How to shutdown the machine? I'm building a tiny OS of my own [duplicate]

Question

Can the hlt instruction in assembly shutdown a computer it as it halts the processor? if it can be done using what i have told, is it the right way?

Can hlt shutdown the machine?

start:
    xor ax, ax; ;clear ax
    mov bx, ax; ;clear bx
    cli ;stop all interrupts
    hlt ;halt the cpu

If this is not the way to be done of if this will not shutdown the system please tell me the right way to do it.

Answer 1

By using these two lines of code:

    cli                     ; stop all interrupts
    hlt                     ; halt the cpu

you could halt a bootable program for x86 pc:

    BITS 16

start:
    mov ax, 07C0h           ; Set up 4K stack space after this bootloader
    add ax, 288             ; (4096 + 512) / 16 bytes per paragraph
    mov ss, ax
    mov sp, 4096

    mov ax, 07C0h           ; Set data segment to where we're loaded
    mov ds, ax


    cld                     ; clear direction flag
    mov si, text_string     ; Put string position into SI
    call print_string       ; Call our string-printing routine


    cli                     ; stop all interrupts
    hlt                     ; halt the cpu

    jmp $                   ; Jump here - infinite loop!


    text_string db 'Hello World!', 0


print_string:               ; Routine: output string in SI to screen
    mov ah, 0Eh             ; int 10h 'print char' function

.repeat:
    lodsb                   ; Get character from string
    cmp al, 0
        je .done            ; If char is zero, end of string
    int 10h                 ; Otherwise, print it
    jmp .repeat

.done:
    ret


    times 510-($-$$) db 0   ; Pad remainder of boot sector with 0s
    dw 0xAA55               ; The standard PC boot signature

Save it as "prog.asm", then use "nasm" to create boot sector:

nasm -f bin -o boot.img prog.asm

Now you could use "qemu" to test it:

qemu-system-i386 -drive file=boot.img,index=0,media=disk,format=raw -boot c -net none

Note: Removing those two lines mentioned above, causes your virtual machine to use maximum cpu cycles available.

Edit: Added "cld" instruction. As mentioned by Michael, it was necessary to make sure text_string is loaded from left-to-right.

Answer 2

The hlt instruction stops the x86 until an interrupt occurs. Unless all interrupts are disabled, that will stop the processor for only a millisecond or so.

To power down a modern computer, use the ACPI (Advanced Configuration and Power Interface).