Display System

The display system provides low-level screen manipulation functions for text-mode display output. It interfaces directly with video memory at address 0xb8000 and provides basic text output functionality with color support.

Features

• Text-mode display (25 lines × 80 columns)
• Color text support (16 colors)
• Direct video memory access
• Automatic scrolling
• Cursor management

Screen Specifications

• Screen dimensions: 25 lines × 80 columns
• Each screen element consists of 2 bytes:
  • First byte: ASCII character
  • Second byte: Attribute byte (text color and background color)
• Total screen size: 4000 bytes (25 × 80 × 2)

API Reference

Basic Output

void clear_screen(void)

Clears the entire screen by filling it with spaces and setting the default color attribute (light grey on black).

void print(const char *str)

Prints a null-terminated string to the screen using the default color (light grey on black).

Parameters:

• str: Null-terminated string to print. If NULL, the function returns without doing anything.

Features:

• Handles newline characters (\n)
• Automatically scrolls when reaching the bottom of the screen
• Stops printing if screen buffer is full

Color Output

void printc(const char *str, unsigned char color)

Prints a null-terminated string to the screen using the specified color attribute.

Parameters:

• str: Null-terminated string to print
• color: Color attribute byte specifying text and background colors

Cursor Control

void print_newline(void)

Moves the cursor to the beginning of the next line.

Features:

• Automatically scrolls the screen up when reaching the bottom
• Clears the new line at the bottom after scrolling

void backspace(void)

Removes the character before the current cursor position.

Features:

• Moves cursor back one position
• Clears the character at that position
• Only operates if there is space to move back

Color Constants

Constant	Value	Description
CL_BLACK	0x00	Black
CL_BLUE	0x01	Blue
CL_GREEN	0x02	Green
CL_CYAN	0x03	Cyan
CL_RED	0x04	Red
CL_MAGENTA	0x05	Magenta
CL_BROWN	0x06	Brown
CL_LIGHT_GREY	0x07	Light Grey (Default)
CL_DARK_GREY	0x08	Dark Grey
CL_LIGHT_BLUE	0x09	Light Blue
CL_LIGHT_GREEN	0x0A	Light Green
CL_LIGHT_CYAN	0x0B	Light Cyan
CL_LIGHT_RED	0x0C	Light Red
CL_LIGHT_MAGENTA	0x0D	Light Magenta
CL_YELLOW	0x0E	Yellow
CL_WHITE	0x0F	White

Implementation Details

Memory Layout

• Video memory begins at physical address 0xb8000
• Each character occupies two consecutive bytes:
  • Byte 1: ASCII character code
  • Byte 2: Attribute byte (foreground and background colors)

Scrolling Mechanism

When the screen is full:

1. Copy all lines up one position
2. Clear the bottom line
3. Adjust cursor position
4. Update video memory

Color Attributes

The attribute byte is divided into:

• Bits 0-3: Foreground color
• Bits 4-6: Background color
• Bit 7: Blink bit (not commonly used)

Usage Examples

// Clear the screen
display::clear_screen();

// Print text in default color
display::print("Hello, World!\n");

// Print colored text
display::printc("Error: ", CL_RED);
display::printc("File not found\n", CL_WHITE);

// Print multiple lines
display::print("Line 1\nLine 2\nLine 3");