Archive for linux arm

A simple “bare-metal” program (ARM)

Posted in Arm stuffs with tags , , , , , , , , , , , , , , on June 16, 2012 by UnixmanLinuxboy

Caution: Here be dragons!
For those who feel like writing C code with nice and fancy algorithm, this is not for you. This is for them who wants to dig deeper at the core to see how things run and work.
A “bare-metal” program is simplest of all which does not need special services from underlying OS. It is capable enough to run on its own.

Without much ado, lets get it on, shall we?

This tutorial can be used on both real hardware and an emulator. I have used QEMU for this purpose.

Architecture to emulate: ARM926EJ-S
This is well supported by QEMU and has four UART
serial ports. The first serial port (UART0) works
as a terminal when we will use '-nographic' or
'-serial stdio' option in QMEU

Goal: This program will run inside QEMU emulator and will pass few characters to the UART0 which will, then be redirected by QEMU to the standard console.
Note: On real board (ARM926EJ-S) you have to connect to serial port (Minicom).

QEMU, ARM toolchain, little patience and willingness to learn.

Install QEMU on ubuntu:
sudo apt-get install qemu qemu-kvm-extras

Get ARM toolchain from:
ARM toolchain download link

Note: Do not download the installer (it sucks!)
Instead, get the .tar.gz archieve and unpack it your working directory
and set your PATH accordingly.
If you’re using login shell, modify the .bash_profile,
else modify .bashrc.

No interrupt handling and other fancy stuffs.

The code is well-explained and do contact me if you need more information.

The command to run the final binary in QEMU is:
qemu-system-arm -M versatilepb -m 128M -nographic -kernel output.bin

"-M versatilepb" specifies the machine type: Here we are using ARM9 versatile board.
"-nographic" mode tells to redirect UART0 messages to console.

Note: Without the option "-m 128M" QEMU crashes badly. A bug has been already filed regarding this.

Here we go!!!

File: simple-startup.s

/* ===========================================
Start up routines for bare-metal program

.global _Reset
B Reset_Handler /* Reset handler */
B Undefined /* Undefined mode handler */
B SWI /* Software interrupt */
B Prfetch_Abort /* Prefetch abort handler */
B Data_Abort /* Data abort handler */
B IRQ_Handler /* IRQ handler */
B FIQ_Handler /* FIQ handler */

LDR sp, =stack_top /*set up env for C function */
BL simple_init
B .
Undefined: /* do nothing */
B .
SWI: /* do nothing */
B .
Prfetch_Abort: /* do nothing */
B .
Data_Abort: /* do nothing */
B .
IRQ_Handler: /* do nothing */
B .
FIQ_Handler: /* do nothing */
B .

File: simple-init.c

/* Simple Bare metal program init */

/* Note: QEMU model of PL011 serial port ignores the transmit
FIFO capabilities. When writing on a real SOC, the
"Transmit FIFO Full" flag must be checked in UARTFR register
before writing on the UART register*/

volatile unsigned int* const UART0 = (unsigned int*)0x0101F1000;

static void uart_print(const char *s)
while(*s != '') {
*UART0 = (unsigned int)(*s); /* send to UART */

/* Main entry point */
void simple_init()
uart_print("Welcome to Simple bare-metal program\n");
uart_print("If you're running in QEMU, press Ctrl+a\n");
uart_print("and then x to stop me...\n");

File: simple-linker.ld

/* Simple Bare metal program - linker script
* Note: This is written for QEMU
* QEMU loads the kernel address at 0x00010000
* The emulator starts at 0x00000000 and peripheral interrupts
* are disabled at startup.
* The code must be compiled and linked to be placed at
* 0x00010000
* IRQ handling can be ignored.

. = 0x10000;
.startup . : { simple-startup.o(.text) }
.text : { *(.text) }
.data : { *(.data) }
.bss : { *(.bss) }
. = . + 0x1000; /* move by 4kbyte (our stack memory) */
stack_top = .;

Build commands:

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

And here is the output:

sarbojit@Phrozen:~/timepass/assembly-fun$ qemu-system-arm -M versatilepb -m 128M -nographic -kernel output.bin
Welcome to Simple bare-metal program
If you’re running in QEMU, press Ctrl+a
and then x to stop me…
QEMU: Terminated

Inspired by: