hello-os.asm

[BITS 16]			;Tells the assembler that its a 16 bit code
[ORG 0x7C00]		;Origin, tell the assembler that where the code will
					;be in memory after it is been loaded

MOV SI, HelloString 	;Store string pointer to SI
CALL PrintString	;Call print string procedure
JMP $ 			;Infinite loop, hang it here.


PrintCharacter:	;Procedure to print character on screen
		;Assume that ASCII value is in register AL
MOV AH, 0x0E	;Tell BIOS that we need to print one charater on screen.
MOV BH, 0x00	;Page no.
MOV BL, 0x07	;Text attribute 0x07 is lightgrey font on black background

INT 0x10	;Call video interrupt
RET		;Return to calling procedure



PrintString:	;Procedure to print string on screen
		;Assume that string starting pointer is in register SI

next_character:		;Lable to fetch next character from string
MOV AL, [SI]		;Get a byte from string and store in AL register
INC SI			;Increment SI pointer
OR AL, AL		;Check if value in AL is zero (end of string)
JZ exit_function 	;If end then return
CALL PrintCharacter 	;Else print the character which is in AL register
JMP next_character	;Fetch next character from string
exit_function:		;End label
RET			;Return from procedure


;Data
HelloString db 'Hello World', 0	;HelloWorld string ending with 0

TIMES 510 - ($ - $$) db 0	;Fill the rest of sector with 0
DW 0xAA55			;Add boot signature at the end of bootloader