Basic OS.asm

[BITS 16]      ; 16 bit code generation
[ORG 0x7C00]   ; Origin location

; Main program
main:		; Label for the start of the main program

 mov ax,0x0000	; Setup the Data Segment register
		; Location of data is DS:Offset
 mov ds,ax	; This can not be loaded directly it has to be in two steps.
		; 'mov ds, 0x0000' will NOT work due to limitations on the CPU

 mov si, HelloWorld	; Load the string into position for the procedure.
 call PutStr	; Call/start the procedure

jmp $		; Never ending loop

; Procedures
PutStr:		; Procedure label/start
 ; Set up the registers for the interrupt call
 mov ah,0x0E	; The function to display a chacter (teletype)
 mov bh,0x00	; Page number
 mov bl,0x07	; Normal text attribute

.nextchar	; Internal label (needed to loop round for the next character)
 lodsb		; I think of this as LOaD String Block 
		; (Not sure if thats the real meaning though)
		; Loads [SI] into AL and increases SI by one
 ; Check for end of string '0' 
 or al,al	; Sets the zero flag if al = 0 
		; (OR outputs 0's where there is a zero bit in the register)
 jz .return	; If the zero flag has been set go to the end of the procedure.
		; Zero flag gets set when an instruction returns 0 as the answer.
 int 0x10	; Run the BIOS video interrupt 
 jmp .nextchar	; Loop back round tothe top
.return		; Label at the end to jump to when complete
 ret		; Return to main program

; Data

HelloWorld db 255,255,255,255,255,255,255,255,255,255,255,255,255,'Welcome to Basic OS',13,10,255,255,255,255,255,255,255,255,255,255,255,255,255,'made by :- Mangat Rai',13,10,0

; End Matter
times 510-($-$$) db 0	; Fill the rest with zeros
dw 0xAA55		; Boot loader signature