4_writing_kernel_in_C.md

Writing kernel in C

Writing C program in Protected Mode

Now, we start writing our kernel in C. [commit]

Debugging steps are the same as what we did in the previous note "5. Build, run, and debug".

Writing "Hello, World!" in Protected Mode

What is Text Mode?

• Write ASCII to video memory
  • starting at address 0xB8000 for colors, or
  • starting at address 0xB0000 for monochrome
• Supports 16 unique colors, e.g.,
  • color number 1, Black, RGB = 0 0 0, HEX = 00 00 00
  • color number 2, Blue, RGB = 0 0 170, HEX = 00 00 AA
  • ...
  • color number 15, White, RGB = 255 255 255, HEX = FF FF FF
• No need to set individual screen pixels for printing characters
  • The video card will take the ASCII value and automatically reference it in a font table to output pixels for the letter.

Each character will take 2 bytes.

• Byte 0 = ASCII character
• Byte 1 = Color

Example: Display "AB" in white

0xB8000 = 'A'
0xB8001 = 0x0F
0xB8002 = 'B'
0xB8003 = 0x0F

Writing to video memory in C

char *video_mem = (char *)0xB8000;
video_mem[0] = 'A';
video_mem[1] = '2'; // Green

but, we can optimize this by using uint16_t

#include <stdint.h>

...

uint16_t *video_mem = (uint16_t *)0xB8000;
video_mem[0] = 0x0241; // 'A' (65 decimal or 0x41 hex) + Green (0x02) in the little-endian format

and with a bit more helper functions, we can call print and write "Hello, World!".

[commit]

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