ARMv7-A Bare Bones
|
This tutorial needs to explain what the code does as tutorials are not just copy paste. You can help out by editing this page to include more context to what the code does. |
|
WAIT! Have you read Getting Started, Beginner Mistakes, and some of the related OS theory? |
| Difficulty level |
|---|
 Beginner |
| Kernel Designs |
|---|
| Models |
| Other Concepts |
In this tutorial, you will write a basic ARM kernel and boot it. You will target the ARM Versatile Express development board for the Cortex-A15.
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
You will 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 GAS, GCC, and LD, but it is also suggested to have 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