131 lines
4.7 KiB
ArmAsm
131 lines
4.7 KiB
ArmAsm
/* 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 <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
|
|
|
|
cli
|
|
|
|
call gdt_init
|
|
|
|
movw $0x10, %ax
|
|
movw %ax, %ds
|
|
movw %ax, %es
|
|
movw %ax, %fs
|
|
movw %ax, %gs
|
|
movw %ax, %ss
|
|
# jmp kernel
|
|
ljmp $0x08, $kernel
|
|
|
|
kernel:
|
|
/*
|
|
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
|
|
|
|
.global keyboard_test
|
|
keyboard_test:
|
|
hlt
|
|
|
|
/*
|
|
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
|