Real Mode Assembly: Print Char to Screen without INT Instruction on Boot

Question

The following site "Writing Boot Sector Code" provides a sample of code that prints 'A' to the screen when the system boots. From what I have been reading don't you have to use INT opcode to get BIOS to do certain things? How does the code below, from the site referenced above work without using interrupts? What portion of code actually tells the hardware to print 'A' to the screen?

Code in question:

.code16
.section .text
.globl _start
_start:
  mov $0xb800, %ax
  mov %ax, %ds
  movb $'A', 0
  movb $0x1e, 1
idle:
  jmp idle 

---

APPENDING TO ORIGINAL QUESTION

If I use the following code does the BIOS call write to the text buffer for me? The buffer starting at address 0xb800?

   # Author: Matthew Hoggan
   # Date Created: Tuesday, Mar 6, 2012
   .code16                        # Tell assembler to work in 16 bit mode (directive)
   .section .text
   .globl _start                  # Help linker find start of program
   _start:
       movb $0x0e,     %ah        # Function to print a character to the screen                 
       movb $0x00,     %bh        # Indicate the page number
       movb $0x07,     %bl        # Text attribute
       mov  $'A',      %al        # Move data into low nibble                   
       int  $0x10                 # Video Service Request to Bios                             
   _hang:                         
       jmp  _hang                 
       .end   

Answer 1

Direct answer to your question: The line "movb $'A', 0" effectively completes the print to the screen (and the following line, "movb $0x1e, 1" specifies what color it should be).

Longer answer: The video hardware draws the screen based on the contents of memory. When in text mode, the video hardware starts drawing based on memory segment 0xB800. Whatever is at byte 0 defines the character to be drawn at the first text cell on the screen. The next byte defines the attributes (foreground color, background color, and blink status). This pattern repeats (char - attr - char - attr) throughout the entire screen.

So, technically, my direct answer wasn't true. The 2 'movb' statements simply stage the letter 'A' to be printed. 'A' is not printed until the next time hardware refreshes the display based on the memory.

Answer 2

At boot, you're in a default screen mode - in this case a text screen mode. Your example program is writing directly into the character buffer that's displayed on the screen for that text screen mode. Setting the data segment register to 0xb800 is getting things set up to point at that buffer. Check out this tutorial for more information.

Answer 3

Basically when you call INT 10h, BIOS will execute a routine that does almost the same thing, by writing characters and their attributes to video memory. It is, however useful to know how to write and execute these routines yourself so if and when you decide to switch the CPU into 32-bit protected mode, you can still print characters to the screen because in that mode you will no longer be able to call BIOS interrupts.