Redefining ‘A’ in VGA Mode 03h

In the 8086-era DOS environment, text display typically relied on the BIOS and the graphics adapter’s built-in character generator rather than any sophisticated font rendering system. The fonts were firmly tied to specific text modes implemented by adapters such as CGA, EGA, or VGA. One of the most common modes was text mode 03h, which presented an 80×25 text grid.

In one of our previous deep dives, we explored the infamous VGA-Mode 13h and how to integrate hardware timers to produce simple beeps with the PC-Speaker. This time, we’ll change the appearance of an ASCII character ( in this case 'A' ) by redefining its pixel data. In our next part you will also understand where this is actually heading for.

ASCII and the DOS Character Set

ASCII (American Standard Code for Information Interchange) is originally a 7-bit code that defines characters (letters, digits, punctuation) in the range 0–127. In DOS, the standard extended ASCII set (often called Code Page 437) added an additional 128 characters (128–255), including box-drawing characters, symbols, and other glyphs used frequently in the DOS world.

Text Mode 03h (80×25)

One of the typical display modes used in 8086 DOS for text output is Mode 03h. In Mode 03h, the screen is divided into 80 columns and 25 rows of text characters. Each position on the screen is one text cell which holds one character plus attributes (like foreground and background colors, blinking, etc.).

• With the earliest CGA (Color Graphics Adapter), each text cell was typically an 8×8 pixel matrix (though some adapters might effectively use 8×14 or 9×14 for EGA, and 9×16 for VGA).

• For standard VGA in 80×25 text mode, each character cell is often 9 pixels wide and 16 pixels tall (though the actual glyph itself might only occupy 8 pixels in width, with the 9th column used for spacing or other features).

If you consider the typical VGA text mode of 9×16 character cells in 80×25, the total screen resolution in terms of raw pixels is 720×400 (80 columns × 9 pixels wide, 25 rows × 16 pixels tall). In Mode 03h, each character is stored in video RAM (at ES:DI B800h:0000h) with a Byte for the character Index (00h - FFh) and another byte for defining foreground color and background color (0-F for the Background and 0-F for the Foreground).

How to?

There are times you might want to go beyond the default text-mode look. Maybe you want to display your own symbols, or get creative with ASCII graphics. In DOS and other low-level environments, you have an awesome ability to poke the BIOS or VGA memory directly to define what each character’s pixels look like. We will use the BIOS' interrupt 10h with AL = 11h to load our custom font. Let's break it down step by step:

Step By Step

First let's clone our startup environment again so that you can follow along:

git clone git@github.com:MilesTails01/i8086_boilercode.git

After downloading you need to unpack DOSBOX.zip and TOOLS.zip

cd \bin tar -xf TOOLS.zip tar -xf DOSBOX.zip build.bat // Compile, Link and Start the source debug.bat // Starts AFDEBUG and loads the compiled exe start.bat // Start the program without rebuilding it

Remove anything that is not needed. Keep the code simple and start with the minimum

; ################################# ; # ASCII.ASM # ; ################################# ; ################################# ; # STACK # ; ################################# STACK SEGMENT PARA STACK db 256 dup(0) STACK ENDS ; ################################# ; # DATA # ; ################################# DATA SEGMENT PARA 'DATA' DATA ENDS ; ################################# ; # CODE # ; ################################# CODE SEGMENT PARA 'CODE' ASSUME cs:CODE, ds:DATA, ss:STACK mov ax, DATA mov ds, ax mov ax, STACK mov ss, ax mov sp, 256 CODE ENDS

Step 1

Let's define our character pixel data first. 0 will become the background color and 1 will become the foreground color. Keep in mind in normal 80x25 Mode the dimension for a character are 9x16. We will define 14 rows each row containing 8bit cause (keep in mind to make the 9th bit 0 cause db is limited to 1 byte). This character will be striped block if you wonder what it will look like later.

CHAR_DATA db 010101010b db 010101010b db 010101010b db 010101010b db 010101010b db 010101010b db 010101010b db 010101010b db 010101010b db 010101010b db 010101010b db 010101010b db 010101010b db 010101010b

Step 2

Let's define a print function to fill the whole screen with the character A

PRINT PROC NEAR mov ax, 0B800h ; VGA text mode memory segment mov es, ax mov di, 0 ; Start at the top-left of the screen mov cx, 2000 ; 80x25 screen = 2000 characters mov al, 'A' ; ASCII code for 'A' mov ah, 19h ; Attribute byte (blue on white) rep stosw ; Fill screen with 'A' ret PRINT ENDP

Step 3

Now the important part: We will use interrupt 10h (Bios Interrupt) with AH set to 11h. I wrote the definition of the parameter for AX, BX, CX, DX and BP in the comments.

