Jump to Protected Mode not working after linking the kernel

问题

I have been developing a Bootloader and have run into a problem when linking c++ code to my assembly stage2 code before I linked the files the second stage would jump to protected mode then to long mode without any problems but now after I have linked it there seems to be a problem when jumping to protected mode Here is the code I use to jump to protected mode:

main:            
;first stage of bootloader is loaded at the address 0x07c0:0 
        ;second stage of bootloader is loaded at address 0x200:0x0 


        cli 
   xor ax, ax         ; All segments set to 0, flat memory model 
   mov ds, ax 
   mov es, ax 
   mov gs, ax 
   mov fs, ax 
   mov ss, ax 
   ; 
   ; Set stack top SS:0xffff 
   ; 
   mov sp, 0x0FFFF 
   ; 


        mov [CDDriveNumber], dl 

        SwitchToProtectedMode: 
        lgdt [GDT_32];load the gdt 

        call EnableA20 



        mov eax, cr0 
        or eax, 1 
        mov cr0, eax 


        ; Flush CS and set code selector 
        ;   

        jmp 0x8:Protected_Mode 

        [BITS 32];Declare 32 bits 

        Protected_Mode:

Here is the GDT:

GDT_START: 
;null descriptor 
dd 0 
dd 0 
;data descriptor 
dw 0xFFFF 
dw 0 
db 0 
db 10011010b 
db 11001111b 
db 0 
;code descriptor 
dw 0xFFFF 
dw 0 
db 0 
db 10010010b 
db 11001111b 
db 0 
GDT_END: 
align 4 
GDT_32: 
dw GDT_END - GDT_START - 1 
dd GDT_START

Here is the linker script I use to link my c and assembly code

KernAddr = 0x200;
ENTRY(_Start)
SECTIONS
{
    . = KernAddr;

    .text : AT(ADDR(.text) - KernAddr)
    {
        _code = .;
        *(.text)
        *(.rodata*)
        . = ALIGN(4096);
    }

   .data : AT(ADDR(.data) - KernAddr)
   {
        _data = .;
        *(.data)
        . = ALIGN(4096);
   }

   .eh_frame : AT(ADDR(.eh_frame) - KernAddr)
   {
       _ehframe = .;
       *(.eh_frame)
        . = ALIGN(4096);
   }

   .bss : AT(ADDR(.bss) - KernAddr)
   {
       _bss = .;
       *(.bss)

       /*
        * You usually need to include generated COMMON symbols
        * under kernel BSS section or use gcc's -fno-common
        */

        *(COMMON)
       . = ALIGN(4096);
   }

   _end = .;

   /DISCARD/ :
   {
        *(.comment)
   }
}

Here is the batch program I made to build everything:

nasm Stage1.asm -o Stage1.bin
nasm -f elf64 Stage2.asm -o Stage2.o
x86_64-elf-g++ -ffreestanding -mcmodel=large -mno-red-zone -mno-mmx -mno-sse -mno-sse2 -mno-sse3 -mno-3dnow -c -o kernel.o kernel.cpp
x86_64-elf-ld -T linkerscript.ld -o Anmu.bin Stage2.o kernel.o -nostdlib
copy Stage1.bin Root
copy Anmu.bin Root
mkisofs -b Stage1.bin -no-emul-boot -boot-info-table -o BootLoader.iso ./Root

The rest of the code is available here: https://github.com/AnonymousUser1337/Anmu

回答1:

You say that Stage2 is loaded at segment 0x0200, which is address 0x2000, but your linker says it starts at offset 0x0200.

Also, despite naming your output "Anmu.bin", your file is actually still an ELF executable, which means all of the headers and whatnot are still present in the file. Instead, you need to use objcopy to strip all of the headers and debugging symbols, giving you a flat binary:

objcopy -S -O binary Anmu.bin Anmu-flat.bin

Now, "Anmu-flat.bin" is nothing but code and data, and the first byte of the file is the start of the first instruction.

来源：https://stackoverflow.com/questions/25272080/jump-to-protected-mode-not-working-after-linking-the-kernel

标签

c++

assembly

x86

nasm

osdev