Need some help with ASM text driver

[phillid]

Hello, again. I know this is an OS dev forum, and what I'm asking isn't entirely OS - related, but I need some help writing a text driver in ASM. I know about the memory addresses and all of the rest, but I need a simple, example driver for any type of monitor - color or B&W. It doesn't have to scroll the text or anything, I just need an example loop that I can adapt to my requirements.

Thanks a lot for any guidance.

phillid

[Tosi]

Do you know a high level language, such as C(++)?
Try writing text mode print routines in C or some other high level language first, and converting that to assembly. Then optimize what you came up with.
But if you don't know how to write something like that in assembly, I would recommend just sticking to a HLL and using assembly only when necessary.

[NickJohnson]

Are you using VGA text mode (which is the default when the computer boots), or are you talking about a full-fledged graphics driver with custom fonts? Either way, you can probably find a lot of information on the wiki, either under VGA Hardware or Text mode. Writing the code for the former should be trivial even in assembly.

[Brendan]

Hi,

Hello, again. I know this is an OS dev forum, and what I'm asking isn't entirely OS - related, but I need some help writing a text driver in ASM. I know about the memory addresses and all of the rest, but I need a simple, example driver for any type of monitor - color or B&W. It doesn't have to scroll the text or anything, I just need an example loop that I can adapt to my requirements.

You mean, something like this:

Code: Select all

;Input
; esi = address of zero terminated UTF32 string describing what should be displayed

displayLog:
    pushad
    xor edi,edi                        ;edi = screen address = "unknown"
    cld
    mov ebx,[cursorX]                  ;ebx = cursorX
    mov ecx,[cursorY]                  ;ecx = cursorY

.nextChar:
    lodsd                              ;eax = next unicode character
    test eax,eax                       ;Has the end of the string been reached?
    je .done                           ; yes

    cmp eax,NEWLINE_CHAR               ;Is it newline?
    je .handleNewLineChar              ; yes
    cmp eax,TAB_CHAR                   ;Is it tab?
    je .handleTabChar                  ; yes
    cmp eax,SPACE_CHAR                 ;Is it space?
    je .handleSpaceChar                ; yes

    cmp ebx,[screenWidth]              ;Is the cursor past the right edge of the screen?
    jb .doneLineWrap                   ; no
    xor edi,edi                        ; yes, edi = screen address = "unknown"
    inc ecx                            ;      Increase cursorY
    xor ebx,ebx                        ;      cursorX = 0
.doneLineWrap:

    cmp ecx,[screenHeight]             ;Is the cursor past the bottom edge of the screen?
    jb .doneScroll                     ; no
    ;**** Add scrolling code here ****
    mov ecx,[screenHeight]             ;ecx = screenHeight
    dec ecx                            ;ecx = new cursorY = last line of screen
    jmp .calcScreenAddress
.doneScroll:

    test edi,edi                       ;Is the current screen address known?
    jne .gotScreenAddress              ; yes
                                       ; no, need to calculate new screen address
.calcScreenAddress:
    push eax
    mov eax,[screenWidth]              ;eax = screenWidth
    mul ecx                            ;edx:eax = cursorY * screenWidth
    mov edi,[screenAddress]            ;edi = address of display memory
    add eax,ebx                        ;eax = cursorY * screenWidth + cursorX
    lea edi,[edi+eax*2]                ;edi = address of display memory + 2 * (cursorY * screenWidth + cursorX)
    pop eax
.gotScreenAddress:

    call convertUnicodeToExtASCII      ;Convert the unicode character to an "extended ASCII" character
    mov ah,[currentAttribute]          ;ah = current attribute
    stosw                              ;Put the character on the screen, and increase the screen address
    inc ebx                            ;Increase cursorX
    jmp .nextChar

.handleNewLineChar:
    xor edi,edi                        ;edi = screen address = "unknown"
    inc ecx                            ;Increase cursorY
    xor ebx,ebx                        ;cursorX = 0
    jmp .nextChar

.handleSpaceChar:
    inc ebx                            ;Increase cursorX
    test edi,edi                       ;Is screen address known?
    je .nextChar                       ; no, leave it as unknown
    add edi,2                          ; yes, update it
    jmp .nextChar

.handleTabChar:
    xor edi,edi                        ;edi = screen address = "unknown"
    add ebx,4                          ;ebx = cursorX + 4
    and bl,0xFC                        ;ebx = (cursorX + 4) % 4 = next tab position
    jmp .nextChar

.done:
    mov [cursorX],ebx                  ;Set new cursorX
    mov [cursorY],ecx                  ;Set new cursorY
    popad
    ret

Cheers,

Brendan

[phillid]

Yes! Thank you, brendan! I will look through the code and learn exactly what each instruction does tonight.

Thanks again

phillid