Building Busybox to get root file system to Linux kernel

Previously, we have built linux kernel, Qemu and booted them using qemu.

But, in previous attempts, we loaded a simple hello world program in the kernel.

Now, we can try to create a further more improved linux system.

For that, we need to get a Root FileSystem.

Here is a explanation on what is a Root File System.

So, we need a set of utilities according to our need to run using the linux kernel.

To create all these utilities, it will take certain effort. To make it simple, we can use a project called,

BusyBox – The Swiss Army Knife of Embedded Linux

Busybox, is a set of utilities, compiled into a single executable and shown as multiple executable’s are available, by creating soft links with different names.

TODO : Here is a example of how it can be achieved.

Creating Busybox based Root FS.

Here is an overview of what we are going to do.

  1. Download the busybox source.
  2. Configure
  3. Build
  4. Create Root File System image
  5. Load using Qemu.

Here, its detailed.

1) Download the busybox source.

Busybox source can be downloaded from busybox FTP site, here.

mkdir src;
cd src;
wget -c http://www.busybox.net/downloads/busybox-1.21.0.tar.bz2
tar -xf busybox-1.21.0.tar.bz2
cd busybox-1.21.0;

2) Configure busybox

BusyBox has menu based configuration. Any ways, to make it simpler to configure, busybox provides a set of configurations, as below.

make target Description
help Show the complete list of make options
defconfig Enable a default (generic) configuration
allnoconfig Disable all applications (empty configuration)
allyesconfig Enable all applications (complete configuration)
allbareconfig Enable all applications, but no subfeatures
config Text-based configurator
menuconfig N-curses (menu-based) configurator
all Build the BusyBox binary and documentation (./docs)
install Build and make it ready to install at INSTALL PREFIX directory (./_install)
busybox Build the BusyBox binary
clean Clean the source tree
distclean Completely clean the source tree
sizes Emit the text/data sizes of the enabled applications

Out of these, we are going to use, defconfig.

export ARCH=arm;
export CROSS_COMPILE=arm-none-linux-gnueabi-;
make defconfig;

Now, we need to customize this busybox, so that we can run it along with linux kernel as individual.

Build busybox as a static library, as we wont have libc in the linux kernel we are booting.

This can be done by following.

make menuconfig
# Enable   Busybox Settings--->Build Options---> [*] Build BusyBox as a static binary (no shared libs)

3) Build busybox

Now, we can build the busybox, using make install command

make -j4 install

Now,after build, we would have got the busybox install directory at busyboz-1.21.0/_install/

[Update] Note : If the build fails with error as below, Source

networking/lib.a(inetd.o): In function `register_rpc':
inetd.c:(.text.register_rpc+0x2c): undefined reference to `pmap_unset'
inetd.c:(.text.register_rpc+0x42): undefined reference to `pmap_set'
networking/lib.a(inetd.o): In function `prepare_socket_fd':
inetd.c:(.text.prepare_socket_fd+0x52): undefined reference to
`bindresvport'
collect2: error: ld returned 1 exit status
make: *** [busybox_unstripped] Error 1

Disable RPC support as below

Via menuconfig:
 Networking Utilities  --->
   []   Support RPC services

4) Create Root Filesystem image.

[TODO] As we have seen, initramfs supports newc file format. We are going to create a cpio archive to hold the root filesystem.

$ mkdir ../init/
$ cd _install;
$ find . | cpio -o --format=newc > ../../init/rootfs.img
$ cd ../../init;
$ file rootfs.img
   ASCII cpio archive (SVR4 with no CRC)
$ cd ../

5) Booting using initramfs we have built.

Now, we have the kernel built previously, rootfs that we have built now.

Lets boot it using qemu

$ qemu-system-arm -M versatilepb -kernel zImage -initrd init/rootfs.img -append "root=/dev/ram rdinit=/bin/sh"

The system would have booted and the shell prompt would be shown.
You can try ls on this to find something like this..

