MS-DOS VGA Video Register & Text Mode Corruption

msdos-textmode-corruption.txt

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          MS-DOS VGA VIDEO HARDWARE & DIRECT MEMORY MANIPULATION
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[1] BIOS INTERFACE FONT MISMATCHES (INT 10h)
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Forces the VGA BIOS to map non-native physical ROM font heights into the active 
16-pixel high text grid. Forcing mismatched metrics causes row duplication, 
truncation, and structural distortions.

; --- Skinny/Thorned Font (Map 7x14 Font into 16-pixel Grid) ---
mov ax, 1101h       ; Subfunction 11h: Load ROM 7x14 Font
mov bx, 0000h       ; BH = Font height (0=Default), BL = Font Block (0)
int 10h             ; Result: Truncates columns; leaves sharp, jagged serifs
int 20h             ; Terminate Program

; --- Row Doubling Font (Map 8x14 Font into 16-pixel Grid) ---
mov ax, 1102h       ; Subfunction 11h: Load ROM 8x14 Font
mov bx, 0000h       ; BH = Font height (0=Default), BL = Font Block (0)
int 10h             ; Result: Hardware duplicates bottom pixel rows to fill cell
int 20h

; --- Squashed Faux-Gothic Font (Map 8x8 Font into 16-pixel Grid) ---
mov ax, 1104h       ; Subfunction 11h: Load ROM 8x8 Font
mov bx, 0000h       ; BH = Font height (0=Default), BL = Font Block (0)
int 10h             ; Result: Flattens font to top 8 pixels; lower half blank
int 20h

; --- Split Block Character Map Selection ---
mov ax, 1103h       ; Subfunction 11h: Set Block Specifier
mov bx, 0004h       ; BL bitmask: Selects character map source blocks in Plane 2
int 10h             ; Result: Cross-contaminates maps; injects glitch artifacts
int 20h


[2] HARDWARE PORT REGISTER MANIPULATION (CRTC / ATTRIBUTE CONTROLLER)
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Bypasses the BIOS entirely to talk directly to VGA I/O ports. Manipulates 
horizontal pixel clocks, raster beam scanline timing, and column cell widths.

; --- Scanline Compression (CRTC Registers) ---
mov dx, 03d4h       ; DX = CRTC Controller Address Port
mov al, 09h         ; Index 09h: Maximum Scan Line Register
out dx, al
inc dx              ; DX = CRTC Data Port (03d5h)
mov al, 07h         ; Force character cell to 7 pixels tall (Default = 15)
out dx, al          ; Result: Compresses active text block into sharp, tiny grid
int 20h

; --- 9-Dot Stripping & Spacing Shift (Attribute Controller) ---
mov dx, 03dah       ; Status Register 1 (Reading resets 3C0h Address/Data flip-flop)
in al, dx
mov dx, 03c0h       ; DX = Attribute Controller Port
mov al, 10h         ; Index 10h: Attribute Mode Control Register
out dx, al
mov al, 00h         ; Clear Bit 0 (Graphics Mode) & Bit 2 (Line Graphics Enable)
out dx, al          ; Result: Slices 9th column padding; horizontal text squish
int 20h


[3] DIRECT VIDEO MEMORY MANIPULATION (Segment B800h)
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Bypasses both BIOS and video hardware logic by writing directly to screen RAM. 
VGA Text Mode maps memory sequentially as alternating pairs: [Character, Attribute].

; --- Screen Character/Attribute Flood Loop ---
mov ax, b800h       ; Segment B800h = Color Text Video Memory Address
mov es, ax
mov di, 0000h       ; DI = Top-left corner offset of screen
mov cx, 07d0h       ; CX = 2000 loops (80 columns x 25 rows = 4000 bytes)
mov ax, 8a8ah       ; AH = Attribute (8Ah = Blinking Light Green), AL = Char (8Ah)
rep stosw           ; Blast AX into ES:DI, incrementing DI automatically
int 20h             ; Result: Instant fullscreen matrix-style textured glitch wall

; --- Flash-Overwriting Character Bytes Only ---
mov ax, b800h       ; Point ES to video RAM segment
mov es, ax
mov di, 0000h       ; Start at top-left
mov cx, 07d0h       ; Target entire screen
mov al, b0h         ; AL = Shading character (░ Block)
@loop:
mov es:[di], al     ; Overwrite character byte
add di, 2           ; Skip the next byte (preserves existing color attribute)
loop @loop
int 20h


[4] RESTORE / CLEAN RESET OVERRIDE
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Flushes custom register tables, terminates text distortions, reinitializes the 
standard 8x16 font array from ROM, and blanks screen back to plain Mode 3.

; --- Absolute Mode 3 Reinitialization ---
mov ax, 0003h       ; AH = 00h (Set Video Mode Subfunction), AL = 03h (80x25 Text)
int 10h             ; Triggers clean hardware state restore
int 20h
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