"Hello, World!" in x86-64 Assembly for the BIOS environment. Written from scratch (no tutorials). I'm proud of it, but not proud enough for it to go in it's own repository; instead, it will live here in a Gist instead.

boot.asm

[BITS 16]                       ; 16-bit code
[ORG 0x7C00]                    ; BIOS loads the boot sector at 0x7C00
jmp start                       ; Jump to the `start` label

message db "Hello, World!", 0   ; The message to print
length equ ($ - message)        ; The length of `message` (difference of the
                                ; current address and the address of `message`)

start:
    mov si, message             ; Load the address of `message` into SI
    xor bx, bx                  ; Zero out BX to use it as a counter
    mov cx, length              ; Load the length of the message into CX
    mov ah, 0x0E                ; TeleType (TTY) output function
next:
    mov al, byte [si + bx]      ; Dereference the address of (`message` + BX)
    int 0x10                    ; Call BIOS interrupt 0x10 (TTY output)
    inc bx                      ; Increment BX to point to the next character
    cmp bx, cx                  ; Have we reached the specified length?
    jne next                    ; If not, jump back to `next`
end:
    jmp $                       ; Infinite loop

times 510 - ($ - $$) db 0       ; Pad the rest of the boot sector with zeros
dw 0xAA55                       ; Boot signature (A.K.A. magic number)