB in my experience) with ls -lh u-boot* — we will use it later in this article. I assume that you have set up QEMU, networking and the ARM tool chain, as explained in previous articles in this series (1, 2, 3). If not, then I suggest you read the last three articles.

Boot U-Boot in QEMU

Now we can boot the U-Boot binary in QEMU, which is simple. Instead of specifying the Linux kernel as the file to boot in QEMU, use the U-Boot binary:

# qemu-system-arm -M versatilepb -nographic -kernel u-boot.bin

Run some commands in U-Boot, to check if it is working:

Versatile # printenv
bootargs=root=/dev/nfs mem=128M ip=dhcp netdev=25,0,0xf1010000,0xf1010010,eth0
Environment size: 184/65532 bytes

Figure 1: U-Boot

The next step is to boot a small program from U-Boot. In the previous article, we wrote a small bare-metal program — so let us use that.

We will create a flash binary image that includes u-boot.bin and the bare-metal program in it. The test program from the last article will be used here again with some modification. As the u-boot.bin size is around 72 KB, we will move our sample program upward in memory. In the linker script, change the starting address of the program:

. = 0x100000;
startup : { startup.o(.text)}
.data : {*(.data)}
.bss : {*(.bss)}
. = . + 0x500;
sp_top = .;

Compile the test program as shown below:

# arm-none-eabi-gcc -c -mcpu=arm926ej-s init.c -o init.o
# arm-none-eabi-as -mcpu=arm926ej-s startup.s -o startup.o
# arm-none-eabi-ld -T linker.ld init.o startup.o -o test.elf
# arm-none-eabi-objcopy -O binary test.elf test.bin

Now, our test program’s binary file and the u-boot.bin must be packed in a single file. Let’s use the mkimage tool for this; locate it in the U-Boot source-code directory.

# mkimage -A arm -C none -O linux -T kernel -d test.bin -a 0x00100000 -e 0x00100000 test.uimg
Image Name:
Created:      Wed Jul 6 13:29:54 2011
Image Type:   ARM Linux Kernel Image (uncompressed)
Data Size:    148 Bytes = 0.14 kB = 0.00 MB
Load Address: 0x00100000
Entry Point:  0x00100000

Our sample binary file is ready. Let’s combine it with u-boot.bin to create the final flash image file:

#cat u-boot.bin test.uimg > flash.bin

Calculate the starting address of the test program in the flash.bin file:

# printf "0x%X" $(expr $(stat -c%s u-boot.bin) + 65536)

Boot the flash image in QEMU:

# qemu-system-arm -M versatilepb -nographic -kernel flash.bin

Now verify the image address in U-Boot:

Versatile # iminfo 0x21C68
## Checking Image at 00021c68 ...
Image Name:
Image Type:   ARM Linux Kernel Image (uncompressed)
Data Size:    136 Bytes =  0.1 kB
Load Address: 00100000
Entry Point:  00100000
Verifying Checksum ... OK

The image is present at the address 0x21C68. Boot it by executing the bootm command:

Versatile # bootm 0x21C68
## Booting image at 00021c68 ...
Image Name:
Image Type:   ARM Linux Kernel Image (uncompressed)
Data Size:    148 Bytes =  0.1 kB
Load Address: 00100000
Entry Point:  00100000

Starting kernel ...

Hello Open World

That’s all folks!


This article is inspired by the following blog post: “U-boot for ARM on QEMU“.


  1. Hi your article is very useful. I have one doubt at this step..

    # printf “0x%X” $(expr $(stat -c%s u-boot.bin) + 65536)

    $(stat -c%s u-boot.bin)-> this command gives the size of the uboot.bin

    65536 -> i would like to know more about this magic number and how the start address is calculated

    Thanks in Advance

    • I guess it come from the previous command “printenv” which was use in the first u-boot boot with the u-boot bin file. We can see in the status reported that there is :
      Environment size: 184/65532 bytes

      I guess this is an area reserved by u-boot to store some environnement data in memory. And because arm is a 32 bit wide machine, the next byte address for the first byte of the test.bin shoud be at an offset of 65532+4=65536
      But I’m not sure about this… Is there anybody who could confirm this ?


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