/ # ls
bin    dev    linuxrc    root    sbin    usr
/ #

Voila!! Very basic utility is working!!

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Building and Booting Linux using Qemu

Previously, we have Built and Booted U-Boot through Qemu.
Now, let us build and boot Linux using Qemu.
Get the latest kernel source from https://www.kernel.org/
I took Stable 3.9.3 as on writing.

mkdir original
mkdir src
cd original
wget -c https://www.kernel.org/pub/linux/kernel/v3.x/linux-3.9.3.tar.xz
cd ../src
tar -xf ../original/linux-3.9.3.tar.xz

Let us define the enviroinment variables that kernel build uses..

export ARCH=arm
export CROSS_COMPILE=arm-none-eabi-

Now, Let us configure this kernel build for Versatile Express.
This config is available at

./arch/arm/configs/vexpress_defconfig

For list of available configs, you can further explore in arch/ directory

make vexpress_defconfig;

Now, We need to make few changes, to make this kernel usable for our
needs in latter times.For this, We can remov module support (for
simplicity) and enabled EABI support as a binary format (allowing also old ABI).
This is necessary to run software compiled with the CodeSourcery toolchain.
Enable

kernel Features ---> Use the ARM EABI to compile the kernel and  
Kernel Features ---> Allow old ABI binaries to run with this kernel 

make menuconfig

We are all set to build the kernel. Now run

make -j4 all

Here, -j4 informs the build to use 4 build Jobs == Number of cores in your machine.
It will take some time to build.

Meanwhile, let us see what is the root file system and why do we need one, and
how to boot the kernel with a simple program.

Here is a definition of Root File System from Linux Information project

	
The root filesystem is the filesystem that is contained on the same 
partition on which the root directory is located, and it is the 
filesystem on which all the other filesystems are mounted (i.e.,
logically attached to the system) as the system is booted up 
(i.e., started up). 

The exact contents of the root filesystem will vary according to the 
computer, but they will include the files that are necessary for 
booting the system and for bringing it up to such a state that
the other filesystems can be mounted as well as tools for fixing a 
broken system and for recovering lost files from backups. The 
contents will include the root directory together with a minimal set 
of subdirectories and files including /boot, /dev, /etc, /bin, /sbin 
and sometimes /tmp (for temporary files).

Hope the kernel would have been built and ready by now.
The build should have completed with a message like,

  OBJCOPY arch/arm/boot/zImage
  Kernel: arch/arm/boot/zImage is ready
rajkumar.r@14:56:56:~/kernel/3_linux_qemu/src/linux-3.9.3$

The kernel will be available at

arch/arm/boot/zImage

Now, we can try to boot the kernel using qemu as below.

qemu-system-arm -M vexpress-a9 -kernel arch/arm/boot/zImage -append\
 "console=tty1"

-M vexpress-a9 : Emulate V Express Board
-kernel arch/arm/boot/zImage : Use this file as kernel
-append "console=tty1" console acts as the tty1
- generall Linux uses tty interface to display console messages

Here, you can read what is tty

But, now, the kernel will end up in panic telling something like,

VFS: Cannot open root device "(null)" or unknown block(0,0): error -6
Please append a correct "root=" boot option;
Kernel panic - not syncing: VFS: Unable to mount root fs on unknown 
block(0,0)

Now, What we discussed about File system comes useful.
As the kernel message is telling, we are missing a root file system. To build
a complete set of root file system is a complex task [ Relatively 😛 ] So, we
are going to generate a simple file system.

Creating a Simple Filesystem :

create file test.c in the src directory with the below content.

