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This article will demonstrate how to write a small Limine-compliant x86-64 kernel in (GNU) [[C]], and boot it using the [[Limine]] bootloader.
 
ItAdditionally, isit alsois veryhighly recommended to check out [https://github.com/limine-bootloader/limine-c-template this template project] as it provides example buildable code to go along with this guide.
 
===Overview===
 
For this example, we will create these 2 files to create the basic directory tree of our project:
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This is the kernel "main".
 
<sourcesyntaxhighlight lang="c">
#include <stdint.h>
#include <stddef.h>
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}
 
</syntaxhighlight>
</source>
 
===linker.ld===
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This is going to be our linker script describing where our sections will end up in memory.
 
<sourcesyntaxhighlight lang="c">
/* Tell the linker that we want an x86_64 ELF64 output file */
OUTPUT_FORMAT(elf64-x86-64)
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PHDRS
{
requeststext PT_LOAD FLAGS((1PT_LOAD << 1) | FLAGS(1 << 20x05)) ; /* WriteExecute + Read */
text rodata PT_LOAD FLAGS((1 << 00x04) | (1 << 2)) ; /* ExecuteRead + Readonly */
rodatadata PT_LOAD FLAGS((1 << 20x06)) ; /* Write /*+ Read only */
data PT_LOAD dynamic PT_DYNAMIC FLAGS((1 << 10x06) | (1 << 2)) ; /* WriteDynamic PHDR +for Readrelocations */
dynamic PT_DYNAMIC FLAGS((1 << 1) | (1 << 2)) ; /* Dynamic PHDR for relocations */
}
 
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/* that is the beginning of the region. */
. = 0xffffffff80000000;
 
/* Define a section to contain the Limine requests and assign it to its own PHDR */▼
.requests : {
KEEP(*(.requests_start_marker))▼
KEEP(*(.requests))▼
KEEP(*(.requests_end_marker))▼
} :requests
 
/* Move to the next memory page for .text */
. += CONSTANT(MAXPAGESIZE);
 
.text : {
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/* Move to the next memory page for .rodata */
. += ALIGN(CONSTANT(MAXPAGESIZE));
 
.rodata : {
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/* Move to the next memory page for .data */
. += ALIGN(CONSTANT(MAXPAGESIZE));
 
.data : {
*(.data .data.*)
 
▲ /* DefinePlace athe sectionsections tothat contain the Limine requests andas assignpart itof to its ownthe PHDR.data */
/* output section. */
▲ KEEP(*(.requests_start_marker))
▲ KEEP(*(.requests))
▲ KEEP(*(.requests_end_marker))
} :data
 
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} :data
 
/* Discard .note.* and .eh_frame* since they may cause issues on some hosts. */
/* Also discard the program interpreter section since we do not need one. This is */
/* more or less equivalent to the --no-dynamic-linker linker flag, except that it */
/* works with ld.gold. */
/DISCARD/ : {
*(.eh_frame*)
*(.note .note.*)
*(.interp)
}
}
 
</syntaxhighlight>
</source>
 
==Building the kernel and creating an image==
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GNU make will process it.
 
<sourcesyntaxhighlight lang="make">
# Nuke built-in rules and variables.
override MAKEFLAGS += -rR
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# We are using the standard "cc" here, it may work by using
# the host system's toolchain, but this is not guaranteed.
override DEFAULT_CCDEFAULT_KCC := cc
$(eval $(call DEFAULT_VAR,CCKCC,$(DEFAULT_CCDEFAULT_KCC)))
 
# Same thing for "ld" (the linker).
override DEFAULT_LDDEFAULT_KLD := ld
$(eval $(call DEFAULT_VAR,LDKLD,$(DEFAULT_LDDEFAULT_KLD)))
 
# User controllable C flags.
override DEFAULT_CFLAGSDEFAULT_KCFLAGS := -g -O2 -pipe
$(eval $(call DEFAULT_VAR,CFLAGSKCFLAGS,$(DEFAULT_CFLAGSDEFAULT_KCFLAGS)))
 
# User controllable C preprocessor flags. We set none by default.
override DEFAULT_CPPFLAGSDEFAULT_KCPPFLAGS :=
$(eval $(call DEFAULT_VAR,CPPFLAGSKCPPFLAGS,$(DEFAULT_CPPFLAGSDEFAULT_KCPPFLAGS)))
 
# User controllable nasm flags.
override DEFAULT_NASMFLAGSDEFAULT_KNASMFLAGS := -F dwarf -g
$(eval $(call DEFAULT_VAR,NASMFLAGSKNASMFLAGS,$(DEFAULT_NASMFLAGSDEFAULT_KNASMFLAGS)))
 
