Can't paint a pixel using VESA. How it works?

[mrjbom]

Hey. I asked a similar question earlier, but never found an answer. Please help again.

I use GRUB as my bootloader. In the kernel file.asm I run VESA mode with the screen 800x600 8 bbp. The file itself looks like this:

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bits 32
%define MULTIBOOT_MAGIC 0x1BADB002
%define MULTIBOOT_FLAGS (1<<0 | 1<<1 | 1<<2)

section .text
align 4
multiboot_header:
    dd MULTIBOOT_MAGIC
    dd MULTIBOOT_FLAGS
    dd -(MULTIBOOT_MAGIC + MULTIBOOT_FLAGS)
    dd 0
    dd 0
    dd 0
    dd 0
    dd 0
    dd 0
    dd 800 ; width
    dd 600 ; height
    dd 8 ; bbp
 
global start
extern kmain
 
start:
  cli
  mov esp, stack_space
  call kmain
  hlt

section .bss
resb 8192
stack_space:

Here's my kernel.c:

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#include "kernel.h"

void kmain(void)
{
    memset((void*)0xFD000000, 0xFF0000, sizeof(0xFD000000));
    return;
}

The QEMU window changes in size, which means that VESA mode is enabled.
According to this article, the video memory address of QEMU is 0xFD000000.
Using

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memset

I try to paint the whole screen red, but nothing comes out.
Please, help.

[klange]

According to what article? I will correct it.

QEMU does not use a fixed address for the framebuffer. Grub will pass the framebuffer address to you in the multiboot struct (and you can also locate it through PCI).

[iansjack]

What do you suppose sizeof(0xFD000000) is?

So how many pixels are you setting?

(I can confirm that 0xFD000000 is the correct value for the address of the framebuffer in this case.)

[pvc]

Your memset is wrong. First, memset operates on bytes (at lest on logical level). So your color 0xFF0000 is truncated to byte 0x00.
Second, sizeof operator is used incorrectly. sizeof(0xFD000000) returns 4, which is the same as sizeof(int) So you would change just 4 bytes. You should calculate size of the framebuffer using fomula

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width * height * bits_per_pixel / 8  // not always correct

or

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height * pitch // this is correct way to do it

Where pitch is number of bytes per scanline (can be found in multiboot_info structure).
Next, 8 bit mode doesn't know about RGB, it uses palette instead. I think 32 bit color would be a better option.
Also assuming fixed framebuffer address (0xFD000000 in your case) is reaaaaly bad thing to do. Proper way here is to parse multiboot_info structure which is pointed to by ebx.

[mrjbom]

Drawing In Protected Mode

According to what article? I will correct it.

[mrjbom]

QEMU does not use a fixed address for the framebuffer. Grub will pass the framebuffer address to you in the multiboot struct (and you can also locate it through PCI).

I tried to do it, but it didn't work. I used the code for multiboot info from this site.

Complement of its kernel.asm up to this:

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bits 32

%define MULTIBOOT_MAGIC 0x1BADB002
%define MULTIBOOT_FLAGS (1<<0 | 1<<1 | 1<<2)
%define KERNEL_BASE     0xFFFF800000000000

section .text
align 4
multiboot_header:
    dd MULTIBOOT_MAGIC
    dd MULTIBOOT_FLAGS
    dd -(MULTIBOOT_MAGIC + MULTIBOOT_FLAGS)
    dd 0
    dd 0
    dd 0
    dd 0
    dd 0
    dd 0
    dd 800 ; width
    dd 600 ; height
    dd 32 ; bbp
 
global start
extern kmain
 
start:
  cli
  mov esp, stack_space
  mov [multiboot_info_ptr - KERNEL_BASE], ebx
  call kmain
  hlt

section .bss
multiboot_info_ptr
resb 8192
stack_space:

kernel.c looks like this:

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#include "kernel.h"

#define VESA_WIDTH 800
#define VESA_HEIGHT 600
#define VESA_BBP 32
#define VESA_SIZE VESA_WIDTH * VESA_HEIGHT * VESA_BBP / 8

void kmain(unsigned long magic, unsigned long addr)
{
    multiboot_info_t *mbi;
    mbi = (multiboot_info_t *) addr;
    memset((void*)mbi->framebuffer_addr, 0xFF0000, VESA_SIZE);
    return;
}

But I see only a black screen again. I suspect grub doesn't give me the address on the multiboot info. But I can't check it.