#include 

void main() {
	while(1) 
		printf("Hello World!\n");  
}

As the program looks, it will run continouesly as kernel expects the first
program to run forever. Compile this program, using cross compiler for Arm
running Linux
[ This is not same as bare metal toolchain. Bare metal toolchain is for ARM
which has no OS. i.e like arm-none-eabi-, which we have exported while
building kernel.]

arm-none-linux-gnueabi-gcc -static test.c -o test

This will Compile and creates an ELF, staticaly liked to all required code,
in a single binary. We need a Filesystem, but we have a binary file now.
So we need to generate Filesystem using some tool. Before that, we should know,
What is initramfs?

https://wiki.ubuntu.com/Initramfs

initramfs, as the name tells, its the Initial Ram File System. This is
introduced for Linux 2.6 kernel, before which initrd is being used.

From ubuntu Wiki

The key parts of initramfs are:

1) CPIO archive, so no filesystems at all are needed in kernel. 
   The archive is simply unpacked into a ram disk.
2) This unpacking happens before do_basic_setup is called. This means 
   that firmware files are available before in-kernel drivers load.
3) The userspace init is called instead of prepare_namespace. All 
   finding of the root device, and md setup happens in userspace.
4) An initramfs can be built into the kernel directly by adding it to 
   the  ELF archive under the section name .init.ramfs initramfs' can be
   stacked. Providing an initramfs to the kernel using the traditional
   initrd mechanisms causes it to be unpacked along side the initramfs'
   that are built into the kernel.
5) All magic naming of the root device goes away. Integrating udev into 
   the initramfs means that the exact same view of the /dev tree can be 
   used throughout the boot sequence. This should solve the majority of 
   the SATA failures that are seen where an install can succeed, but the
   initrd cannot boot.

This initramfs uses a format called newc. Now, to get the cpio archive,
initramfs from the binary, run the below command.

echo test | cpio -o --format=newc > rootfs

Now, we have the zImage kernel and rootfs – Initramfs. Let us load the kernel

qemu-system-arm -M vexpress-a9 -kernel linux-3.9.3/arch/arm/boot/zImage\
-initrd rootfs -append "root=/dev/ram rdinit=/test"

Here, 

-initrd rootfs : Qemu option which tells, rootfs is the Filesystem 
binary image.

root=/dev/ram and 
rdinit=/test are kernel options passed to the kernel we load.

rdinit=/test tells the kernel to run "test" executable we built as init.

Now, we can see the “hello world” being printed.

Voila!! Done!!

7. TFTP Setup in ubuntu

Goal : To setup TFTP Server in ubuntu and test the same.

Installing TFTP Server in Ubuntu

$ sudo apt-get install tftp tftpd-hpa

Once the setup is done, you can view/edit configuration at

/etc/default/tftpd-hpa

It should look something like,

$ cat /etc/default/tftpd-hpa
TFTP_USERNAME="tftp"
TFTP_DIRECTORY="/var/lib/tftpboot"
TFTP_ADDRESS="0.0.0.0:69"
TFTP_OPTIONS="--secure"

Starting and stoping the service: We can start and stop tftp service through the following commands.

service tftpd-hpa status
service tftpd-hpa stop
service tftpd-hpa start
service tftpd-hpa restart
service tftpd-hpa force-reload

Testing TFTP For testing, we are going to try to download a file from tftp server. For this, we need to copy some file to the location given at TFTP_DIRECTORY variable in /etc/default/tftpd-hpa I took a uImage file to the location.

$ cp uImage /var/lib/tftpboot/

Now, start the tftp client as below,

raj@raj-VirtualBox:~$ tftp localhost
tftp>

To check the status, run status command

tftp> status
Connected to localhost.
Mode: netascii Verbose: off Tracing: off
Rexmt-interval: 5 seconds, Max-timeout: 25 seconds
tftp>

Now, try to get the file you have copied to the TFTP_DIRECTORY

tftp> get uImage
Received 2169792 bytes in 0.3 seconds
tftp>

Voila!! You are done.. TFTP is configured and working.

