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It saves you the struggle to write and properly link bootstrap code or to implement your own boot loader entirely from scratch. Unlike GRUB (which does not support switching to long mode), bootloaders such as Limine or BOOTBOOT can load your 64-bit kernel directly by doing all the things listed in the previous section (and more). Open Source boot loaders written for long mode kernels already exist, you don't have to reinvent the wheel. Enter Long Mode by far jump to the kernel entry point in (virtual) memory.Set up Long Mode readying stuff (PAE, PML4, etc.) - Remember to set up the higher-half addressing!.Parse kernel ELF headers (if kernel is separate from executable).Enter Protected Mode (or skip this step and enter long mode directly).Do Protected Mode readying stuff (GDT, IDT, A20 gate, etc.).I won't give any code, but the basic outline is: This method is the simplest (since you write all the code), though it requires the most work. Note: Obviously there is no bootstrap assembly yet, which is the hard part of starting out, and you can't link without it.īefore you can actually use your kernel, you need to deal with the hard job of loading it. X86_64-elf-gcc -ffreestanding -T -o -nostdlib -lgcc Set ENTRY(.) to your entry function, and KERNEL_VMA to your base virtual address.

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* under kernel BSS section or use gcc's -fno-commonįeel free to edit this linker script to suit your needs. * You usually need to include generated COMMON symbols eh_frame : AT(ADDR(.eh_frame) - KERNEL_VMA) The kernel will be linked as an x86_64 executable, to run at a virtual higher-half address. Otherwise, several #UD and #NM exceptions will be triggered. They need special %cr0 and %cr4 setup that we're not ready for. Check this thread on the forums for extra context. You will need to instruct GCC not to use the the AMD64 ABI 128-byte 'red zone', which resides below the stack pointer, or your kernel will be interrupt unsafe. Check the SysV AMD64 ABI document for extra details. In fact, using the 'large' code model is discouraged due to its inefficiency, but it can be fine as a start. The -mcmodel=large argument enables us to run the kernel at any 64-bit virtual memory address we want. X86_64-elf-gcc -ffreestanding -mcmodel=large -mno-red-zone -mno-mmx -mno-sse -mno-sse2 -c foo.c -o foo.o

The kernel should run in a uniform environment.

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Decide now on how to load your kernel - your own bootloader, GRUB (with separate loader executable), GRUB2 (elf64 + 32-bit bootstrap code), or a 64-bit capable bootloader such as Limine or BOOTBOOT.Read up on long mode and how to initialize/use it (if you intend on not using a 64-bit aware bootloader or rolling your own).Have built a cross-compiler for the x86_64-elf target.Make sure that you have the following done before proceeding: 5.4 With a 32-bit bootstrap in your kernel.

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