ARMv7-A Bare Bones

Difficulty level
Beginner

Kernel Designs
Models
Monolithic Kernel Microkernel Exokernel Nano/Picokernel Cache Kernel Virtualizing Kernel Megalithic Kernel
Other Concepts
Modular Kernel Higher Half Kernel 64-bit Kernel Hybrid Kernel

In this tutorial, we will write a basic ARM kernel and boot it. We target the ARM Versatile Express development board for the Cortex-A15.

You should read Getting Started and Beginner Mistakes before starting this tutorial.

Rationale

• Why ARMv7-A?

ARMv7 is likely the last purely 32-bit iteration of the ARM architecture, so processors based on it are likely to be supported the longest. The 'A' profile is targeted at complex computing devices like smartphones, and is therefore of the most interest to OS developers.

• Why the Cortex-A15?

Largely because it's the most powerful ARMv7-A processor supported by QEMU, and it also has its own development board.

• Why the Versatile Express?

This board was designed by ARM Holdings as a prototyping board, so it makes sense to target a relatively neutral platform built with the Cortex-A15 in mind.

Toolchain

We'll be using the GNU toolchain for this, so go read GCC Cross-Compiler if you haven't already. The target platform is arm-none-eabi. You will need at least as, gcc, and ld, but I suggest also having gdb, objcopy, and objdump.

Code

_start.arm:

.global _start
_start:
    ldr sp, =STACK_TOP
    bl start
1:
    b 1b
.size _start, . - _start

start.c:

#include <stdint.h>

#define UART0_BASE 0x1c090000
void start()
{
    *(volatile uint32_t *)(UART0_BASE) = 'A';
}

linker.ld:

ENTRY(_start)

SECTIONS {
    /* QEMU loads the kernel in Flash here. I strongly suggest you look at the
     * memory map provided in the CoreTile TRM (see below).
     */
    . = 0x80010000;

    /* Make sure our entry stub goes first */
    .stub   : { _start.o(.text) }
    .text   : { *(.text) }
    .rodata : { *(.rodata) }
    .data   : { *(.data) }
    .bss    : { *(.bss COMMON) }

    STACK_BASE = .;
    . += 0x10000;
    STACK_TOP = .;
}

Building and Running

$ arm-none-eabi-as -march=armv7-a -mcpu=cortex-a15 _start.arm -o _start.o
$ arm-none-eabi-gcc -ffreestanding -Wall -Wextra -Werror -c start.c -o start.o
$ arm-none-eabi-ld -T linker.ld _start.o start.o -o kernel.elf
$ qemu-system-arm -M vexpress-a15 -cpu cortex-a15 -kernel kernel.elf -nographic

Resources

It's a lot of material, but these are invaluable references for developing on ARM platforms.

• ARMv7 Cortex-A Series Programmer's Guide
• ARMv7-A Architecture Reference Manual
• ARM Cortex-A15 Technical Reference Manual
• CoreTile Express A-15 Technical Reference Manual
• Motherboard Express μATX Technical Reference Manual