Reference

6. Connecting Qemu to network

Previously, we have Built and booted U-Boot in Qemu as in this post.  Now, We are going to connect the Qemu to the virtual network for further usage. This is based on this post. We are going to connect Qemu to Network bridge and TUN/TAP For this, we need the following installed in out ubuntu machine

$ sudo apt-get install bridge-utils uml-utilities openvpn firestarter

Insert the TUN device module to the kernel

$ sudo modprobe tun

The node ?tun? must have been automatically created in /dev/net folder. If not try to create it manually by typing the following commands

$ sudo mkdir /dev/net
$ sudo mknod /dev/net/tun c 10 200
$ sudo chmod 660 /dev/net/tun

Our machine has the ethernet or Wireless Lan connection. In case of Ethernet connection, it should be named like eth0. We need to tap network from this to our virtual network, tap0. We can do it using following scripts.

Contents of /etc/qemu-ifup

#!/bin/sh

#
# script to bring up the tun device in QEMU in bridged mode
# first parameter is name of tap device (e.g. tap0)
#
# some constants specific to the local host ? change to suit your host
#

ETH0IP=10.0.2.15
GATEWAY=10.0.2.2
BROADCAST=10.0.2.255

#
# First take eth0 down, then bring it up with IP 0.0.0.0
#
ifconfig eth0 down
ifconfig eth0 0.0.0.0 promisc up

#
# Bring up the tap device (name specified as first argument, by QEMU)
#
openvpn --mktun --dev $1 -user 'id -un'
ifconfig $1 0.0.0.0 promisc up

#
# create the bridge between eth0 and the tap device
#
brctl addbr br0
brctl addif br0 eth0
brctl addif br0 $1

#
# only a single bridge so loops are not possible, turn off spanning
# tree protocol
#
brctl stp br0 off

#
# Bring up the bridge with ETH0IP and add the default route
#
ifconfig br0 $ETH0IP netmask 255.255.0.0 broadcast $BROADCAST
route add default gw $GATEWAY

#
# stop firewall ? comment this out if you don?t use Firestarter
#
service firestarter stop

Here, you may need to edit the following according to your network configurations.

ETH0IP=10.0.2.15 		
GATEWAY=10.0.2.2		
BROADCAST=10.0.2.255

Save the file as executables. Current ifconfig shoud return something like this.

$ ifconfig
eth0	  Link encap:Ethernet  HWaddr 08:00:27:d6:16:37  
          inet addr:10.0.2.15  Bcast:10.0.2.255  Mask:255.255.255.0
          inet6 addr: fe80::a00:27ff:fed6:1637/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:78 errors:0 dropped:0 overruns:0 frame:0
          TX packets:156 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:12215 (12.2 KB)  TX bytes:22482 (22.4 KB)

lo        Link encap:Local Loopback  
          inet addr:127.0.0.1  Mask:255.0.0.0
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING  MTU:16436  Metric:1
          RX packets:42 errors:0 dropped:0 overruns:0 frame:0
          TX packets:42 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0 
          RX bytes:3354 (3.3 KB)  TX bytes:3354 (3.3 KB)

Now, we can run the script to connect bridge the network.

raj@raj-VirtualBox:~$ sudo /etc/qemu-ifup tap0
[sudo] password for raj: 
Tue May 21 10:29:29 2013 TUN/TAP device tap0 opened
Tue May 21 10:29:29 2013 Persist state set to: ON
raj@raj-VirtualBox:~$

If you check the ifconfig now, it should look like below with br0 and tap0 added..

