Hello World NASM boot loader example.

build.bat

@echo on
echo Bulding bin file...
nasm loader.asm -f bin -o loader.bin

echo Writing bin file to ploppy
partcopy loader.bin 0 200 -f0

build.sh

# Build bin file
nasm loader.asm -f bin -o loader.bin

# Write bin file to floppy
dd if=loader.bin bs=512 of=/dev/fd0

loader.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

Why 16 bit?

Why 16 bit?

Because it is simple to load 16 bit code. Then you can switch CPU to 32/64 bit mode, use protected mode and etc.

Thanks

Can smbody help? How I might to call my extern C function from assembly code?

Can smbody help? How I might to call my extern C function from assembly code?

Just add extern functionName and call functionName wherever needed. Make sure to link the files first