vesa - VESA linear framebuffer driver

Header: kernel/include/kernel/vesa.h
Source: kernel/arch/i386/vesa.c
Config: kernel/include/kernel/vesa_config.h

Locates the Multiboot 2 framebuffer tag and exposes the linear framebuffer for pixel-level drawing. The GRUB Multiboot 2 header (in boot.S) requests a direct-colour RGB framebuffer at a preferred resolution defined in vesa_config.h.

Configuration (vesa_config.h)

The bootloader may supply a different resolution if the hardware does not support the preferred one; code must always use the values from vesa_fb_t, not the compile-time constants.

Data structures

vesa_fb_t

typedef struct {
    uint32_t *addr;
    uint32_t  pitch;
    uint32_t  width;
    uint32_t  height;
    uint8_t   bpp;
    uint8_t   red_shift;
    uint8_t   red_bits;
    uint8_t   green_shift;
    uint8_t   green_bits;
    uint8_t   blue_shift;
    uint8_t   blue_bits;
} vesa_fb_t;

Describes the framebuffer as reported by the bootloader. The RGB shift and bit-width fields are used to compose pixel values for any packed-pixel format; see vesa_put_pixel.

Functions

vesa_init

bool vesa_init(multiboot2_info_t *mbi);

Walk the Multiboot 2 tag list to find the framebuffer tag (type 8). If found and the framebuffer type is direct-colour RGB, populate the global vesa_fb_t descriptor.

Returns true on success. Returns false and logs a reason if:

• mbi is NULL.
• No framebuffer tag is present.
• The framebuffer is not direct-colour RGB (e.g. EGA text or indexed colour).
• bpp is zero or not byte-aligned.

vesa_get_fb

const vesa_fb_t *vesa_get_fb(void);

Return a pointer to the global framebuffer descriptor, or NULL if vesa_init() has not been called successfully. Read-only; callers must not modify the returned struct.

vesa_put_pixel

void vesa_put_pixel(uint32_t x, uint32_t y, uint32_t colour);

Write a pixel at (x, y). colour is a 32-bit packed value in 0x00RRGGBB format; the function shifts and masks the channels to match the framebuffer’s reported layout before writing the correct number of bytes.

No bounds checking is performed. Out-of-bounds coordinates will corrupt adjacent framebuffer memory or crash the system.

vesa_clear

void vesa_clear(uint32_t colour);

Fill the entire framebuffer with a single colour by calling vesa_put_pixel for every pixel. Used by vesa_tty_init() to clear the screen.

Future work

• Hardware-accelerated memset-style fill using REP STOSD for faster clears and rectangle fills.
• Support for bpp values other than 32 (e.g. 16-bit 5-6-5 colour).
• Double-buffering: render to an off-screen buffer and flip to reduce visible tearing.
• Mode-setting: negotiate resolution with the firmware at runtime rather than relying solely on the Multiboot 2 framebuffer tag.