raj@raj-VirtualBox:~$ ifconfig 
br0       Link encap:Ethernet  HWaddr 08:00:27:d6:16:37  
          inet addr:10.0.2.15  Bcast:10.0.2.255  Mask:255.255.0.0
          inet6 addr: fe80::a00:27ff:fed6:1637/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:4 errors:0 dropped:0 overruns:0 frame:0
          TX packets:33 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0 
          RX bytes:328 (328.0 B)  TX bytes:7126 (7.1 KB)

eth0      Link encap:Ethernet  HWaddr 08:00:27:d6:16:37  
          inet6 addr: fe80::a00:27ff:fed6:1637/64 Scope:Link
          UP BROADCAST RUNNING PROMISC MULTICAST  MTU:1500  Metric:1
          RX packets:118 errors:0 dropped:0 overruns:0 frame:0
          TX packets:267 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:17802 (17.8 KB)  TX bytes:40277 (40.2 KB)

lo        Link encap:Local Loopback  
          inet addr:127.0.0.1  Mask:255.0.0.0
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING  MTU:16436  Metric:1
          RX packets:150 errors:0 dropped:0 overruns:0 frame:0
          TX packets:150 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0 
          RX bytes:10986 (10.9 KB)  TX bytes:10986 (10.9 KB)

tap0      Link encap:Ethernet  HWaddr 9e:43:58:5d:bb:3f  
          UP BROADCAST PROMISC MULTICAST  MTU:1500  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:100 
          RX bytes:0 (0.0 B)  TX bytes:0 (0.0 B)

raj@raj-VirtualBox:~$

Now, let us test the setup with qemu. For this, we need the u-boot.bin that we generated using this.

sudo qemu-system-arm -M versatilepb -nographic \
-net nic -net tap,ifname=tap0 -kernel u-boot.bin

This should give Uboot prompt as below.

U-Boot 2010.03-dirty (May 02 2013 - 10:00:51)

DRAM:   0 kB
## Unknown FLASH on Bank 1 - Size = 0x00000000 = 0 MB
Flash:  0 kB
*** Warning - bad CRC, using default environment

In:    serial
Out:   serial
Err:   serial
Net:   SMC91111-0
VersatilePB #

Now, we have to set an IP address for the qemu system. This can be done by running this command in uboot prompt

VersatilePB # setenv ipaddr 10.0.2.3

Note down your machines IP and now, We can test the connection by doing a ping to our machine.

VersatilePB # ping 10.0.2.15
SMC91111: PHY auto-negotiate timed out
SMC91111: MAC 52:54:00:12:34:56
Using SMC91111-0 device
host 10.0.2.15 is alive
VersatilePB #

Voila!! your system is connected if its telling the host is alive..

Booting Uboot in QEMU

U-Boot is universal bootloader, which is used very widely. Its a starting point to learn the low level hardware interactions for me. Qemu supports Versatile PB and we can try to emulate uboor on versatile PB using QEMU.

Creating workbench:

$ mkdir uboot
$ cd uboot;
$ mkdir original
$ mkdir src
$ cd original

Download U-Boot source code from U-Boot FTP.

$ wget -c ftp://ftp.denx.de/pub/u-boot/u-boot-latest.tar.bz2
$ cd ../src/
$ tar -xf ../original/u-boot-latest.tar.bz2
$ cd u-boot-2010.03

Compile the u-Boot code.

$ export ARCH=arm;
$ export CROSS_COMPILE=arm-none-eabi-;

$ make versatilepb_config 
$ make all

Now, if build went right, you should have u-boot.bin in the folder, and to boot this, run,

$ qemu-system-arm -M versatilepb -m 128M -nographic -kernel u-boot.bin;

Now you will be in uBoot prompt!!

U-Boot 2010.03-dirty (May 17 2013 - 15:52:24)

DRAM:   0 kB
## Unknown FLASH on Bank 1 - Size = 0x00000000 = 0 MB
Flash:  0 kB
*** Warning - bad CRC, using default environment

In:    serial
Out:   serial
Err:   serial
Net:   SMC91111-0
VersatilePB #

Try command like ‘?’, ‘help’ etc..

Bare Metal Programming for ARM VersatilePB II : Hello world

Reference : http://balau82.wordpress.com/2010/02/28/hello-world-for-bare-metal-arm-using-qemu/

As we are working on a bare silicon, we dont have any windowing system to display the prints. But in the silicon, we can verify some signal flow using some LED highs and lows.

