Bare Bones with NASM
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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
MBFLAGS equ MBALIGN | MEMINFO ; this is the Multiboot 'flag' field
MAGIC equ 0x1BADB002 ; 'magic number' lets bootloader find the header
CHECKSUM equ -(MAGIC + MBFLAGS) ; 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 MBFLAGS
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 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.
; 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:
You can then assemble boot.asm using:
nasm -felf32 boot.asm -o boot.o
Kernel
BITS 32
VGA_WIDTH equ 80
VGA_HEIGHT equ 25
VGA_COLOR_BLACK equ 0
VGA_COLOR_BLUE equ 1
VGA_COLOR_GREEN equ 2
VGA_COLOR_CYAN equ 3
VGA_COLOR_RED equ 4
VGA_COLOR_MAGENTA equ 5
VGA_COLOR_BROWN equ 6
VGA_COLOR_LIGHT_GREY equ 7
VGA_COLOR_DARK_GREY equ 8
VGA_COLOR_LIGHT_BLUE equ 9
VGA_COLOR_LIGHT_GREEN equ 10
VGA_COLOR_LIGHT_CYAN equ 11
VGA_COLOR_LIGHT_RED equ 12
VGA_COLOR_LIGHT_MAGENTA equ 13
VGA_COLOR_LIGHT_BROWN equ 14
VGA_COLOR_WHITE equ 15
global kernel_main
kernel_main:
mov dh, VGA_COLOR_LIGHT_GREY
mov dl, VGA_COLOR_BLACK
call terminal_set_color
mov esi, hello_string
call terminal_write_string
jmp $
; IN = dl: y, dh: x
; OUT = dx: Index with offset 0xB8000 at VGA buffer
; Other registers preserved
terminal_getidx:
push ax; preserve registers
shl dh, 1 ; multiply by two because every entry is a word that takes up 2 bytes
mov al, VGA_WIDTH
mul dl
mov dl, al
shl dl, 1 ; same
add dl, dh
mov dh, 0
pop ax
ret
; IN = dl: bg color, dh: fg color
; OUT = none
terminal_set_color:
shl dl, 4
or dl, dh
mov [terminal_color], dl
ret
; IN = dl: y, dh: x, al: ASCII char
; OUT = none
terminal_putentryat:
pusha
call terminal_getidx
mov ebx, edx
mov dl, [terminal_color]
mov byte [0xB8000 + ebx], al
mov byte [0xB8001 + ebx], dl
popa
ret
; IN = al: ASCII char
terminal_putchar:
mov dx, [terminal_cursor_pos] ; This loads terminal_column at DH, and terminal_row at DL
call terminal_putentryat
inc dh
cmp dh, VGA_WIDTH
jne .cursor_moved
mov dh, 0
inc dl
cmp dl, VGA_HEIGHT
jne .cursor_moved
mov dl, 0
.cursor_moved:
; Store new cursor position
mov [terminal_cursor_pos], dx
ret
; IN = cx: length of string, ESI: string location
; OUT = none
terminal_write:
pusha
.loopy:
mov al, [esi]
call terminal_putchar
dec cx
cmp cx, 0
je .done
inc esi
jmp .loopy
.done:
popa
ret
; IN = ESI: zero delimited string location
; OUT = ECX: length of string
terminal_strlen:
push eax
push esi
mov ecx, 0
.loopy:
mov al, [esi]
cmp al, 0
je .done
inc esi
inc ecx
jmp .loopy
.done:
pop esi
pop eax
ret
; IN = ESI: string location
; OUT = none
terminal_write_string:
pusha
call terminal_strlen
call terminal_write
popa
ret
; Exercises:
; - Newline support
; - Terminal scrolling when screen is full
; Note:
; - The string is looped through twice on printing.
hello_string db "Hello, kernel World!", 0xA, 0 ; 0xA = line feed
terminal_color db 0
terminal_cursor_pos:
terminal_column db 0
terminal_row db 0
Similar as before, to assemble it:
nasm -felf32 kernel.asm -o kernel.o