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Changed bootloader to use NASM.

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neviyn 2017-10-20 18:54:43 +01:00
parent 1f2819aeeb
commit 3da4d3538d
1 changed files with 85 additions and 105 deletions
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190
boot.s
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@ -1,110 +1,90 @@
/* http://wiki.osdev.org/Bare_Bones */ ; Declare constants for the multiboot header.
/* Declare constants for the multiboot header. */ MBALIGN equ 1<<0 ; align loaded modules on page boundaries
.set ALIGN, 1<<0 /* align loaded modules on page boundaries */ MEMINFO equ 1<<1 ; provide memory map
.set MEMINFO, 1<<1 /* provide memory map */ FLAGS equ MBALIGN | MEMINFO ; this is the Multiboot 'flag' field
.set FLAGS, ALIGN | MEMINFO /* this is the Multiboot 'flag' field */ MAGIC equ 0x1BADB002 ; 'magic number' lets bootloader find the header
.set MAGIC, 0x1BADB002 /* 'magic number' lets bootloader find the header */ CHECKSUM equ -(MAGIC + FLAGS) ; checksum of above, to prove we are multiboot
.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
Declare a multiboot header that marks the program as a kernel. These are magic ; search for this signature in the first 8 KiB of the kernel file, aligned at a
values that are documented in the multiboot standard. The bootloader will ; 32-bit boundary. The signature is in its own section so the header can be
search for this signature in the first 8 KiB of the kernel file, aligned at a ; forced to be within the first 8 KiB of the kernel file.
32-bit boundary. The signature is in its own section so the header can be section .multiboot
forced to be within the first 8 KiB of the kernel file. align 4
*/ dd MAGIC
.section .multiboot dd FLAGS
.align 4 dd CHECKSUM
.long MAGIC
.long FLAGS ; The multiboot standard does not define the value of the stack pointer register
.long CHECKSUM ; (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
The multiboot standard does not define the value of the stack pointer register ; downwards on x86. The stack is in its own section so it can be marked nobits,
(esp) and it is up to the kernel to provide a stack. This allocates room for a ; which means the kernel file is smaller because it does not contain an
small stack by creating a symbol at the bottom of it, then allocating 16384 ; uninitialized stack. The stack on x86 must be 16-byte aligned according to the
bytes for it, and finally creating a symbol at the top. The stack grows ; System V ABI standard and de-facto extensions. The compiler will assume the
downwards on x86. The stack is in its own section so it can be marked nobits, ; stack is properly aligned and failure to align the stack will result in
which means the kernel file is smaller because it does not contain an ; undefined behavior.
uninitialized stack. The stack on x86 must be 16-byte aligned according to the section .bss
System V ABI standard and de-facto extensions. The compiler will assume the align 16
stack is properly aligned and failure to align the stack will result in
undefined behavior.
*/
.section .bss
.align 16
stack_bottom: stack_bottom:
.skip 16384 # 16 KiB resb 16384 ; 16 KiB
stack_top: stack_top:
/* ; The linker script specifies _start as the entry point to the kernel and the
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
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.
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
.section .text global _start:function (_start.end - _start)
.global _start
.type _start, @function
_start: _start:
/* ; The bootloader has loaded us into 32-bit protected mode on a x86
The bootloader has loaded us into 32-bit protected mode on a x86 ; machine. Interrupts are disabled. Paging is disabled. The processor
machine. Interrupts are disabled. Paging is disabled. The processor ; state is as defined in the multiboot standard. The kernel has full
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
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
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
function, unless the kernel provides its own <stdio.h> header and a ; printf implementation. There are no security restrictions, no
printf implementation. There are no security restrictions, no ; safeguards, no debugging mechanisms, only what the kernel provides
safeguards, no debugging mechanisms, only what the kernel provides ; itself. It has absolute and complete power over the
itself. It has absolute and complete power over the ; machine.
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.
To set up a stack, we set the esp register to point to the top of our mov esp, stack_top
stack (as it grows downwards on x86 systems). This is necessarily done
in assembly as languages such as C cannot function without a stack. ; This is a good place to initialize crucial processor state before the
*/ ; high-level kernel is entered. It's best to minimize the early
mov $stack_top, %esp ; 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
This is a good place to initialize crucial processor state before the ; yet. The GDT should be loaded here. Paging should be enabled here.
high-level kernel is entered. It's best to minimize the early ; C++ features such as global constructors and exceptions will require
environment where crucial features are offline. Note that the ; runtime support to work as well.
processor is not fully initialized yet: Features such as floating
point instructions and instruction set extensions are not initialized ; Enter the high-level kernel. The ABI requires the stack is 16-byte
yet. The GDT should be loaded here. Paging should be enabled here. ; aligned at the time of the call instruction (which afterwards pushes
C++ features such as global constructors and exceptions will require ; the return pointer of size 4 bytes). The stack was originally 16-byte
runtime support to work as well. ; 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
Enter the high-level kernel. The ABI requires the stack is 16-byte extern kernel_main
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.
*/
call kernel_main call kernel_main
/* ; If the system has nothing more to do, put the computer into an
If the system has nothing more to do, put the computer into an ; infinite loop. To do that:
infinite loop. To do that: ; 1) Disable interrupts with cli (clear interrupt enable in eflags).
1) Disable interrupts with cli (clear interrupt enable in eflags). ; They are already disabled by the bootloader, so this is not needed.
They are already disabled by the bootloader, so this is not needed. ; Mind that you might later enable interrupts and return from
Mind that you might later enable interrupts and return from ; kernel_main (which is sort of nonsensical to do).
kernel_main (which is sort of nonsensical to do). ; 2) Wait for the next interrupt to arrive with hlt (halt instruction).
2) Wait for the next interrupt to arrive with hlt (halt instruction). ; Since they are disabled, this will lock up the computer.
Since they are disabled, this will lock up the computer. ; 3) Jump to the hlt instruction if it ever wakes up due to a
3) Jump to the hlt instruction if it ever wakes up due to a ; non-maskable interrupt occurring or due to system management mode.
non-maskable interrupt occurring or due to system management mode.
*/
cli cli
1: hlt .hang: hlt
jmp 1b jmp .hang
.end:
/*
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