VGA: only part of screen

[Awe2K]

So, I've switched to 800x600x32 mode, and tried to fill screen with white color.
Hovewer, it doesn't work for some reason, maybe I am wrong? Please help me.
Thanks, Awe2K.

Screenshot:
http://imgur.com/LNKt64f

Code:
My putpixel:

Code: Select all

static void putpixel(int x, int y, int color) {
	unsigned char * screen = vid_mem;
	unsigned where = x * 3 + y * 2400;
	screen[where] = color & 255;              // BLUE
	screen[where + 1] = (color >> 8) & 255;   // GREEN
	screen[where + 2] = (color >> 16) & 255;  // RED
}

My fill:

Code: Select all

int x,y;
	for (x=0;x<800;x++)
	for (y=0;y<600;y++)
	putpixel(x,y,bgc); // bgc==0xFFFFFF

[Octocontrabass]

Did you switch to a mode with a linear frame buffer? If not, you'll have to flip between pages in the frame buffer in order to fill the entire screen.

Does vid_mem point to the correct address?

800x600x32

unsigned where = x * 3 + y * 2400;

How many bytes are 32 bits? How many bits are in 3 bytes?

[Awe2K]

Did you switch to a mode with a linear frame buffer? If not, you'll have to flip between pages in the frame buffer in order to fill the entire screen.

I've used GRUB to switch mode (specified in multiboot header).

Does vid_mem point to the correct address?

Yes, it is 0xA0000

800x600x32
unsigned where = x * 3 + y * 2400;

How many bytes are 32 bits? How many bits are in 3 bytes?

Yes, I know 32 bits is for 4 bytes, and 3 bytes are 24 bits. But if I change 3 to 4 and 2400 to 3200, then it just gets worse.

Screenshot (after editing code as you mentioned):
http://imgur.com/LXqhhmI

[Awe2K]

Also, here are the screenshots of how fonts work both ways (my crash screen):
x*3+y*2400 http://imgur.com/LIY5oEL
x*4+y*3200 http://imgur.com/y6PerOv

[Octocontrabass]

Does vid_mem point to the correct address?

Yes, it is 0xA0000

A framebuffer at 800x600x32 is about 1.8 megabytes. There is only 128 kilobytes of address space available for a framebuffer at that address. You either have the wrong address or you aren't using a LFB mode.

Yes, I know 32 bits is 4 bytes, and 3 bytes are 24 bits. But if I change 3 to 4 and 2400 to 3200, then it just gets worse.

That's because you're actually in a 24bpp mode, not 32bpp.

[Awe2K]

In boot.asm

Code: Select all

   dd 0    ; # 0 = set graphics mode
   dd 800  ; # 800 = width
   dd 600  ; # 600 = height
   dd 32   ; # 32 = bpp

Second flag in multiboot header is set.
May my GRUB be trashing vga mode?

[Awe2K]

Also, how can I get linear frame buffer with GRUB?

[Awe2K]

Bump?

[Octocontrabass]

May my GRUB be trashing vga mode?

GRUB is not guaranteed to set the exact mode you've requested.

Also, how can I get linear frame buffer with GRUB?

How do you know it's not already a linear frame buffer?

Bump?

Six hours is not very long. Some of us have things to do outside of posting on internet forums.

[Awe2K]

I know that setting video mode manually won't give linear frame buffer.But I thought grub does so. Or I have to switch back to real mode and set this mode manually?

[Brendan]

Hi,

I know that setting video mode manually won't give linear frame buffer.But I thought grub does so. Or I have to switch back to real mode and set this mode manually?

If the "video mode number" that GRUB told you it used (in the multi-boot information structure) has bit 14 set (which is very likely) then the video mode is using LFB; and in that case you've probably got the address 0xA0000 from the wrong part of the VBE mode information structure (e.g. from the "window A base address" field and not from the "LFB base address" field).


Cheers,

Brendan

[Awe2K]

Hi,

I know that setting video mode manually won't give linear frame buffer.But I thought grub does so. Or I have to switch back to real mode and set this mode manually?

If the "video mode number" that GRUB told you it used (in the multi-boot information structure) has bit 14 set (which is very likely) then the video mode is using LFB; and in that case you've probably got the address 0xA0000 from the wrong part of the VBE mode information structure (e.g. from the "window A base address" field and not from the "LFB base address" field).


Cheers,

Brendan

But if bit 14 is not set, how do I achieve that?

[Brendan]

Hi,

If the "video mode number" that GRUB told you it used (in the multi-boot information structure) has bit 14 set (which is very likely) then the video mode is using LFB; and in that case you've probably got the address 0xA0000 from the wrong part of the VBE mode information structure (e.g. from the "window A base address" field and not from the "LFB base address" field).

But if bit 14 is not set, how do I achieve that?

If bit 14 isn't set, then GRUB is buggy and you're mostly screwed. You'd either have to refuse to boot, switch to real mode and set a video mode yourself to work around the problem, or setup segments for (one of) VBE's protected mode interfaces so that you can do bank switching (and write code that switches banks whenever it needs to access a different 64 KiB bank of display memory).

Of course it's extremely likely that bit 14 is set.


Cheers,

Brendan

[Awe2K]

Ok, now I've got my mboot->vbe_mode bit 14 set, how do I know linear address then, or where are framebuffer segments located?

[Awe2K]

Okay, it seems I've solved it. Grub simply gave me wrong vga memory offset. (I've forgot packed attr. within mboot struct)