I started with creating an x86_64-elf-* cross compiler. I also installed QEMU x86_64 emulator. So far so good. Then I started laying in code from Higher-Half Bare Bones. When that was less than successful, I turned to Creating a 64-bit kernel, 64-bit Higher Half Kernel with GRUB 2, and even the original Bare Bones wikis. I had trouble either getting the code to link or boot with each one. Admittedly, I was pulling from several sources, but I had done this before with a 32-bit kernel. I should be able to get at least the multiboot 1 header to work!!
After lots of readelf, x86_64-elf-objdump, and hexdump commands, I finally started to see the issues (I was too close to the problem for 2 full days). I had several issues to fix:
- My multiboot section was not the first one in the file.
- My multiboot header was not in the required 8K.
- My elf page size was larger than I wanted it to be.
The multiboot 2 documentation has a lot of holes in it, especially when you compare it to Grub's implementation. Frankly, it is my opinion that the documentation leaves enough holes that Grub has taken liberties that break the specification in its implementation. At any rate, I was finally able to get an image compiled that contained both multiboot headers and is able to be booted by Grub2 by either header.
I am considering adding my code as a wiki (without the back-story) since it took a lot of deep digging for me to pull this together. However, with the risk of creating "another bare bones how to" wiki, I thought I might ask for some feedback here first. The code is included below as the comments might help explain the issues I uncovered.
First the loader.s:
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;==============================================================================================
;
; loader.s
;
; This file contains the initial functions and structures required by the Multiboot loader to
; properly transfer control to the kernel after loading it in memory. There are structures
; that are required to be 4-byte aligned and must be in the first 8K of the file for the final
; binary to be Multiboot compliant. These structures are implemented here.
;
; Date Tracker Pgmr Description
; ---------- ------ ---- ------------------------------------------------------------------
; 2014/09/24 Initial ADCL Leveraged from osdev.org -- "Higher Half Bare Bones" wiki
; 2014/09/25 Initial ADCL Since "Higher Half Bare Bones" did not work, scrapped it all for
; a run at "64-bit Higher Half Kernel with GRUB2.
; 2014/09/26 Initial ADCL After a lot of work and a sleepless night, I have finally been
; able to get an elf64 image to boot with GRUB2 using the multiboot
; 1 header. The key to making this work was changing the page size
; for the linker to be 0x800 (2K). The multiboot header is found
; on the third page and needs to be in the first 8K of the file.
; With a page size of 2K, this offset is at 4K. If I bump the page
; size to the next power of 2 (0x1000 or 4K) when the multiboot
; header is at 8K. You would think that is right, but it turns out
; that the entire header bust be contained within 8K, putting it
; just out of reach. The parameter on x86_64-elf-gcc that makes
; this successful is "-z max-page-size=0x800".
;
; Due to issues in the linker.ld script which have been resolved,
; the multiboot header is now in the second page, not the third.
; Changing back to "-z max-page-size=0x1000".
;
;==============================================================================================
bits 32
section .multiboot
align 8
MBMODALIGN equ 1<<0
MBMEMINFO equ 1<<1
MBVIDINFO equ 1<<2
MBAOUT equ 1<<16 ; this will not be used in our kernel
MBMAGIC equ 0x1badb002
MBFLAGS equ (MBMODALIGN | MBMEMINFO | MBVIDINFO)
MBCHECK equ -(MBMAGIC + MBFLAGS)
MultibootHeader: ; Offset Description
dd MBMAGIC ; 0 multiboot magic number
dd MBFLAGS ; 4 multiboot flags
dd MBCHECK ; 8 checksum
dd 0 ; c header address (flag bit 16)
dd 0 ; 10 load address (flag bit 16)
dd 0 ; 14 load end address (flag bit 16)
dd 0 ; 18 bss end address (flag bit 16)
dd 0 ; 1c entry address (flag bit 16)
dd 1 ; 20 vidoe mode (1 == text) (flag bit 2)
dd 80 ; 24 video columns (flag bit 2)
dd 25 ; 28 video rows (flag bit 2)
dd 8 ; 2c video color depth (flag bit 2)
;----------------------------------------------------------------------------------------------
; So, I was thinking, why would I not be able to provide BOTH the multiboot header 1 and the
; multiboot header 2 in the same file? I could not think of any technical reason except that
; grub needs to know which signature to look for. The command in grub.cfg is multiboot2 for
; the newer version. The following multiboot2 header also works. The key is going to make
; sure we get the system in a state that we can get into a consistent state for the rest of
; the kernel initialization. The following is the multiboot2 header for this implementation.
;
; **NOTE** the docuentation does not tell you that each of tag structures must be 8-byte
; aligned. Therefore you will see 'align 8' lines throughout this structure.
