Question: I'm trying to learn assembly for school and there is this part at the beginning of the example code:

```
mov al, 0
mov ah, 5
int 10

```

Before this there is a procedure :

```
.386
instructions SEGMENT use16
        ASSUME  CS:instructions

interrupt_handler PROC
; some code
interrupt_handler ENDP

```

What is the `int 10` line doing? Is it calling the `interrupt_handler` procedure? Why is it exactly 10?

This is all running in DoSBox and is assembled using masm.

Answer: 
I found what appears to be a complete copy of the code in the OPs native language (Polish). <https://ideone.com/fork/YQG7y> . There is this section of code (run through Google translate):

```
; ================================================= =======

; main program - installation and uninstallation of the procedure
; interrupt handling

; determining page number 0 for text mode
start:
mov al, 0
mov ah, 5
int 10

```

It is clear from this code that it is a bug. It should be [`int 10h`](http://ctyme.com/intr/rb-0091.htm) and not [`int 10`](http://www.ctyme.com/intr/rb-0048.htm) (same as `int 0ah`). `int 10h` is documented as:

> 
> 
> ```
> VIDEO - SELECT ACTIVE DISPLAY PAGE
> AH = 05h
> AL = new page number (00h to number of pages - 1) (see #00010)
> 
> Return:
> Nothing
> 
> Desc: Specify which of possibly multiple display pages will be visible
> 
> ```
> 
> 

`int 10` is something completely different:

> 
> 
> ```
> IRQ2 - LPT2 (PC), VERTICAL RETRACE INTERRUPT (EGA,VGA)
> 
> ```
> 
> 

Calling IRQ2 interrupt handler with `int 10` will effectively do nothing from the standpoint of the program. Since the default text page is likely already 0 the program works as expected.

---

The correct code:

```
mov al, 0
mov ah, 5
int 10h

```

would set the text mode display page to 0 using the BIOS service 10h function 5.
Simple answer is `int 10h` (I think your code has a typo) typically calls a real mode interrupt handler at the vector that provides video services. [`int 10h` services](http://www.ctyme.com/intr/int-10.htm) include setting the video mode, character and string output, and graphics primitives (reading and writing pixels in graphics mode).