REPLACE_ASCII PROC NEAR ; mov ax, 03h ; Set video mode to text mode (80x25, color) ; int 10h ; 9x16 font size ; ; mov ax, 1112h ; Force 8x8 font size for character cells ; xor bl, bl ; RAM block ; int 10h ; Change text mode character (int 10h) ; AH = 11h ; BH = Number of bytes per character ; CX = Number of characters to change ; DX = Starting character to change ; ES:BP = Offset of character data push ds pop es ; make sure es = ds lea bp, CHAR_DATA ; Pointer to custom font data mov ax, 1100h ; Load user-defined font mov bh, 0Eh ; Number of bytes per character xor bl,bl ; RAM block mov cx, 01h ; Number of characters to replace = 1 for now mov dx, 41h ; Starting character to change (41h = 'A' in ASCII) int 10h ; Call BIOS to load the font ret REPLACE_ASCII ENDP

Step 4

Optional: If you like you can print the current ASCII table in the middle of the screen ;)

ASCIIPRINT PROC NEAR mov ax, 0B800h ; VGA text mode memory segment mov es, ax xor di, di ; Reset offset mov cx, 5 ; Total number of characters (0-255) mov ah, 4Fh ; Attribute byte (red on white) mov dl, 10 ; Padding on the left (adjust as needed) mov dh, 60 ; Total width of the centered line (adjust as needed) mov di, 1620 ; Row (10 x 80 + padding) * 2, starting with left padding PrintLoop: stosw ; Store the character (AL) and attribute (AH) inc al ; Move to the next ASCII character dec dh ; Decrement width counter jnz PrintLoop ; Continue until row width is filled add di, 40 ; Move to the next row with padding ((80 - 60) * 2) mov dh, 60 ; Reset row width loop PrintLoop ; Repeat for all 256 characters ret ASCIIPRINT ENDP

Result

Every character of A got replaced with the new striped block. This Glyph will come in very handy in a later article.

Complete Code

; ################################# ; # ASCII.ASM # ; ################################# ; ################################# ; # STACK # ; ################################# STACK SEGMENT PARA STACK db 256 dup(0) STACK ENDS ; ################################# ; # DATA # ; ################################# DATA SEGMENT PARA 'DATA' CHAR_DATA db 010101010b db 010101010b db 010101010b db 010101010b db 010101010b db 010101010b db 010101010b db 010101010b db 010101010b db 010101010b db 010101010b db 010101010b db 010101010b db 010101010b DATA ENDS ; ################################# ; # CODE # ; ################################# CODE SEGMENT PARA 'CODE' ASSUME cs:CODE, ds:DATA, ss:STACK mov ax, DATA mov ds, ax mov ax, STACK mov ss, ax mov sp, 256 ; ################################# ; # MAIN # ; ################################# call REPLACE_ASCII call PRINT call ASCIIPRINT ret ; ============================= ; | FUNCTIONS | ; ============================= PUBLIC REPLACE_ASCII REPLACE_ASCII PROC NEAR ; mov ax, 03h ; Set video mode to text mode (80x25, color) ; int 10h ; 9x16 font size ; ; mov ax, 1112h ; Force 8x8 font size for character cells ; xor bl, bl ; RAM block ; int 10h ; Change text mode character (int 10h) ; AH = 11h ; BH = Number of bytes per character ; CX = Number of characters to change ; DX = Starting character to change ; ES:BP = Offset of character data push ds pop es ; make sure es = ds lea bp, CHAR_DATA ; Pointer to custom font data mov ax, 1100h ; Load user-defined font mov bh, 0Eh ; Number of bytes per character xor bl,bl ; RAM block mov cx, 01h ; Number of characters to replace = 1 for now mov dx, 41h ; Starting character to change (41h = 'A' in ASCII) int 10h ; Call BIOS to load the font ret REPLACE_ASCII ENDP PRINT PROC NEAR mov ax, 0B800h ; VGA text mode memory segment mov es, ax mov di, 0 ; Start at the top-left of the screen mov cx, 2000 ; 80x25 screen = 2000 characters mov al, 'A' ; ASCII code for 'A' mov ah, 19h ; Attribute byte (blue on white) rep stosw ; Fill screen with 'A' ret PRINT ENDP ASCIIPRINT PROC NEAR mov ax, 0B800h ; VGA text mode memory segment mov es, ax xor di, di ; Reset offset mov cx, 5 ; Total number of characters (0-255) mov ah, 4Fh ; Attribute byte (red on white) mov dl, 10 ; Padding on the left (adjust as needed) mov dh, 60 ; Total width of the centered line (adjust as needed) mov di, 1620 ; Row (10 x 80 + padding) * 2, starting with left padding PrintLoop: stosw ; Store the character (AL) and attribute (AH) inc al ; Move to the next ASCII character dec dh ; Decrement width counter jnz PrintLoop ; Continue until row width is filled add di, 40 ; Move to the next row with padding ((80 - 60) * 2) mov dh, 60 ; Reset row width loop PrintLoop ; Repeat for all 256 characters ret ASCIIPRINT ENDP CODE ENDS END

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