Bootsector that prints hex

boot_sector.asm

;
; String printing boot sector
;
[org 0x7c00] ; Tell the assembler where to load code

mov dx, 0x1fb6 ; store value to print in dx
call print_hex ; call the function

jmp $ ; hang

%include "print_string.asm"
%include "print_hex.asm"

; Data
HEX_OUT: db '0x0000', 0

; Padding and magic number
times 510-($-$$) db 0
dw 0xaa55

print_hex.asm

;
; print_hex.asm
;

; prints the value of DX as hex
print_hex:
  pusha

  mov bx, HEX_OUT ; store pointer to hex value
  add bx, 0x5     ; move pointer to end of string

  loop:
    mov cl, dl  ; copy dx  value into cl
    cmp cl, 0   ; check if 0
    je end_loop ; if 0 then we're done

    and cl, 0xF ; mask first half byte
    cmp cl, 0xA ; compare to A
    jl lt_A     ; if <  A go to lt_A
    jmp gte_A   ; if >= A go to gte_A

  lt_A: ; add 48 
    add cl, 48  ; add 48 to bring it to ascii '0'
    jmp end_cmp ; go to end of this code block
  gte_A: ; add 55
    add cl, 55  ; add 55 to bring it to ascii 'A'
    jmp end_cmp ; go to end of this code block

  end_cmp:
  mov [bx], cl  ; put value into dereferenced pointer
  sub bx, 1     ; decrement pointer
  shr dx, 4     ; shift right 4 bits to get the next half byte
  jmp loop      ; go back to start of loop

  end_loop:
  mov bx, HEX_OUT   ; print the string pointed to
  call print_string ; by BX

  popa
  ret

print_string.asm

; print_string.asm

print_string:
  pusha           ; store all register values on stack
  mov ah, 0x0e    ; BIOS tele-type output

  while:
    mov al, [bx]
    cmp al, 0     ; check for null value
    je end_while  ; jump to end if null

    int 0x10      ; Output value in al
    add bx, 1     ; Increment pointer
    jmp while     ; jump to emit loop

  end_while:
  popa            ; restore all register values
  ret