From 8a719ab74dd35e0e753c0c4320ac10180ab72970 Mon Sep 17 00:00:00 2001 From: Joursoir Date: Fri, 13 Aug 2021 23:25:50 +0000 Subject: x86/boot: move and rename asm code with multiboot header --- arch/x86/boot/multiboot.s | 114 ++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 114 insertions(+) create mode 100644 arch/x86/boot/multiboot.s (limited to 'arch/x86') diff --git a/arch/x86/boot/multiboot.s b/arch/x86/boot/multiboot.s new file mode 100644 index 0000000..33e54f1 --- /dev/null +++ b/arch/x86/boot/multiboot.s @@ -0,0 +1,114 @@ +# https://www.gnu.org/software/grub/manual/multiboot/multiboot.html + +/* +Declare constants for the multiboot header. +*/ +.set ALIGN, 1 << 0 # align loaded modules on page boundaries +.set MEMINFO, 1 << 1 # provide memory map +.set FLAGS, ALIGN | MEMINFO # this is the Multiboot 'flag' field +.set MAGIC, 0x1BADB002 # 'magic number' lets bootloader find the header +.set CHECKSUM, -(MAGIC + FLAGS) # checksum of above, to prove we are multiboot + +/* +Declare a multiboot header that marks the program as a kernel. +These are magic values that are documented in the multiboot standard. +The bootloader will search for this signature in the first 8 KiB of +the kernel file, aligned at a 32-bit boundary. The signature is in +its own section so the header can be forced to be within the first +8 KiB of the kernel file. +*/ +.section .multiboot +.align 4 +.long MAGIC +.long FLAGS +.long CHECKSUM + +/* +The multiboot standard does not define the value of the +stack pointer register (esp) and it is up to the kernel to provide a stack. +This allocates room for a small stack by creating a symbol at the bottom +of it, then allocating 16384 bytes for it, and finally creating a symbol +at the top. The stack grows downwards on x86. The stack is in its own +section so it can be marked nobits, which means the kernel file is smaller +because it does not contain an uninitialized stack. The stack on x86 +must be 16-byte aligned according to the System V ABI standard and +de-facto extensions. The compiler will assume the stack is properly +aligned and failure to align the stack will result in undefined behavior. +*/ +.section .bss +.align 16 +stack_bottom: +.skip 16384 # 16 KiB +stack_top: + +/* +The linker script specifies _start as the entry point to the kernel and the +bootloader will jump to this position once the kernel has been loaded. It +doesn't make sense to return from this function as the bootloader is gone. +*/ +.section .text +.global _start +.type _start, @function +_start: + /* + The bootloader has loaded us into 32-bit protected mode on a x86 + machine. Interrupts are disabled. Paging is disabled. The processor + state is as defined in the multiboot standard. The kernel has full + control of the CPU. The kernel can only make use of hardware features + and any code it provides as part of itself. There's no printf + function, unless the kernel provides its own header and a + printf implementation. There are no security restrictions, no + safeguards, no debugging mechanisms, only what the kernel provides + itself. It has absolute and complete power over the + machine. + */ + + /* + To set up a stack, we set the esp register to point to the top of the + stack (as it grows downwards on x86 systems). This is necessarily done + in assembly as languages such as C cannot function without a stack. + */ + mov $stack_top, %esp + + /* + This is a good place to initialize crucial processor state before the + high-level kernel is entered. It's best to minimize the early + environment where crucial features are offline. Note that the + processor is not fully initialized yet: Features such as floating + point instructions and instruction set extensions are not initialized + yet. The GDT should be loaded here. Paging should be enabled here. + C++ features such as global constructors and exceptions will require + runtime support to work as well. + */ + + /* + Enter the high-level kernel. The ABI requires the stack is 16-byte + aligned at the time of the call instruction (which afterwards pushes + the return pointer of size 4 bytes). The stack was originally 16-byte + aligned above and we've pushed a multiple of 16 bytes to the + stack since (pushed 0 bytes so far), so the alignment has thus been + preserved and the call is well defined. + */ + call kernel_main + + /* + If the system has nothing more to do, put the computer into an + infinite loop. To do that: + 1) Disable interrupts with cli (clear interrupt enable in eflags). + They are already disabled by the bootloader, so this is not needed. + Mind that you might later enable interrupts and return from + kernel_main (which is sort of nonsensical to do). + 2) Wait for the next interrupt to arrive with hlt (halt instruction). + Since they are disabled, this will lock up the computer. + 3) Jump to the hlt instruction if it ever wakes up due to a + non-maskable interrupt occurring or due to system management mode. + */ + cli +1: hlt + jmp 1b + +/* +Set the size of the _start symbol to the current location '.' minus its +start. This is useful when debugging or when you implement call tracing. +*/ +.size _start, . - _start -- cgit v1.2.3-18-g5258