Switch VGA Video Modes in Real Mode

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

nasm
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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.

Implement VGA video mode switching and direct memory access in boot sector code.

Requirements:

Use INT 0x10, AH=0x00 to switch between text modes
Print text in at least 2 different modes (e.g., 40x25 and 80x25)
Set cursor position using INT 0x10, AH=0x02
Write characters with attributes using direct VGA memory access
Print colored text in multiple colors
Show screen clearing
Include a simple text-based "UI" or border
Demonstrate cursor movement
Switch back to default mode before halting

Test:

Show mode switch
Show colored text
Show cursor positioning
Show direct VGA memory write
Show screen clear

VGA modes in real mode

Mode switching

nasm
Loading…

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

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.

Implement VGA video mode switching and direct memory access in boot sector code.

Requirements:

Use INT 0x10, AH=0x00 to switch between text modes
Print text in at least 2 different modes (e.g., 40x25 and 80x25)
Set cursor position using INT 0x10, AH=0x02
Write characters with attributes using direct VGA memory access
Print colored text in multiple colors
Show screen clearing
Include a simple text-based "UI" or border
Demonstrate cursor movement
Switch back to default mode before halting

Test:

Show mode switch
Show colored text
Show cursor positioning
Show direct VGA memory write
Show screen clear

Topic Overview

VGA modes in real mode

Mode switching

Restoring text mode

Why for this exercise

Implementation Task

Progressive Hints

Resources

Open Source

Switch VGA Video Modes in Real Mode

Topic Overview

VGA modes in real mode

Mode switching

Restoring text mode

Why for this exercise

Implementation Task

Progressive Hints

Resources

Open Source