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assada 2024-05-21 11:38:26 +03:00
parent 259cb49e17
commit b2bc2f6da1
Signed by: assada
GPG Key ID: D4860A938E541F06
1 changed files with 0 additions and 58 deletions
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58
boot.s
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@ -16,74 +16,16 @@ stack_bottom:
.skip 16384 # 16 KiB .skip 16384 # 16 KiB
stack_top: 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 .section .text
.global _start .global _start
.type _start, @function .type _start, @function
_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 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 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 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 cli
1: hlt 1: hlt
jmp 1b 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 .size _start, . - _start