Switch VGA Video Modes in Real Mode

Topic Overview

VGA modes in real mode

INT 0x10 can switch video modes before an OS graphics driver exists. Mode 0x13 gives 320x200x256 graphics; mode 0x03 is standard 80x25 text. For example, after mov ax,0x0013; int 0x10, byte writes to 0xA000:0000 paint pixels in planar layout unless you treat it as chain-4.

Mode switching

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• Text mode: Color cells at 0xB8000 (attribute in high byte)
• Mode 13h: Linear-ish 256-color buffer, one byte per pixel
• Mode 12h: 640x480x16 planar, requires VGA register setup
• Always set DS/ES to 0xA000 before plotting in graphics modes

Return to text with mov ax,0x0003; int 0x10 before rebooting.

Restoring text mode

If you leave graphics mode without resetting, the BIOS boot menu on the next warm reboot may be invisible. Document palette changes when using mode 13h so graders see intentional colors. Writing directly to 0xA0000 without setting graphics mode produces garbage in text mode because the memory maps to different decode windows depending on the sequencer state.

Why for this exercise

You will switch VGA modes and draw pixels or characters through the framebuffer. This exercise requires using INT 0x10 for mode set and addressing video RAM with the correct segment.

Compute where a character lands in the 0xB8000 text buffer and decode its attribute byte.

Input (stdin, one line): column row charhex attrhex: the target cell of the 80x25 screen (mode 0x03), the ASCII code of the character in hex, and the attribute byte in hex. Example: "0 24 41 1F" puts a white-on-blue 'A' in the bottom-left corner.

Requirements:

• Cell offset = (row * 80 + column) * 2 (80 columns per row, 2 bytes per cell: char byte first, then attribute byte)
• char_addr = 0xB8000 + offset, attr_addr = char_addr + 1
• Attribute byte: bits 0-3 foreground color, bits 4-7 background color (bit 7 doubles as the blink bit)
• Standard 16-color palette: 0 black, 1 blue, 2 green, 3 cyan, 4 red, 5 magenta, 6 brown, 7 light grey, 8 dark grey, 9 light blue, 10 light green, 11 light cyan, 12 light red, 13 light magenta, 14 yellow, 15 white
• Print the printable ASCII character itself; for codes outside 32-126 print '?'

Output exactly these lines: offset=0x<4 hex digits uppercase> char_addr=0x<5 hex digits uppercase> attr_addr=0x<5 hex digits uppercase> char=<c> fg=<decimal> <color name> bg=<decimal> <color name> blink=on|off