Pmode print in asm

[kattmat]

Hey there!

I have been playing around with making OSs for a little while now. Made my own 16bit prompt which could handle some commands and stuff.
Now I'm going to do the same thing in protected mode, but with more features.
It's all written in ASM (I know C, but I want to learn the pure basics), and there is a problem I've had for like a week now and still haven't been able to solve:
How to print stuff to the screen from ASM?

Code: Select all

mov si, MyString
call Print
nop
Print:
    pusha
    .NextChar:
        lodsb
        mov byte [0xB8000+bx], al
        inc bx
        mov byte [0xB8000+bx], 0x07
        cmp al, al
            jz .Return
        jmp .NextChar
    .Return
        popa
        ret

That stuff just doesn't work.. No output to the screen whatsoever..[/code]

[bubach]

< swedish >hej kattmat</ swedish >
I guess that you are from sweden with that name.
You can have a look on my source to see how i have done it..

http://bos.asmhackers.net/

[kattmat]

<Swedish> Bingo-lingo. =) Kul att det finns svenskar som h?ller p? och meckar! </Swedish>

I'll check it out. I guess it'll work out just fine. Cheers!

[kattmat]

Okey, I have been trying and trying and trying now, without any success whatsoever.

The cls (clearscreen) function works just fine, but none of the print functions work at all.

What's wrong?

I'm compiling with nasm, but the only difference as far as I can tell is
use32 -> [BITS 32]
org xxx -> [ORG xxx]

Help!

[kattmat]

Code: Select all

print:
	push ax
	push bx
	push eax
	
	mov bh, al
	mov eax, 0x00
	
	.displaychar:
		lodsb
		or al, al
			jz .done
		
		mov [(eax*2) + 0xB8000], al
		mov [(eax*2) + 0xB8001], bh
		inc eax
		jmp .displaychar
	
	.done:
		pop eax
		pop bx
		pop ax
		ret

Why isn't it working?[/code]

[frizzz]

I just published my own assembler OS. You will find there a lot of examples, how to write to the screen in VGA3-mode. Look at my homepage:
www.rcfriz.de
But in fact it is very simple:
mov ax,[attribute_and_letter] ; attribute is H-byte
mov WORD [es:esi+0B800h],ax ;...and You will see it
You need to store a selector in es, which adresses a descriptor with absolute startadress 0
The value in esi ranges from 0...0F00h
... seems to me, that You forgot the selector in es...

[bubach]

I took a break from osdev for about 6 months becasue i couldn't get my text output to work, so you are not alone!

First you will need some way of handling where the cursor is on the screen. In my example code I'll just have a variable which contains the cursor position.
You should have "get_cursor" and "set_cursor" functions later on (to update the cursor on screen, not only your variable)...

Then you move the content of bx, which is char and color, to the adress of the cursor position that you have stored multiplied with 2 (1 byte char+1 byte attribute=cursor position*2) + 0xB8000 which is the text mode memory.

Formula: (cursor position*2)+0xB8000=char+attribute

Example:

Code: Select all

          cursor_offset    dw   0

;-----------------------------------------;
;  print char                             ;
;            in: bl = char, bh = attrib   ;
;-----------------------------------------;
print_char:
	  push	  eax

	  cmp	  bl, 13
	  jne	  .cont
;  call	  new_line           ; This is where you should call your "new_line" 
	  jmp	  .done

     .cont:
	  cmp	  bl, 10			      ; ignore
	  je	  .done

	  mov     ax, word [cursor_offset]            ; somehow get the cursor
                                                      ; position and add it to eax
	  mov	  [es:(eax*2 + 0xB8000)], bx
	  add     [cursor_offset], 1                  ; add 1 to cursor position

     .done:
	  pop	  eax
	  ret

To test this code you do like this:

Code: Select all

          mov     bh, 0x07
          mov     bl, 'T'
          call    print_char

          mov     bh, 0x07
          mov     bl, 'e'
          call    print_char

          mov     bh, 0x07
          mov     bl, 's'
          call    print_char

          mov     bh, 0x07
          mov     bl, 't'
          call    print_char

I hope that it helps you..
/ Christoffer

[bubach]

Another note:

Code: Select all

print:
	push ax
	push bx
	push eax
	
	mov bh, al
	mov eax, 0x00
	
	.displaychar:
                push    eax     ; added: save eax
		lodsb                 ;this gets a byte from esi and puts it
		or al, al              ; into al, which mess up the cursor position
	        jz .done             ;better save and restore eax....
                mov    bl, al    ; added: move value to bl instead of al
                pop   eax            ; added: restore cursor position

		mov [(eax*2) + 0xB8000], bx    ; like this 

		inc eax
		jmp .displaychar
	
	.done:
		pop eax
		pop bx
		pop ax
		ret