Working with Make and Linker Script

In this section, we will learn how to develop an operating system on the Raspberry Pi 5 by creating multiple source files and linking them with a linker. We will also use a Makefile to automate the build process. Additionally, we will cover C and C++.

Creating a Makefile

A Makefile helps us automate the build process and keep track of our files. Here is an example Makefile:

CSRCS := $(wildcard *.c)
CPPSRCS := $(wildcard *.cpp)
ASRCS := $(wildcard *.S)
COBJS := $(CSRCS:.c=.o)
CPPOBJS := $(CPPSRCS:.cpp=.o)
AOBJS := $(ASRCS:.S=.o)
AllOBJS := $(COBJS) $(CPPOBJS) $(AOBJS)
LOADADDR = 0x80000

GCCFLAGS = -mcpu=cortex-a76 -mlittle-endian -Wall -O0 -ffreestanding \
           -nostartfiles -nostdlib -nostdinc -g -I ./include

AFLAGS = -mcpu=cortex-a76 -mlittle-endian  -I ./include -O0 -g

CFLAGS = -mcpu=cortex-a76 -mlittle-endian -Wall -fsigned-char -ffreestanding -g \
         -I ./include -O0 -fno-exceptions 

CPPFLAGS = -fno-exceptions -fno-rtti -nostdinc++ -mcpu=cortex-a76 -mlittle-endian -Wall -fsigned-char \
			  -ffreestanding -g -I ./include -O0 -mstrict-align -std=c++14 -Wno-aligned-new

all: clean new kernel_2712.img

%.o: %.S
	@echo "as $@"
	@aarch64-none-elf-gcc $(AFLAGS) -c $< -o $@

%.o: %.c
	@echo "gcc $@"
	@aarch64-none-elf-gcc $(CFLAGS) -c $< -o $@

%.o: %.cpp
	@echo "g++ $@"
	@aarch64-none-elf-g++ $(CPPFLAGS) -c $< -o $@

kernel_2712.img: $(AllOBJS)
	@echo "============================================================================="
	@echo "Linking..."
	@aarch64-none-elf-ld -o kernel_2712.elf -Map kernel_2712.map -nostdlib \
		--section-start=.init=$(LOADADDR) --no-warn-rwx-segments \
		-g -T linker.ld $(AllOBJS)
	aarch64-none-elf-objcopy -O binary kernel_2712.elf kernel_2712.img

clean:
	/bin/rm -f kernel_2712.elf *.o *.img > /dev/null 2> /dev/null || true

new:
	/bin/clear

Explanation of the Makefile

Defining Variables: The first lines of the Makefile define variables containing the source files (C, C++, and Assembly) and their object files.

Compiler Flags: The flags "GCCFLAGS", "AFLAGS", "CFLAGS", and "CPPFLAGS" contain the compiler options for the various file types.

Build Targets:

all: This target deletes old files (clean), clears the screen (new), and builds the kernel (kernel_2712.img).

%.o: %.S, %.o: %.c, %.o: %.cpp: These rules define how to compile Assembly, C, and C++ source code into object files.

kernel_2712.img: This target links all object files and creates the final binary file kernel_2712.img.

clean: Deletes all generated files.

new: Clears the screen.

The Linker Script

A linker script determines how the linker arranges the various sections of the program in memory. Here is an example of a linker script:

ENTRY(_start)

SECTIONS
{
	.init : {
		*(.init)
	}
	.text : {
		*(.text*)
		_etext = .;
	}
	.rodata : {
		*(.rodata*)
	}
	.init_array : {
		__init_start = .;
		KEEP(*(.init_array*))
		__init_end = .;
	}
	.ARM.exidx : {
		__exidx_start = .;
		*(.ARM.exidx*)
		__exidx_end = .;
	}
	.eh_frame : {
		*(.eh_frame*)
	}
	.data : {
		*(.data*)
	}
	.bss : {
		__bss_start = .;
		*(.bss*)
		*(COMMON)
		__bss_end = .;
	}
}
__bss_size = (__bss_end - __bss_start) >> 3;

Name this file "linker.ld".

Explanation of the Linker Script

ENTRY(_start): Defines the entry point of the program.

SECTIONS: Defines the various sections of the program and their memory locations.

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