Line feed (0AH) messing up with video interrupt INT 10H

Question

I am trying to write a simply code that uses nested loop, prints the character in the following pattern,

XXXXXXXXXX
XXXXXXXXX
XXXXXXXX
XXXXXXX
XXXXXX
XXXXX
XXXX
XXX
XX
X

and here is the code, and The code works absolutely fine

   MOV AX, @DATA                ; INITIALIZE DS
    MOV DS, AX



    ; PRINT X

    MOV CX,10
    MOV BX,10

    L2:
        PUSH CX
        MOV CX,BX
            L1:
                MOV DX, OFFSET HW            ; LOAD THE STRING
                MOV AH,09H
                INT 21H
            Loop L1
        SUB BX,01
        POP CX

        MOV DX,0AH
        MOV AH,02H
        INT 21H
    Loop L2


    MOV AH, 4CH                  ; RETURN CONTROL TO DOS
    INT 21H

but as soon as i use clear screen using video interrupt the output completely mess up,

Here is the output,

and here is the code for it,

.MODEL SMALL
 .STACK 100H

 .DATA
    HW  DB  "X$"

 .CODE
   MAIN PROC

    MOV AX, @DATA                ; INITIALIZE DS
    MOV DS, AX

    ; CLEAR SCREEN

    MOV AH, 06H
    MOV AL, 00H
    MOV CX, 00H
    MOV DH, 25
    MOV DL, 80
    MOV BH, 0FH 
    INT 10H

    ; PRINT X

    MOV CX,10
    MOV BX,10

    L2:
        PUSH CX
        MOV CX,BX
            L1:
                MOV DX, OFFSET HW            ; LOAD THE STRING
                MOV AH,09H
                INT 21H
            Loop L1
        SUB BX,01
        POP CX

        MOV DX,0AH
        MOV AH,02H
        INT 21H
    Loop L2





    MOV AH, 4CH                  ; RETURN CONTROL TO DOS
    INT 21H

   MAIN ENDP
 END MAIN

but when i remove the line feed that leave a line after each iteration, i.e

MOV DX,0AH
MOV AH,02H
INT 21H

the output is following as expected,

XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

I donot understand what the clear screen code has to do with the line feed code ? why they are messing up with each other. ?

Same issue occur when i am using Cursor Position set code instead of clear screen,

MOV AH, 02H
MOV BH, 00H
MOV CX, 0000H
MOV DX, 0C22H    
INT 10H

The output suppose to be the following in the middle of my screen,

XXXXXXXXXX
XXXXXXXXX
XXXXXXXX
XXXXXXX
XXXXXX
XXXXX
XXXX
XXX
XX
X

but it is displaying as below,

huseyin tugrul buyukisik · Accepted Answer

Part of your code:

MOV AH, 02H
MOV BH, 00H  ----> this is page number
MOV CX, 0000H
MOV DX, 0C22H   --->why such big number for column number?
INT 10H

Why are you using page 0h? Sure about you are using that page? How? I know its done with:

VIDEO - GET CURRENT VIDEO MODE

AH = 0Fh
int 10h
Return:
AH = number of character columns
AL = display mode (see #00010 at AH=00h)
BH = active page (see AH=05h) ---------------->you will need this

VIDEO - SET CURSOR POSITION

AH = 02h
BH = page number  <--------------now you know what to put here
0-3 in modes 2&3
0-7 in modes 0&1
0 in graphics modes
DH = row (00h is top)
DL = column (00h is left)
int 10h

Return:
Nothing

Source: Ralph Brown's interrupts page.

Display Pages

Graphics adapters can store several screens of text data (this is because displaying one screen of graphics requires significantly more memory than text). To fully use the display memory, it is divided into display pages. One display page can hold the data for one screen. The pages are numbered starting with 0; the number of pages available depends on the adapter and the display mode selected.

For 80 x 25 text mode, each display page is 4 KB. Display page 0 for text mode starts at address B800:0000h.

The active display page is the page currently being displayed. For 80 x 25 text mode, the memory requirement is 80 x 25 = 2000 words = 4000 bytes (i.e., the display does not use all of the 4 KB or 4096 bytes in the display page memory).

The video controller displays the first WORD in the active display page at the upper left corner of the screen (0,0), then displays the next WORD at (1,0), etc., displaying the screen row by row. The screen display can be looked at as the image of a two-dimensional array.