These LED’s gets signal through certain peripherals and we decided to use UART as the peripheral to experiment as its available in UBOOT. Communicating to a peripheral is through memory mapped I/O. i.e, every peripheral is given a address and that is used to communicate as its a location.

Few more details for Memory mapped IO for this experiment..

1) QEMU supports versatilePB board. UART0 of verstailePB is mapped to terminal when using -nographic or -serial stdio option in qemu

2) The memory map of the VersatilePB board is implemented in QEMU in this board-specific C source http://git.savannah.gnu.org/cgit/qemu.git/tree/hw/versatilepb.c and we got address of UART0 as 0x101f1000

Okay.. Lets start to write the code..

Contents of test.c
volatile unsigned int * const UART0DR = (unsigned int *)0x101f1000;

void print_uart0(const char *s) {
 while(*s != '') { /* Loop until end of string */
 *UART0DR = (unsigned int)(*s); /* Transmit char */
 s++; /* Next char */
 }
}

void c_entry() {
 print_uart0("Hello world!\n");
 }

As the UART is a dynamic device, we declare it as volatile and we assign the mapped address as we found earlier.

Hope the code is self explanatory.

Contents of test.ld

ENTRY(_Reset)
SECTIONS
{
 . = 0x10000;
 .startup . : { startup.o(.text) }
 .text : { *(.text) }
 .data : { *(.data) }
 .bss : { *(.bss COMMON) }
 . = ALIGN(8);
 . = . + 0x1000; /* 4kB of stack memory */
 stack_top = .;
}

We start at adress 0x100000 as qemu loads the binary from this address.

Contents of starup.s
.global _Reset
_Reset:
 LDR sp, =stack_top
 BL c_entry
 B .

Next, we can compile this code..

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

Here, you can notice an addition to last build, that is, creation of binary image..

Its because, U-Boot loads raw binary files..

And now, we can run this using the command,

$ qemu-system-arm -M versatilepb -m 128M -nographic -kernel test.bin

-M versatilepb : specifies qemu to emulate Versatile PB.
-m 128M tells qemu to use 128MB Ram for this system
-nographic : Dont use graphic and provide terminal output
-kernel test.bin : Load this binary as the system image.

You can see Hello World Printed in the screen., Now, to close this, Hit,

'Ctl + a' and then 'x'

Bare Metal Programming for ARM VersatilePB I : Getting Started

Goal :

  1. To understand Bare Metal Programming.
  2. To create, compile, and run a bare metal program, for versatilePB board using Qemu.

Requirements :

  1. qemu,
  2. Ubuntu,
  3. Cross compilation toolchain

What is Bare metal Programming??

From : http://www.catb.org/jargon/html/B/bare-metal.html

bare metal: n.

1. [common] New computer hardware, unadorned with such snares and 
    delusions as an operating system, an HLL, or even assembler. 
    Commonly used in the phrase programming on the bare metal, which
    refers to the arduous work of bit bashing needed to create 
    these basic tools for a new machine. Real bare-metal programming
    involves things like building boot proms and BIOS chips, 
    implementing basic monitors used to test device drivers, 
    and writing the assemblers that will be used to write the 
    compiler back ends that will give the new machine a real 
    development environment.

 2. “Programming on the bare metal” is also used to describe a style
 of hand-hacking that relies on bit-level peculiarities of a particular
 hardware design, esp. tricks for speed and space optimization that 
 rely on crocks such as overlapping instructions (or, as in the famous
 case described in The Story of Mel' (in Appendix A), interleaving of
 opcodes on a magnetic drum to minimize fetch delays due to the device's 
 rotational latency). This sort of thing has become rare as the relative
 costs of programming time and machine resources have changed, but is 
 still found in heavily constrained environments such as industrial
 embedded systems. See Real Programmer.

