Multiboot1 Bare Bones with NASM: Difference between revisions
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=== Bootstrap Assembly (NASM) === |
=== Bootstrap Assembly (NASM) === |
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We will now create a file called boot.asm and discuss its contents. In this example, we are using the [[NASM|Netwide Assembler]] which is not part of your previously built cross-compiler toolchain and you will have to install it separately. |
We will now create a file called boot.asm and discuss its contents. In this example, we are using the [[NASM|Netwide Assembler]] which is not part of your previously built cross-compiler toolchain and you will have to install it separately. |
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The very most important piece to create is the multiboot header, as it must be very early in the kernel binary, or the bootloader will fail to recognize us. |
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<source lang="asm"> |
<source lang="asm"> |
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; Declare constants |
; Declare constants for the multiboot header. |
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MBALIGN |
MBALIGN equ 1<<0 ; align loaded modules on page boundaries |
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MEMINFO |
MEMINFO equ 1<<1 ; provide memory map |
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FLAGS |
FLAGS equ MBALIGN | MEMINFO ; this is the Multiboot 'flag' field |
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MAGIC |
MAGIC equ 0x1BADB002 ; 'magic number' lets bootloader find the header |
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CHECKSUM |
CHECKSUM equ -(MAGIC + FLAGS) ; checksum of above, to prove we are multiboot |
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; Declare a header |
; Declare a multiboot header that marks the program as a kernel. These are magic |
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⚫ | |||
; section so we can force the header to be in the start of the final program. |
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; search for this signature in the first 8 KiB of the kernel file, aligned at a |
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; You don't need to understand all these details as it is just magic values that |
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; 32-bit boundary. The signature is in its own section so the header can be |
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⚫ | |||
; |
; forced to be within the first 8 KiB of the kernel file. |
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section .multiboot |
section .multiboot |
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align 4 |
align 4 |
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Line 26: | Line 24: | ||
dd CHECKSUM |
dd CHECKSUM |
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; The multiboot standard does not define the value of the stack pointer register |
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; Currently the stack pointer register (esp) points at anything and using it may |
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; (esp) and it is up to the kernel to provide a stack. This allocates room for a |
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; cause massive harm. Instead, we'll provide our own stack. We will allocate |
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; |
; small stack by creating a symbol at the bottom of it, then allocating 16384 |
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; |
; bytes for it, and finally creating a symbol at the top. The stack grows |
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; downwards on x86. The stack is in its own section so it can be marked nobits, |
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section .bootstrap_stack, nobits |
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; which means the kernel file is smaller because it does not contain an |
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; uninitialized stack. The stack on x86 must be 16-byte aligned according to the |
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; System V ABI standard and de-facto extensions. The compiler will assume the |
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; stack is properly aligned and failure to align the stack will result in |
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; undefined behavior. |
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section .bss |
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align 4 |
align 4 |
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stack_bottom: |
stack_bottom: |
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resb 16384 |
resb 16384 ; 16 KiB |
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stack_top: |
stack_top: |
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Line 42: | Line 46: | ||
global _start |
global _start |
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_start: |
_start: |
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; The bootloader has loaded us into 32-bit protected mode on a x86 |
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; Welcome to kernel mode! We now have sufficient code for the bootloader to |
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; machine. Interrupts are disabled. Paging is disabled. The processor |
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; load and run our operating system. It doesn't do anything interesting yet. |
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; state is as defined in the multiboot standard. The kernel has full |
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; Perhaps we would like to call printf("Hello, World\n"). You should now |
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; |
; control of the CPU. The kernel can only make use of hardware features |
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; |
; and any code it provides as part of itself. There's no printf |
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; |
; function, unless the kernel provides its own <stdio.h> header and a |
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⚫ | |||
; yourself. And that is one of the best things about kernel development: |
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⚫ | |||
; you get to make the entire system yourself. You have absolute and complete |
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; itself. It has absolute and complete power over the |
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⚫ | |||
; machine. |
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⚫ | |||
⚫ | |||
; By now, you are perhaps tired of assembly language. You realize some |
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⚫ | |||
; things simply cannot be done in C, such as making the multiboot header in |
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; in assembly as languages such as C cannot function without a stack. |
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; the right section and setting up the stack. However, you would like to |
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; write the operating system in a higher level language, such as C or C++. |
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; To that end, the next task is preparing the processor for execution of |
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; such code. C doesn't expect much at this point and we only need to set up |
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⚫ | |||
; such as floating point instructions are not available yet. |
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⚫ | |||
⚫ | |||
mov esp, stack_top |
mov esp, stack_top |
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; This is a good place to initialize crucial processor state before the |
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; We are now ready to actually execute C code. We cannot embed that in an |
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; high-level kernel is entered. It's best to minimize the early |
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; assembly file, so we'll create a kernel.c file in a moment. In that file, |
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; environment where crucial features are offline. Note that the |
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; we'll create a C entry point called kernel_main and call it here. |
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⚫ | |||
; point instructions and instruction set extensions are not initialized |
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; yet. The GDT should be loaded here. Paging should be enabled here. |
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; C++ features such as global constructors and exceptions will require |
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; runtime support to work as well. |
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; Enter the high-level kernel. The ABI requires the stack is 16-byte |
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; aligned at the time of the call instruction (which afterwards pushes |
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; the return pointer of size 4 bytes). The stack was originally 16-byte |
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; aligned above and we've since pushed a multiple of 16 bytes to the |
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; stack since (pushed 0 bytes so far) and the alignment is thus |
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; preserved and the call is well defined. |
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extern kernel_main |
extern kernel_main |
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call kernel_main |
call kernel_main |
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; |
; If the system has nothing more to do, put the computer into an |
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; infinite loop. To do that |
; infinite loop. To do that: |
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; |
; 1) Disable interrupts with cli (clear interrupt enable in eflags). |
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; They are already disabled by the bootloader, so this is not needed. |
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⚫ | |||
; Mind that you might later enable interrupts and return from |
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; continues execution, just to be safe. |
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; kernel_main (which is sort of nonsensical to do). |
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⚫ | |||
; Since they are disabled, this will lock up the computer. |
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; 3) Jump to the hlt instruction if it ever wakes up due to a |
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; non-maskable interrupt occurring or due to system management mode. |
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cli |
cli |
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.hang: |
.hang: hlt |
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hlt |
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jmp .hang |
jmp .hang |
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</source> |
</source> |
Revision as of 19:12, 25 July 2016
This article is an extension to the Bare Bones article and describes how to use NASM in a Hello World kernel. Mentally add the following changes to the base article.
Booting the Operating System
Bootstrap Assembly (NASM)
We will now create a file called boot.asm and discuss its contents. In this example, we are using the Netwide Assembler which is not part of your previously built cross-compiler toolchain and you will have to install it separately.
; 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 4
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.
section .text
global _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 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 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.
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
You can then assemble boot.asm using:
nasm -felf32 boot.asm -o boot.o