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'
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