Hardware VGA Text Mode IO in old dos assembly Issue_问答_开发者

After reading about at least the first 3 or 4 chapters of about 4 different books on assembly programming I got to a stage where I can put "Hello World" on a dosbox console using MASM 6.11. Imagine my delight!!

The first version of my program used DOS Function 13h. The second version of my program used BIOS Function 10h

I now want to do the third version using direct hardware output. I have read the parts of the books that explain the screen is divided into 80x25 on a VGA monitors (not bothered about detecting CGA and all that so my program uses memory address 0B800h for colour VGA, because DOSBox is great and all, and my desire to move to Win Assembler sometime before im 90 years old). I have read that each character on the hardware screen is 2 bytes (1 for the attribute and one for the character itself, therefore you have 80x25x2=4000 bytes). The odd bytes describe the attribute, and the even bytes the ASCII character.

But my problem is this. No matter how I try, I cant get my program to output a simple black and white (which is just the attribute, I assume I can change this reasonably easily) string (which is just an array of bytes) 5 lines from the top of the screen, and 20 characters in from the left edge (which is just the number of blank characters away from a zero based index with 4000 bytes long). (if my calc is correct that is 5x80=400+20=420x2=840 is the starting position of my string within the array of 4000 bytes)

How do I separate the attribute from the character (I got it to work partially but it only shows every second character, then a bunch of random junk (thats how I figured I need some sort of byte pair for the attribute and text), or how do I set it up such that both are recognised together. How do I control he position of the text on the screen once the calcs are made. Where am I going wrong.

I have tried looking around the web for this seemingly simple question but am unable to find a solution. Is there anyone who used to program in DOS and x86 Assembly that can tell me how to do this easy little program by not using BIOS or DOS functions, just with the hardware.

I would really appreicate a simple code snippet if possible. Or a refrence to some site or free e-book. I dont want buying a big book on dos console programming which will end up useless when I move to windows shortly. The only reason I am focused on this is because I want to learn true assembly, not some macro language or some pretensious high level language that claims to be assembly.

I am trying to build a library of routines that will make Assembly easier to learn so people dont have to work though all the 3 to 6 chapters across 10 books of theory esentially explaining again and again the same stuff when really all that is needed is enough to know how to get some output, assign values to variables, get some input, and do some loops and decisions. The theory can come along later, and by the time they get to loops and decisions most people will have done enough assembler to have all the theory anyway. I beleive assembly should be taught no different than any other language starting with a simple hello world program then getting input ect. I want to make this possible. But hey, I'm just a beginner, maybe my taughts will change when I learn more.

One other note, I know for a fact the problem is NOT with DOSBox as I have a very old PC running true MS-DOS V6.2 and the program still doesnt work (bu开发者_运维技巧t gives almost identical output). In fact, DOSBox actually runs some of my old programs even better than True dos. Gem desktop being one example. Just wanted to get that cleared before people try suggesting its a problem with the emulator. It cant be, not with such simple programs. No im afraid the problem is with my little brain not fully understanding what is needed.

Can anyone out there please help!!

Below is the program I used (MASM 6.1 Under DOSBox on Win 7 64-bit). It uses BIOS Intrrupt 10h Function 13h sub function 0. I want to do the very same using direct hardware IO.

.model small
.stack
.data           ;part of the program containing data
    ;Constants - None
    ;Variables
    MyMsg   db    'Hello World'

.code
Main:
GetAddress:
    mov ax,@data        ;Gets address of data segment
    mov es,ax           ;Loads segment address into es regrister
    mov bp,OFFSET MyMsg ;Load Offset into DX

SetAttributes:
    mov bl,01001111b    ;BG/FG Colour attributes
    mov cx,11           ;Length of string in data segment

SetRowAndCol:
    mov dh,24       ;Set the row to start printing at
    mov dl,68       ;Set the column to start printing at

GetFunctionAndSub:
    mov ah,13h      ;BIOS Function 10h - String Output
    mov al,0        ;BIOS Sub-Function (0-3)

Execute:
    int 10h         ;BIOS Interrupt 10h

EndProg:
    mov ax,4c00h    ;Terminate program return 0 to OS
    int 21h         ;DOS Interrupt 21h

end Main
end

I want to have this in a format that is easy to explain. So here is my current workings. I've almost got it. But it only prints the attributes, getting the characters on screen is a problem. (Ocasionally when I modify it slightly, I get every second character with random attributes (I think I know the technicalities of why, but dont know enough assembler to fix it)).

.model small
.stack
.data
    ;Constants
    ScreenSeg   equ     0B800h

    ;Variables
    MyMsg   db  'Hello World'
    StrLen  equ $-MyMsg

.code
Main:               

SetSeg:
    mov ax, ScreenSeg   ;set segment register:
    mov ds, ax

InitializeStringLoop:   ;Display all characters: - Not working :( Y!
    mov cx, StrLen      ;number of characters.
    mov di, 00h         ;start from byte 'h'

OutputString:
    mov [di], offset byte ptr MyMsg[di]
    add di, 2           ;skip over next attribute code in vga memory.
    loop OutputString

InitializeAttributeLoop:;Color all characters: - Atributes are working fine.
    mov cx, StrLen      ;number of characters.
    mov di, 01h         ;start from byte after 'h'

;Assuming I have all chars with same attributes - fine for now - later I would make this
;into a procedure that I will just pass the details into. - But for now I just want a
;basic output tutorial.

