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Diffstat (limited to 'boot.asm')
| -rw-r--r-- | boot.asm | 90 |
1 files changed, 0 insertions, 90 deletions
diff --git a/boot.asm b/boot.asm deleted file mode 100644 index 94b4c57..0000000 --- a/boot.asm +++ /dev/null @@ -1,90 +0,0 @@ -; Declare constants for the multiboot header. -MBALIGN equ 1 << 0 ; align loaded modules on page boundaries -MEMINFO equ 1 << 1 ; provide memory map -FLAGS equ MBALIGN | MEMINFO ; this is the Multiboot 'flag' field -MAGIC equ 0x1BADB002 ; 'magic number' lets bootloader find the header -CHECKSUM equ -(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 - dd MAGIC - dd FLAGS - dd 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: -resb 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. -; Declare _start as a function symbol with the given symbol size. -section .text -global _start:function (_start.end - _start) -_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 <stdio.h> 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 our - ; 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 esp, stack_top - - ; 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 isnot 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 since pushed a multiple of 16 bytes to the - ; stack since (pushed 0 bytes so far) and the alignment is thus - ; preserved and the call is well defined. - ; note, that if you are building on Windows, C functions may have "_" prefix in assembly: _kernel_main - extern kernel_main - 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 -.hang: hlt - jmp .hang -.end: |