如何在Raspberry Pi上运行没有操作系统的C程序？

Question

我想尝试将Raspberry Pi用于一些不同的低级嵌入式应用程序.唯一的问题是,与可用的AVR和PIC微控制器板不同,Raspberry Pi通常运行一个操作系统(如Raspbian),它在所有正在运行的程序中分配CPU时间,并使其对某些实时应用程序不切实际.

我最近了解到,假设你有一个像GRUB一样的bootloader,在x86上运行一个C程序(以内核的形式)只需要很少的实际设置,只需要一个汇编程序来调用main函数和实际的C代码.

有没有办法用Raspberry Pi实现这一目标？这是学习低级ARM编程的好方法,它已经有一些复杂的外围设备(USB,以太网等).

Answer 1

虽然在Pi上可以使用裸机,但我会避免使用它,因为Linux变得如此轻量级并为您处理了大量内容.

如果您还想学习裸机,可以使用以下教程:http://www.valvers.com/open-software/raspberry-pi/step01-bare-metal-programming-in-cpt1/

尽管如此,我只是加载你最喜欢的嵌入式Linux发行版(根据你的要求,RT补丁可能是首选)并称之为好.

Answer 2

全自动最小裸金属闪光灯示例

在Ubuntu 16.04主机上测试,Raspberry Pi 2.

dwelch是最全面的例子,但这是一个简单易用的hello世界.

用法:

1. 在主机上插入SD卡

2. 制作图片:

   ./make.sh /dev/mmblck0 p1
   

   哪里:

   • /dev/mmblck0 是SD卡的设备
   • p1是设备的第一个分区(/dev/mmblck0p1)

3. PI上插入SD卡

4. 关闭电源然后再打开电源

GitHub上游:https://github.com/cirosantilli/raspberry-pi-bare-metal-blinker/tree/d20f0337189641824b3ad5e4a688aa91e13fd764

start.S中

.global _start
_start:
    mov sp, #0x8000
    bl main
hang:
    b hang

main.c中

#include <stdint.h>

/* This is bad. Anything remotely serious should use timers
 * provided by the board. But this makes the code simpler. */
#define BUSY_WAIT __asm__ __volatile__("")
#define BUSY_WAIT_N 0x100000

int main( void ) {
    uint32_t i;
    /* At the low level, everything is done by writing to magic memory addresses.
    The device tree files (dtb / dts), which are provided by hardware vendors,
    tell the Linux kernel about those magic values. */
    volatile uint32_t * const GPFSEL4 = (uint32_t *)0x3F200010;
    volatile uint32_t * const GPFSEL3 = (uint32_t *)0x3F20000C;
    volatile uint32_t * const GPSET1  = (uint32_t *)0x3F200020;
    volatile uint32_t * const GPCLR1  = (uint32_t *)0x3F20002C;

    *GPFSEL4 = (*GPFSEL4 & ~(7 << 21)) | (1 << 21);
    *GPFSEL3 = (*GPFSEL3 & ~(7 << 15)) | (1 << 15);
    while (1) {
        *GPSET1 = 1 << (47 - 32);
        *GPCLR1 = 1 << (35 - 32);
        for (i = 0; i < BUSY_WAIT_N; ++i) { BUSY_WAIT; }
        *GPCLR1 = 1 << (47 - 32);
        *GPSET1 = 1 << (35 - 32);
        for (i = 0; i < BUSY_WAIT_N; ++i) { BUSY_WAIT; }
    }
}

ldscript

MEMORY
{
    ram : ORIGIN = 0x8000, LENGTH = 0x10000
}

SECTIONS
{
    .text : { *(.text*) } > ram
    .bss : { *(.bss*) } > ram
}

make.sh

#!/usr/bin/env bash

set -e

dev="${1:-/dev/mmcblk0}"
part="${2:-p1}"
part_dev="${dev}${part}"
mnt='/mnt/rpi'

sudo apt-get install binutils-arm-none-eabi gcc-arm-none-eabi

# Generate kernel7.img
arm-none-eabi-as start.S -o start.o
arm-none-eabi-gcc -Wall -Werror -O2 -nostdlib -nostartfiles -ffreestanding -c main.c -o main.o
arm-none-eabi-ld start.o main.o -T ldscript -o main.elf
# Get the raw assembly out of the generated elf file.
arm-none-eabi-objcopy main.elf -O binary kernel7.img

# Get the firmware. Those are just magic blobs, likely compiled
# from some Broadcom proprietary C code which we cannot access.
wget -O bootcode.bin https://github.com/raspberrypi/firmware/blob/597c662a613df1144a6bc43e5f4505d83bd748ca/boot/bootcode.bin?raw=true
wget -O start.elf https://github.com/raspberrypi/firmware/blob/597c662a613df1144a6bc43e5f4505d83bd748ca/boot/start.elf?raw=true

# Prepare the filesystem.
sudo umount "$part_dev"
echo 'start=2048, type=c' | sudo sfdisk "$dev"
sudo mkfs.vfat "$part_dev"
sudo mkdir -p "$mnt"
sudo mount "${part_dev}" "$mnt"
sudo cp kernel7.img bootcode.bin start.elf "$mnt"

# Cleanup.
sync
sudo umount "$mnt"

QEMU友好的裸机示例

闪光灯的问题是在QEMU中很难观察到LED:https://raspberrypi.stackexchange.com/questions/56373/is-it-possible-to-get-the-state-of-the-leds-和个GPIO-IN-A-QEMU仿真样-T

在这里,我描述了一些可能感兴趣的裸机QEMU设置:如何制作裸机ARM程序并在QEMU上运行它们？写入UART是从QEMU输出输出的最简单方法.

奖金

这是一个奇怪的x86示例:如何在没有操作系统的情况下运行程序？