So, BMP, is a kind of programming, which is direclty linked to low level hardware interaction with minimalistic support of external tools customized to launch the application on the silicon.

Bare Metal == Bare silicon

From : http://balau82.wordpress.com/2010/02/14/simplest-bare-metal-program-for-arm/

Bare metal programs run without an operating system beneath;

coding on bare metal is useful to

deeply understand how a hardware architecture works and

what happens in the lowest levels of an operating system.

Why C For Baremetal Programming??

From : http://mathscitech.org/articles/c-for-systems

 Bare Metal Programming

Like assembly, in C you are forced to work directly with memory, 
pointers, and the underlying arrangement of the bits and bytes 
that make up your data structures and their layout (packed 
structures, byte ordering, data types and conversions, etc.) 
No garbage collection, no forgiveness for buffer overruns, no 
regular expressions. C is for bare metal programming — which 
typically require safe coding practices, tight program design, 
and a good understanding of the toolchain and the target environment.

But when you want the underlying elements of the data structures 
to be visible and your algorithms to be working with these bare
metal details, C is a great choice for turning out programs that
interact with hardware, networks, sensors, and peripherals, and 
that are small (compiled size), fast (execution time), and have 
a small footprint (few dependencies, low loading overhead).

With the aid of bare bones compilation modes like
gcc -S %1.c -fno-exceptions -s -Os, and of working syntax 
converters like A2I, whenever you wish to see the kind the
assembly code that C is generating for you, you can.

Embrace C for systems level development, and you’ll retain 
the power of assembly but from a more efficient point of view.

So, Let me know what does a Executable file contain? [ Example a.out] http://www.freebsd.org/cgi/man.cgi?query=a.out&sektion=5

Okay. Enough of this boredom. Let me get my dirty hand on code….

Simplest Bare metal code..

Contents of test.c

int c_entry() {
return 0;
}

Contents of starup.s

.section INTERRUPT_VECTOR, "x"
.global _Reset
_Reset:
B Reset_Handler /* Reset */
B . /* Undefined */
B . /* SWI */
B . /* Prefetch Abort */
B . /* Data Abort */
B . /* reserved */
B . /* IRQ */
B . /* FIQ */
Reset_Handler:
LDR sp, =stack_top
BL c_entry
B .

Contents of test.ld

ENTRY(_Reset)
SECTIONS
{
. = 0x0;
.text : {
startup.o (INTERRUPT_VECTOR)
*(.text)
}
.data : { *(.data) }
.bss : { *(.bss COMMON) }
. = ALIGN(8);
. = . + 0x1000; /* 4kB of stack memory */
stack_top = .;
}

Explanation of these contents :

 test.c
1 : functiona named c_entry taking no arguments and returning int
2 : returns 0
3:  function ends

startup.s
1: generates a section named INTERRUPT_VECTOR containing executable 
   (“x”) code. Later we can see we use this in linker script.
2: exports the name _Reset to the linker in order to set the program 
   entry point.
3: to 11 is the interrupt vector table that contains a series of branches. 
   The notation “B .” means that the code branches on itself and  stays 
   there forever like an endless for(;;);
14:initializes the stack pointer, that is necessary when calling C 
   functions. The top of the stack (stack_top) will be defined during
   linking.
15:calls the c_entry function, and saves the return address in the link 
   register (lr).

test.ld
1 : Enter / Start execution at this symbol inside binary
2 - 14 : Defines section layout
  3 : Begining of section definition
  4 : Start the address at 0x0
  5 : Place the .text segment next
     6 : Place startup.o in text segment.
	     It puts INTERUPT_VECTOR at 0x0
	 7,8 : End of text segment
  9 : Start .data segment here.
 10 : Start .bss Segment following data segment
 11 : Memory allignment is 8 bits
 12 : Allocate 4Kb of stack memory starting from here
 13 : Point the top of stack at this location
14 : Section layout definition ends here

