Multiboot1 Bare Bones with NASM

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