Kernel: Use a multiboot header instead of a convoluted two-part bootl…

…oader.

The old bootloader was hilariously complicated, requiring a floppy disk with
the kernel on it, and a hard drive with the file system. This patch removes
the floppy disk from the equation and replaces it with a multiboot header.
This means the kernel can now be booted with qemu-system-i386 -kernel kernel

Kernel/.gitignore

@@ -1,7 +1,6 @@
 *.o
 *.d
-.floppy-image
-Boot/boot.bin
+Boot/boot.ao
 kernel
 kernel.map
 _fs_contents

Kernel/Boot/boot.S

@@ -0,0 +1,64 @@
+.set MULTIBOOT_MAGIC, 0x1badb002
+.set MULTIBOOT_PAGE_ALIGN, 0x1
+.set MULTIBOOT_MEMORY_INFO, 0x2
+.set MULTIBOOT_VIDEO_MODE, 0x4
+.set multiboot_flags, MULTIBOOT_PAGE_ALIGN | MULTIBOOT_MEMORY_INFO | MULTIBOOT_VIDEO_MODE
+.set multiboot_checksum, -(MULTIBOOT_MAGIC + multiboot_flags)
+
+.section .multiboot
+.align 4
+
+.long MULTIBOOT_MAGIC
+.long multiboot_flags
+.long multiboot_checksum
+
+
+/* for MULTIBOOT_MEMORY_INFO */
+.long 0x00000000 /* header_addr */
+.long 0x00000000 /* load_addr */
+.long 0x00000000 /* load_end_addr */
+.long 0x00000000 /* bss_end_addr */
+.long 0x00000000 /* entry_addr */
+
+/* for MULTIBOOT_VIDEO_MODE */
+.long 0x00000000 /* mode_type */
+.long 0 /* width */
+.long 0 /* height */
+.long 32 /* depth */
+
+.section .stack, "aw", @nobits
+stack_bottom:
+.skip 32768
+stack_top:
+
+.section .text
+
+.global start
+.type start, @function
+
+.extern init 
+.type init, @function
+
+start:
+ mov $stack_top, %esp
+
+ and $-16, %esp
+
+ pushl %esp
+ pushl %eax /* Multiboot header magic */
+ pushl %ebx /* Multiboot header pointer */
+
+ cli
+ call init
+
+ pushl $exit_message
+ call kprintf
+
+ cli
+
+loop:
+ hlt
+ jmp loop
+
+exit_message:
+ .asciz "Kernel exited."

Kernel/Makefile

@@ -1,5 +1,4 @@
 KERNEL_OBJS = \
- _start.o \
  init.o \
  kmalloc.o \
  StdLib.o \
@@ -65,12 +64,10 @@ AK_OBJS = \
  ../AK/FileSystemPath.o \
  ../AK/StdLibExtras.o
 
-OBJS = $(KERNEL_OBJS) $(VFS_OBJS) $(AK_OBJS)
+CXX_OBJS = $(KERNEL_OBJS) $(VFS_OBJS) $(AK_OBJS)
+OBJS = $(CXX_OBJS) Boot/boot.ao
 
-NASM = nasm
 KERNEL = kernel
-BOOTLOADER = Boot/boot.bin
-IMAGE = .floppy-image
 ARCH_FLAGS =
 STANDARD_FLAGS = -std=c++17 -nostdinc++ -nostdlib -nostdinc
 KERNEL_FLAGS =
@@ -87,30 +84,25 @@ CXXFLAGS = -MMD -MP $(WARNING_FLAGS) $(OPTIMIZATION_FLAGS) $(KERNEL_FLAGS) $(FLA
 #CXX = clang $(CLANG_FLAGS)
 CXX = i686-elf-g++
 LD = i686-elf-ld
+AS = i686-elf-as
 LDFLAGS = -T linker.ld
 
-all: $(KERNEL) $(IMAGE) kernel.map
+all: $(KERNEL) kernel.map
 
 kernel.map: kernel
  @echo "MKMAP $@"; sh mkmap.sh
 
 $(KERNEL): $(OBJS)
  @echo "LD $@"; $(LD) $(LDFLAGS) -o $@ -Ttext 0x10000 $(OBJS)
 
-$(IMAGE): $(KERNEL) $(BOOTLOADER)
- @echo "CREATE $@"; cat $(BOOTLOADER) $(KERNEL) > .tmp-floppy-image
- @dd if=/dev/zero bs=2M count=1 >> .tmp-floppy-image 2> /dev/null
- @dd if=.tmp-floppy-image of=.floppy-image bs=1440k count=1 2>/dev/null
- @rm -f .tmp-floppy-image
-
-$(BOOTLOADER): Boot/boot.asm
- @echo "NASM $<"; $(NASM) -f bin -o $@ $<
-
 .cpp.o:
  @echo "CXX $<"; $(CXX) $(CXXFLAGS) -o $@ -c $<
 
--include $(OBJS:%.o=%.d)
+%.ao: %.S
+ @echo "AS $@"; $(AS) -o $@ $<
+
+-include $(CXX_OBJS:%.o=%.d)
 
 clean:
- @echo "CLEAN"; rm -f $(KERNEL) $(OBJS) $(BOOTLOADER) $(IMAGE) *.d
+ @echo "CLEAN"; rm -f $(KERNEL) $(OBJS) *.d
 

Kernel/init.cpp

@@ -136,7 +136,7 @@ VFS* vfs;
  ASSERT_NOT_REACHED();
 }
 
-[[noreturn]] void init()
+extern "C" [[noreturn]] void init()
 {
  cli();