;----------------------------------------------------------------------------------------------
bits 32
align 16
MBMAGIC2 equ 0xe85250d6
MBLEN equ MultibootHeader2End - MultibootHeader2
MBCHECK2 equ (-(MBMAGIC2 + 0 + MBLEN) & 0xffffffff)
MultibootHeader2: ; Description
dd MBMAGIC2 ; multiboot2 magic number
dd 0 ; architecture: 0=32-bit protected mode
dd MBLEN ; total length of the mb2 header
dd MBCHECK2 ; mb2 checksum
align 8 ; 8-byte aligned
Type4Start: ; Console Flags
dw 4 ; type=4
dw 1 ; not optional
dd Type4End-Type4Start ; size = 12
dd 1<<1 ; EGA text support
Type4End:
align 8 ; 8-byte aligned
Type6Start: ; Modue align
dw 6 ; Type=6
dw 1 ; Not optional
dd Type6End-Type6Start ; size = 8 bytes even tho the doc says 12
Type6End:
align 8 ; 8-byte aligned
; termination tag
dw 0 ; type=0
dw 0 ; flags=0
dd 8 ; size=8
MultibootHeader2End:
;==============================================================================================
section .boot
global EntryPoint
align 32
EntryPoint:
mov ebx,0xb8000
mov word [ebx],0x07<<8|'A' ; put an "A" on the screen so we know we booted
.loop:
cli
hlt
jmp .loop
;==============================================================================================
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/*===========================================================================================*/
/* */
/* linker.ld */
/* */
/* This file contains the command script to feed to the linker that will determine how and */
/* where to put the sections of the executable. Most notably, the .multiboot section needs */
/* to come first as it is required to be in the first 8K of the file. */
/* */
/* Date Tracker Pgmr Description */
/* ---------- ------ ---- -------------------------------------------------------------- */
/* 2014/09/24 Initial ADCL Leveraged from osdev.org -- "Higher Half Bare Bones" wiki */
/* 2014/09/25 Initial ADCL Since "Higher Half Bare Bones" did not work, scrapped it all */
/* for a run at "64-bit Higher Half Kernel with GRUB2. */
/* 2014/09/26 Initial ADCL After a lot of work and a sleepless night, I have finally been */
/* able to get an elf64 image to boot with GRUB2 using the */
/* multiboot 1 header. The key to making this work was changing */
/* the page size for the linker to be 0x800 (2K). See loader.s */
/* for more details. In addition, I have been able to force the */
/* multiboot section to be written first in the output file. */
/* In order to accomplish this, I had to remove all the other */
/* sections and VIRT_BASE manipulations that were taking place. */
/* These will have to be added back in carefully as we go on from */
/* here. */
/* */
/*===========================================================================================*/
ENTRY(EntryPoint)
SECTIONS
{
. = 0x100000;
.boot :
{
*(.multiboot)
*(.boot)
}
.text ALIGN(0x1000) :
{
*(.text)
}
/* that's all I have in this kernel. There's no .data section */
}
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TGT-DIR=bin
TGT-BLD=myos.bin
TGT-FILE=$(TGT-DIR)/$(TGT-BLD)
TGT-ISO=$(subst .bin,.iso,$(TGT-BLD))
TGT-CDROM=$(TGT-DIR)/$(TGT-ISO)
LINK-SCRIPT=linker.ld
ASM=nasm -felf64
LD=x86_64-elf-gcc -ffreestanding -O2 -nostdlib -z max-page-size=0x1000
LD-SCRIPT=-T $(LINK-SCRIPT)
LD-LIBS=-lgcc
ASM-SRC=$(wildcard src/*.s)
OBJ=$(sort $(subst .s,.o,$(subst src/,obj/,$(ASM-SRC))))
.PHONY: iso
iso: $(TGT-CDROM)
$(TGT-CDROM): $(TGT-FILE) bin/grub.cfg makefile
echo Creating $@...
mkdir -p iso/boot/grub
cp bin/grub.cfg iso/boot/grub/
cp $(TGT-FILE) iso/boot/
grub2-mkrescue -o $(TGT-CDROM) iso
rm -fR iso
.PHONY: build
build: $(TGT-FILE)
$(TGT-FILE): $(OBJ) $(LINK-SCRIPT) makefile
echo Linking $@...
mkdir -p bin
$(LD) $(LD-SCRIPT) -o $@ $(OBJ) $(LD-LIBS)
obj/%.o: src/%.s makefile
echo Assembling $<...
mkdir -p obj
$(ASM) $< -o $@
bin/grub.cfg: makefile
echo Generating $@...
mkdir -p bin
echo set timeout 5 > bin/grub.cfg
echo set default 0 >> bin/grub.cfg
echo menuentry \"myos\" { >> bin/grub.cfg
echo multiboot /boot/myos.bin >> bin/grub.cfg
echo boot >> bin/grub.cfg
echo } >> bin/grub.cfg
echo menuentry \"muos mb2\" { >> bin/grub.cfg
echo multiboot2 /boot/myos.bin>> bin/grub.cfg
echo boot >> bin/grub.cfg
echo } >> bin/grub.cfg
.PHONY: clean
clean:
echo Cleaning...
rm -fR obj
rm -fR bin
rm -fR iso
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qemu-system-x86_64 -cdrom myos.isoEDIT: of course I have nothing that is 64-bit at this point. That's coming up. With the subject a misnomer, I updated it.