OutputAttributes:
    mov [di], 11101100b     ;light red(1100) on yellow(1110)
    add di, 2               ;skip over next ascii code in vga memory.
    loop OutputAttributes

EndPrg:
    mov ax, 4C00h
    int 21h
end Main

Of course I want to reduce the instructions used to the bare bones essentials. (for proper tuition purposes, less to cover when teaching others). Hense the reason I did not use MOVSB/W/D ect with REP. I opted instead for an easy to explain manual loop using standard MOV, INC, ADD ect. These are instructions that are basic enough and easy to explain to newcommers. So if possible I would like to keep it as close to this as possible.

I know esentially all that seems to be wrong is the loop for the actual string handler. Its not letting me increment the address the way I want it to. Its embarasssing to me cause I am actually quite a good progammer using C++, C#, VB, and Delphi (way back when)). I know you wouldnt think that given I cant even get a loop right in assembler, but it is such a different language. There are 2 or 3 loops in high level languages, and it seems there are an infinate combination of ways to do loops in assembler depending on the instructions. So I say "Simple Loop", but in reality there is little simple about it.

I hope someone can help me with this, you would be saving my assembly carreer, and ensuring I eventually become a good assembly teacher. Thanks in advance, and especially for reading this far.

The typical convention would be to use ds:si as source, and es:di as destination.

So it would end up being similar to (untested):

  mov ax, @data
  mov ds, ax
  mov ax, ScreenSeg
  mov es, ax
  ...
  mov si, offset MyMsg
OutputString:
  mov al, byte ptr ds:[si]
  mov byte ptr es:[di], al
  add si, 1           ; next character from string
  add di, 2           ; skip over next attribute code in vga memory.
  loop OutputString

I would suggest getting the Masm32 Package if you don't already have it. It is mainly geared towards easily using Assembly Language in "Windows today", which is very nice, but also will teach you a lot about Asm and also shows where to get the Intel Chip manuals that were mentioned in an earlier reply that are indispensable.

I started programming in the 80's and so I can see why you are so interested in the nitty gritty of it, I know I miss it. If more people were to start out there, it would pay off for them very much. You are doing a great service!

I am playing with exactly what you are talking about, Direct Hardware, and I have also learned that Windows has changed some of the DOS services and BIOS services have changed too, so that some don't work any more. I am in fact writing a small .com program and running it from Win7 in a Command Prompt Window, Prints a msg and waits for a key, Pretty cool considering it's Win7 in 2012!

In fact it was BIOS 10h - 0Eh that did not work and so I tried Dos 21h 02h to write to the screen and it worked. The code is below because it is a .com (Command Program) i thought it might be of use to you.

; This makes a .com program (64k Limit, Code, Data and all ; have to fit in this space. Used for small utilities and ; good for very fast tasks. In fact DOS Commands are mostly ; small .com programs like this (except more useful)! ; ; Assemble with Masm using ; c:\masm32\bin\ml /AT /c bfc.asm ; Link with Masm's Link16 using ; c:\masm32\bin\link16 bfc.obj,bfc.com; ; ; Link16 is the key to making this 16bit .com (Command) file

SEGMT SEGMENT
org 100h

Start:
        push    CS
        pop     DS

        MOV SI, OFFSET Message
Next:
        MOV ah, 02h             ; Write Char to Standard out
        MOV dl, [si]            ; Char
        INT 21h                 ; Write it
        INC si                  ; Next Char
        CMP byte ptr[si], 0     ; Done?
        JNE Next                ; Nope

WaitKey:
        XOR ah, ah              ; 0
        INT 16h                 ; Wait for any Key
ExitHere:        
        MOV ah, 4Ch             ; Exit with Return Code
        xor al, al              ; Return Code
        INT 21h

Message db  "It Works in Windows 7!", 0

SEGMT ENDS
END Start

I used to do all of what you are talking about. Trying to remember the details. Michael Abrash is a name you should be googling for. Mode-X for example a 200 something by 200 (240x200?) 256 color mode was very popular as it broke the 16 color boundary and at the time the games looked really good.

I think that the on the metal register programming was doable but painful and you likely need to get the programmers reference/datasheet for the chip you are interested in. As time passed from CGA to EGA to VGA to VESA the way things worked changed as well. I think typically you use int something calls to get access to the page frame then you could fill that in directly. VESA I think worked that way, VESA was a big livesaver when it came to video card support, you used to have to write your own drivers for each chip before then (if you didnt want the ugly standard modes).

I would look at mode-x or vesa and go from there. You need to have a bit of a hacker inside to get through some of this anyway, it is very rare to find a datasheet/programmers reference manual that is complete and accurate, you always have to just shove some bytes around to see what happens. Start filling those memory blocks that are supposed to be the page frames until you see something change on the screen...

I dont see any specific graphics programming books in my library other than the abrash books like the graphics programming black book, which was at the tail end of this period of time. I have bios and dos programmers references and used ralf browns list too. I am sure that I had copies of the manuals for popular video chips (before the internet remember you called a human on that phone thing with a cord hanging out of it, the human took a printed manual, sometimes nicely bound sometimes just a staple in the corner if that, put it in an envelope and mailed it to you and that was your only copy unless you ran it through the copier). I have stacks of printed stuff that, sorry to say, am not going to go through to answer this question. I will keep this question in my mind though and look around some more for info, actually I may have some of my old programs handy, drawing fractals and other such things (direct as practical to the video card/memory).

EDIT.

I know you are looking for text mode stuff, and this is a graphics mode but it may or may not shed some light on what you are trying to do. combination of int calls and filling pages and palette memory directly.

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