Display the binary value of the x86 FLAGS register on boot

flags.asm

bits 16

start:
    ; Set up 512 bytes of stack space. More than plenty!
    mov ax, 0x07C0 ; This is 0x7C00 >> 4
    add ax, 64 ; (512 + 512) / 16, for some weird offset crap
    mov ss, ax ; SS is the stack segment
    mov sp, 512 ; SP is the stack pointer

    ; Call the main routine
    call print_flags

    ; We're finished, so loop-a-noop
    jmp $

print_flags:
    pushfw ; Push x86 FLAGS register to the stack
    pop ax ; Pop x86 FLAGS register from stack to AX
    ; Note that MOV AX, EFLAGS doesn't work for some reason

    ; Call the AX register to binary display routine
    call ax2bin
    ret

; Derived from code by NASM developer, Frank Kotler
; http://forum.nasm.us/index.php?topic=1371.0
ax2bin:
    push cx
    push dx
    mov cx, 16
.top:
    rcr ax, 1 ; Rotate and set the carry with what we rotated off
    mov dl, "0" ; Set DL to "0", the default value
    adc dl, 0 ; If the carry bit was set by the RCR, increase the "0" byte by one to "1"

    push ax ; Save AX
    mov ah, 0x0A ; 0x0A is the "print byte at cursor" argument to INT 0x10
    mov al, dl ; Our result is in DL, but INT 0x10 uses AL
    int 0x10 ; Print the byte
    pop ax ; Restore AX

    loop .top

    pop dx
    pop cx
    ret

times 510 - ($ - $$) db 0
dw 0xAA55