Assembly Using Variables

Question

I am trying to make the text screen print 'h' which is stored in a variable. I am using NASM. x86 Protected Mode, a from scratch kernel.

DisplayMessage:
        ;mov byte[Color], 0xF
        ;mov CFC, EAX;
        ;mov byte[Color], 104
        ;push 104
        ;mov byte[esi], Msg
        ;lodsb
        mov ebx, Msg
        add ebx, 4
        mov [Msg], eax
        mov byte[0xB8000], Msg
        ;mov byte[eax], Color
        ;pop byte[0xB8000]
        ;mov byte[0xB8000], byte Color
        ;mov byte[0xB8000], 0xB500000;
        ;Now return
        ret
EndCode:
Msg: db 104

The letter it displays is never right. Whats the proper way to do this?

so we have to assume that your OS has the text video page mapped to address 0xB8000? And we have to assume that the video card is in text mode? — dthorpe
– dthorpe, Commented Sep 30, 2012 at 7:34
what's the point of including all the commented out code? it just makes it harder to understand what you're trying to do. — dthorpe
– dthorpe, Commented Sep 30, 2012 at 7:36
It was just code that doesn't work. Just left it there for reference. I will remove it once I get this working. =) — civilwarrock
– civilwarrock, Commented Sep 30, 2012 at 14:28

Alexey Frunze · Accepted Answer · 2012-09-30 08:13:35Z

9

    mov ebx, Msg ; this loads ebx with the address of Msg, OK
    add ebx, 4 ; this increments the address by 4, OK, but why?
    mov [Msg], eax ; this stores eax into the first 4 bytes of Msg, OK, but why?
    mov byte[0xB8000], Msg ; this writes the least significant byte of the
                           ; address of Msg to the screen, not OK.
                           ; Does not make any sense.

Why not just?:

mov al, [Msg]
mov [0xB8000], al

This should write the very first character of Msg ('h' has ASCII code 104, correct) at the top-left corner of the screen, if, of course, your data segment has the base address of 0 in its segment descriptor, and if your org is correct.

answered Sep 30, 2012 at 8:13

Alexey Frunze

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11 Comments

Linuxios Over a year ago

What about VGA text mode color codes? I think that's what the +4 is for.

Alexey Frunze Over a year ago

@Linuxios In color text modes, every character cell on the screen is defined by 2 (two) bytes, the first being the character code and the second, a combination of background and foreground colors. Where does 4 come from in here?

civilwarrock Over a year ago

Ok so the +4, ignore that was me testing something. But I tried the <pre> mov al, [Msg] mov [0xB8000], al </pre> And, it displayed nothing. However, I think I didn't set the base address or the segment descriptor, and I don't know what the org is 'org 0x200000'. How do I set the other two? Maybe that's the problem. And why did you use the 'al' register? Thanks - Matt

Linuxios Over a year ago

@civilwarrock: <pre> doesn't work here. Use the backquote (```)(that's 3 of them) to delimitate code in text.

Linuxios Over a year ago

@AlexeyFrunze: Oh wait... It was incrementing the address. Sorry!

|

brenden · Accepted Answer · 2018-04-01 23:15:35Z

0

VGA text mode uses address 0xB8000 as an array of uint16_t With the upper byte being used for color, and the lower byte being the character code. Currently you are storing the character in the upper byte and the lower byte is untouched. It may be random noise that is printing a random character. Try this:

DisplayMessage:
mov al, byte [msg]
mov ah, 0x0F ;White Text on Black Background
mov word [0xB8000], ax
ret

answered Apr 1, 2018 at 23:15

brenden

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