INT 10h Video Functions

00h: Set Video Mode.  Selects the video mode and clears the screen automatically.
Input:
    AH = 0
    AL = 3        ; mode 3 = 80 cols   x 25 rows color text
                  ; to avoid clearing the screen use mode 83h to set high bit
    AL = 6        ; sets medium resolution (640 x 200) graphics mode

When BIOS sets the display mode, it also clears the screen.

01h: Change Cursor Size
Input:
   AH = 1
   CH = starting scan line
   CL = ending scan line

The text mode cursor is displayed as a small dot array at a screen position. For the MDA and EGA, the dot array is 14 rows (0-13). For the CGA, the array is 8 rows (0-7). Normally rows 6 and 7 are lit for the CGA and rows 11 and 12 for the MDA/EGA cursor.

02h: Set Cursor Position.  Move cursor to specified position.
Input:
    AH = 2
    DH = row  (0-24)
    DL = col  (0-79 for 80x25 display)
    BH = video page number (usually 0)

03h: Get Cursor Position.   Return row and column position of cursor.
Input:
    AH = 3
    BH = video page number (usually 0)

Output:
    DH = row                     ; position
    DL = col
    CH = starting scan line     ; size
    CL = ending scan line

06h: Scroll the Screen or a Window Up
Input:
    AH = 6
    AL = number of lines to scroll (0 => whole screen)
    BH = attribute for blank lines
    CH, CL = row, column for upper left corner
    DH, DL = row, column for lower right window

Scrolling the screen up one line means to move each display line UP one row and insert a blank line at the bottom of the screen. The previous top row disappears from the screen.

The whole screen or any rectangular area (window) may be scrolled. AL contains the number of lines to scroll. If AL = 0, all the lines are scrolled and this clears the screen or window.

Example: Clear the screen to black for the 80x25 display.

MOV AH, 6      ; scroll up function
XOR AL, AL     ; clear entire screen
XOR CX, CX     ; upper left corner is (0,0)
MOV DX, 184FH  ; lower right corner is (4Fh, 18H)
MOV BH, 7      ; normal video attribute
INT 10H        ; clear screen

07h: Scroll the Screen/Window down
Input:
    AH = 7
    AL = number of lines to scroll (0 => whole screen)
    BH = attribute for blank lines
    CH, CL = row, column for upper left corner 
    DH, DL = row, column for lower right corner

Same as function 6, but lines are scrolled down instead of up.

08h: Read character at cursor. Can be used in either text or graphics mode.
Input:   AH = 8
         BH = page number
Output:  AH = attribute
         AL = ASCII code of character

09h: Write character and attribute.   Display any ASCII character at current position and set desired attribute.   Can be used in graphics or video mode.
Input:
    AH = 09
    AL = ASCII character code
    BH = video page number (usually 0)
    BL = attribute to be used. (In video mode, sets foreground (pixel) color)
    CX = repetition count

Note: cursor will not be moved beyond the last character displayed.

Also, if AL contains the ASCII code of a control character, a control function is not performed --- a display symbol is shown instead.

Example: change the attribute of the character under the cursor to reverse video for monochrome display.

        MOV AH, 8     ; read character
        XOR BH, BH    ; on page 0
        INT 10H       ; character in AL, attribute in AH

        MOV AH, 9     ; display character
        MOV CX, 1     ; display 1 character
        MOV BL, 70H   ; reverse video attribute
        INT 10H       ; display character  

0Ah: Write character.  Display any ASCII character at current position without changing the current attribute.  Can be used in text or video mode.
Input:
    AH = 0Ah
    AL = ASCII character code
    BH = video page number (usually 0)
    CX = repetition count

0Eh: Display Character and Advance Cursor
Input:   AH = 0Eh
         AL = ASCII code of character
         BH = page number
         BL = foreground color (graphics mode only)

This function displays the character in AL and advances the cursor to the next position in the row, or if at the end of a row, it sends it to the beginning of the next row. If the cursor is in the lower right corner, the screen is scrolled up and the cursor is set to the beginning of the last row. This is the BIOS function used by INT 21h, function 2, to display a character. The control characters bell (07h), backspace (08h), line feed (0Ah), and carriage return (0Dh) cause control functions to be performed.

Source is here.

Line feed (0AH) messing up with video interrupt INT 10H

Answers (1)

Related Questions