 .txt  : Code segment
 .data : Initialized data
 .bss  : Uninitialized data

Compile them ..

raj@19:31:46:$ ls
startup.s  test.c  test.ld
raj@19:31:47:$ arm-none-eabi-as -mcpu=arm926ej-s -g startup.s -o startup.o
raj@19:32:43:$ ls
startup.o  startup.s  test.c  test.ld
raj@19:32:44:$ arm-none-eabi-gcc -c -mcpu=arm926ej-s -g test.c -o test.o
raj@19:32:53:$ ls
startup.o  startup.s  test.c  test.ld  test.o
raj@19:32:54:$

Main Function??

Ohh.. okay.. but the simplest program I know is Hello world which has a Main function.. Where is the main here?? And what is this startup.s??

Yeah.. Yeah.. You are right.. Before pre judging something, please have a look at here to understand what is a Main function??

http://en.wikipedia.org/wiki/Entry_point

So now, You know Main function is not the starting point and before that C Run time library comes into picture..

http://en.wikipedia.org/wiki/Crt0

But we dont have any support for crt0 in our ‘Bare silicon’. So we dont have main and we replace them with startup.s

Lets Link them..

raj@19:32:54:$ arm-none-eabi-ld -T test.ld test.o startup.o -o test.elf
raj@19:32:54:$  ls
startup.o  startup.s  test.c  test.elf  test.ld  test.o
raj@19:32:54:$

Now, we got an ELF file, which is the executable

Okay. How do I run this now??

As you can see in the code, we do nothing but we just return 0. So we can not verify the output if we run this.. Sob..Sob..

So all of what I did so far is utter waste??

No!! You can sill verify your work and this is the foundation you have laid..

Let us examine your binaries and try to find out what it has..

We can use nm command to see symbols in a object files..

From man nm :
nm - list symbols from object files

Lets start with test.o which is our compiled c code

raj@20:05:18:$ ls
startup.o  startup.s  test.c  test.elf  test.ld  test.o
raj@20:05:55:$ nm test.o 
00000000 T c_entry
raj@20:05:55:

Here you can see the c_entry symbol listed against ‘00000000’ and ‘T’

‘T’ means its in Text segment.

Next, lets see our startup.o

 raj@20:15:54:$nm startup.o
00000020 t Reset_Handler
00000000 T _Reset
         U c_entry
         U stack_top
raj@20:15:54:$

Here, we can witness our Reset_Handler as ‘t’ at location 20.

This 20 before Reset_Handler is occupied by the _Reset vector,

which we put at first.

Here you can see we have two cases of ‘T’ and ‘t’.

Might be ‘T’ indicates its global.

Okay. We linked it by our linker script test.ld..

Lets see the result of that..

raj@20:15:54:$nm test.elf 
00000020 t Reset_Handler
00000000 T _Reset
00000030 T c_entry
00001050 A stack_top
raj@20:15:54:$

Good.. We have our data alligned as we have defined.

_Reset at start followed by Reset_Handler, followed by c_entry.

All these are text segment..

Then we can see our stack code..

Yeah.. fine.. But where is my data and .bss segments went??

In above, we cant see this because we have not declared any global data. So obviously we cant see them.. In case if you want to see them, you can declare them and recompile and check.

Buddha said, Dont belive untill you have chcked it.. So, I decided to check it.. I declared three variables as below.

 Contents of test.c 
int global_init_var = 10;
int global_unint_var;
int c_entry() {
    int local_uinit;
    return 0;
}

And here is the corresponding output

00000020 t Reset_Handler
00000000 T _Reset
00000030 T c_entry
0000005c B global_unint_var
00000058 D global_init_var
00001060 A stack_top

0000005c B global_unint_var – uninitialized Global variable which is put

into bss segment..

00000058 D global_init_var – initialized Global variable which is put

into data segment..

Want to explore some more??

Balducci has explained something about using gdb to debug symbols.. Please have the look at them too..