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Now is the time you’ve all been waiting for. Finally we get to the classic “first” program. Every decent programming book has a “Hello, World” program, and now we know enough to make a “Hello, World” operating system. If you have done some experimenting on your own and have already done this, then you may want to skip this lesson. We will create a function to print a string and use it to display our message.
It will get tedious to print one character at a time to the screen, so we’ll create a function to print a zero-terminated string to the screen. This is just a simple loop that prints all the character in a string one at a time.
; ---------------------------------------------
; Print a null-terminated string on the screen
; ---------------------------------------------
putstr:
lodsb ; AL = [DS:SI]
or al, al ; Set zero flag if al=0
jz putstrd ; jump to putstrd if zero flag is set
mov ah, 0x0e ; video function 0Eh (print char)
mov bx, 0x0007 ; color
int 0x10
jmp putstr
putstrd:
retn
Now, a little on how to use this function. First you have to load the address of the first character of the string into the register SI. Then simply call this subroutine putstr.
You can create a string like this in your assembly file.
msg db 'Hello, World!', 0
The zero on the end terminates the string. Then you can print the string to the screen using the following instructions.
mov si, msg ; Load address of message
call putstr ; Print the message
There is just one more thing that needs to be set up before this will work. The address msg, loaded into the register SI, is actually an offset off the beginning of the beginning of the segment that is pointed to by the register DS. So, before you can use the address msg, you must set up the current data segment. For now, we will use flat addressing from the bottom of physical RAM. To set the data segment to start from the bottom, set the DS register to zero. The following two instructions will do this.
xor ax, ax ; Zero out ax
mov ds, ax ; Set data segment to base of RAM
Try putting all of these parts together using the file h.asm from Lesson 3 as a starting point. Then, using the same method described in Lesson 3, assemble your file, copy it to your floppy disk and boot with it. Have fun. If you get stuck, you can look at my solution, helowrld.asm, but it’s no fair peeking until you’ve tried!
Once you have finished, proceed to the next lesson where we will learn how to make our operating system interactive. 
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