Lab 0: Bare Minimum

Just test our toolchain. The resulting kernel8.img should boot on Raspberry Pi, and stop the CPU cores in an infinite loop. You can check that by running

Assembly

When the control is passed to kernel8.img, the environment is not ready for C. Therefore we must implement a small preambule in Assembly.

CPU has 4 cores. All of them are running the same infinite loop for now.

.section ".text.boot"

.global _start

_start:
1:  wfe
    b       1b

Makefile

Our Makefile is very simple. We compile start.S, as this is our only source. Then in linker phase we link it using the linker.ld script. Finaly we convert the resulting elf executable into a raw image.

ARMGNU ?= aarch64-linux-gnu

COPS = -Wall -nostdlib -nostartfiles -ffreestanding -Iinclude -mgeneral-regs-only -std=gnu99 -ggdb
ASMOPS = -Iinclude -ggdb

BUILD_DIR = build
SRC_DIR = src

all : kernel8.img

clean :
	rm -rf $(BUILD_DIR) *.img 

$(BUILD_DIR)/%_c.o: $(SRC_DIR)/%.c
	mkdir -p $(@D)
	$(ARMGNU)-gcc $(COPS) -MMD -c $< -o $@

$(BUILD_DIR)/%_s.o: $(SRC_DIR)/%.S
	mkdir -p $(@D)
	$(ARMGNU)-gcc $(ASMOPS) -MMD -c $< -o $@

C_FILES = $(wildcard $(SRC_DIR)/*.c)
ASM_FILES = $(wildcard $(SRC_DIR)/*.S)
OBJ_FILES = $(C_FILES:$(SRC_DIR)/%.c=$(BUILD_DIR)/%_c.o)
OBJ_FILES += $(ASM_FILES:$(SRC_DIR)/%.S=$(BUILD_DIR)/%_s.o)

DEP_FILES = $(OBJ_FILES:%.o=%.d)
-include $(DEP_FILES)

kernel8.img: $(SRC_DIR)/linker.ld $(OBJ_FILES)
	$(ARMGNU)-ld -T $(SRC_DIR)/linker.ld -o $(BUILD_DIR)/kernel8.elf  $(OBJ_FILES)
	$(ARMGNU)-objcopy $(BUILD_DIR)/kernel8.elf -O binary kernel8.img
run:
	qemu-system-aarch64 -M raspi3 -kernel kernel8.img -serial null -serial pty
gdb:
	qemu-system-aarch64 -M raspi3 -kernel kernel8.img -serial null -serial pty -S -s 

Linker script

Not surpisingly simple too. We just set the base address where our kernel8.img will be loaded, and we put the only section we have there.

For AArch64 the load address is 0x80000, and not 0x8000 as with AArch32.

SECTIONS
{
    . = 0x80000;
    .text : { *(.text.boot) }

   /DISCARD/ : { *(.comment) *(.gnu*) *(.note*) *(.eh_frame*) }
}

How to run

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Next Lab : Multicore C

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Project Source Code: Bare-Metal-Mini-PiOS
Author : andykuo8766