# User controllable linker flags. We set none by default.
override DEFAULT_LDFLAGSDEFAULT_KLDFLAGS :=
$(eval $(call DEFAULT_VAR,LDFLAGSKLDFLAGS,$(DEFAULT_LDFLAGSDEFAULT_KLDFLAGS)))
 
# Internal C flags that should not be changed by the user.
override CFLAGSKCFLAGS += \
-Wall \
-Wextra \
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# Internal C preprocessor flags that should not be changed by the user.
override CPPFLAGSKCPPFLAGS := \
-I src \
$(CPPFLAGSKCPPFLAGS) \
-MMD \
-MP
 
# Internal linker flags that should not be changed by the user.
override LDFLAGSKLDFLAGS += \
-m elf_x86_64 \
-nostdlib \
-static \
-pie \
--no-dynamic-linker \
-z text \
-z max-page-size=0x1000 \
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# Internal nasm flags that should not be changed by the user.
override NASMFLAGSKNASMFLAGS += \
-Wall \
-f elf64
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# Link rules for the final kernel executable.
# The magic printf/dd command is used to force the final ELF file type to ET_DYN.
# GNU binutils, for silly reasons, forces the ELF type to ET_EXEC even for
# relocatable PIEs, if the base load address is non-0.
# See https://sourceware.org/bugzilla/show_bug.cgi?id=31795 for more information.
bin/$(KERNEL): GNUmakefile linker.ld $(OBJ)
mkdir -p "$$(dirname $@)"
$(LDKLD) $(OBJ) $(LDFLAGSKLDFLAGS) -o $@
printf '\003' | dd of=$@ bs=1 count=1 seek=16 conv=notrunc
 
# Include header dependencies.
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obj/%.c.o: src/%.c GNUmakefile
mkdir -p "$$(dirname $@)"
$(CCKCC) $(CFLAGSKCFLAGS) $(CPPFLAGSKCPPFLAGS) -c $< -o $@
 
# Compilation rules for *.S files.
obj/%.S.o: src/%.S GNUmakefile
mkdir -p "$$(dirname $@)"
$(CCKCC) $(CFLAGSKCFLAGS) $(CPPFLAGSKCPPFLAGS) -c $< -o $@
 
# Compilation rules for *.asm (nasm) files.
obj/%.asm.o: src/%.asm GNUmakefile
mkdir -p "$$(dirname $@)"
nasm $(NASMFLAGSKNASMFLAGS) $< -o $@
 
# Remove object files and the final executable.
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clean:
rm -rf bin obj
</syntaxhighlight>
</source>
 
===limine.cfg===
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This file is parsed by Limine and it describes boot entries and other bootloader configuration variables. Further information [https://github.com/limine-bootloader/limine/blob/trunk/CONFIG.md here].
 
<sourcesyntaxhighlight lang="ini">
# Timeout in seconds that Limine will use before automatically booting.
TIMEOUT=5
 
# The entry name that will be displayed in the boot menu.
:myOS (KASLR on)
# We use the Limine boot protocol.
PROTOCOL=limine
 
# Disable KASLR (it is enabled by default for relocatable kernels)▼
KASLR=no▼
 
# Path to the kernel to boot. boot:/// represents the partition on which limine.cfg is located.
KERNEL_PATH=boot:///boot/myos
 
# Same thing, but withoutwith KASLR.
:myOS (KASLRwith offKASLR)
PROTOCOL=limine
 
▲ # Disable KASLR (it is enabled by default for relocatable kernels)
▲ KASLR=no
 
KERNEL_PATH=boot:///boot/myos
</syntaxhighlight>
</source>
 
===Compiling the kernel===
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These are shell commands. They can also be compiled into a script or Makefile.
 
<sourcesyntaxhighlight lang="bash">
# Download the latest Limine binary release for the 7.x branch.
git clone https://github.com/limine-bootloader/limine.git --branch=v7.x-binary --depth=1
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# Install Limine stage 1 and 2 for legacy BIOS boot.
./limine/limine bios-install image.iso
</syntaxhighlight>
</source>
 
====Creating a hard disk/USB drive image====
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These are shell commands. They can also be compiled into a script or Makefile.
 
<sourcesyntaxhighlight lang="bash">
# Create an empty zeroed-out 64MiB image file.
dd if=/dev/zero bs=1M count=0 seek=64 of=image.hdd
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mcopy -i image.hdd@@1M limine/BOOTX64.EFI ::/EFI/BOOT
mcopy -i image.hdd@@1M limine/BOOTIA32.EFI ::/EFI/BOOT
</syntaxhighlight>
</source>
 
==Conclusions==
Retrieved from "https://osdev.wiki/wiki/Special:MobileDiff/6192...28119"