[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 'Hello World',13,10,0
; End Matter
times 510-($-$$) db 0 ; Fill the rest with zeros
dw 0xAA55 ; Boot loader signature |
