Screen

The screen is an output device that allows displaying characters. The way to communicate with the screen is through a system call. This is done for simplicity, as a real screen is much more complex.

With the call INT 7, a string of characters is written to the screen. It receives two parameters:

• AL: length of the string to print
• BX: memory address where the string starts

org 1000h
string db "Hello!"

org 2000h
mov bx, offset string
mov al, 5
int 7
; "Hello!" (without quotes) is printed on the screen.
int 0
end

There are three special characters:

• The backspace character (BS, 8 in decimal) erases the previous character;
• The line feed character (LF, 10 in decimal) effectively prints a line break — useful for not printing everything on a single line;
• The form feed character (FF, 12 in decimal) clears the screen.