[mrjbom]

I tried to do this: memset((void*)0xFD000000, 0xFF0000, VESA_SIZE);
VESA_SIZE is calculated by this formula VESA_SIZE = VESA_WIDTH * VESA_HEIGHT * VESA_BBP / 8

What do you suppose sizeof(0xFD000000) is?

So how many pixels are you setting?

(I can confirm that 0xFD000000 is the correct value for the address of the framebuffer in this case.)

[MichaelPetch]

You never pushed the parameters for kmain before calling it. Since you have the magic number first and the multiboot pointer second you'd have to do push ebx then push eax and then do call kmain.

[mrjbom]

You never pushed the parameters for kmain before calling it. Since you have the magic number first and the multiboot pointer second you'd have to do push ebx then push eax and then do call kmain.

I added the code kernel.asm, now it looks like this:

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bits 32

%define MULTIBOOT_MAGIC 0x1BADB002
%define MULTIBOOT_FLAGS (1<<0 | 1<<1 | 1<<2)
%define KERNEL_BASE     0xFFFF800000000000

section .text
align 4
multiboot_header:
    dd MULTIBOOT_MAGIC
    dd MULTIBOOT_FLAGS
    dd -(MULTIBOOT_MAGIC + MULTIBOOT_FLAGS)
    dd 0
    dd 0
    dd 0
    dd 0
    dd 0
    dd 0
    dd 800 ; width
    dd 600 ; height
    dd 32 ; bbp
 
global start
extern kmain
 
start:
  cli
  mov esp, stack_space
  mov [multiboot_info_ptr - KERNEL_BASE], ebx
  push ebx
  push eax
  call kmain
  hlt

section .bss
multiboot_info_ptr
resb 8192
stack_space:

But it doesn't help, the screen is still black, not red. Apparently, the address on multiboot info on prematurely doesn't turn.

[MichaelPetch]

Although this code falsely assumes the requested video mode of 800x600x32bpp is the one GRUB set, it should be enough to get you going:

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bits 32

%define MULTIBOOT_MAGIC 0x1BADB002
%define MULTIBOOT_FLAGS (1<<0 | 1<<1 | 1<<2)

section .text
align 4
multiboot_header:
    dd MULTIBOOT_MAGIC
    dd MULTIBOOT_FLAGS
    dd -(MULTIBOOT_MAGIC + MULTIBOOT_FLAGS)
    dd 0
    dd 0
    dd 0
    dd 0
    dd 0
    dd 0
    dd 800 ; width
    dd 600 ; height
    dd 32 ; bbp

global start
extern kmain

start:
  cli
  mov esp, stack_space
  push ebx
  push eax
  call kmain
  hlt

section .bss
resb 8192
stack_space:

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#include <stdint.h>
#include "multiboot.h"

void kmain(uint32_t magic, multiboot_info_t *mbi)
{
    (void) magic; /* Silence compiler warning since magic isn't used */
    volatile uint32_t *fb = (uint32_t *)((uint32_t) mbi->framebuffer_addr);
    uint32_t cell;

    for (cell = 0; cell < mbi->framebuffer_width * mbi->framebuffer_height; cell++)
        fb[cell] = 0x00ff0000;

    return;
}

The code above is not generic and assumes 32bpp (not 8 ). I leave it up to the reader to handle pitch and bits per pixels properly.

[mrjbom]

Although this code falsely assumes the requested video mode of 800x600x32bpp is the one GRUB set, it should be enough to get you going

Perfectly. Thank you very much, you helped me very much.