How do i switch from 80*25 vga text mode to 320*200 mode

[GamerWolf548]

in the past 2-3 days. I've been looking for a way to switch between vga modes. With some example code written in C. But i couldn't find anything helpful. Please help

[crosssans]

Have you done any kind of attempt (whether it's just simple code or anything else)? Do you have a hard time trying to understand something in the documentation? Tell us everything!

[rdos]

Nobody should use the 320x200 mode anymore. Bit-plane and palette mapped modes are big no-nos. If you want to use VGA / VBE, make sure you use one that uses RGB components directly. The best alternative is probably to boot with EFI and use the EFI mode linear frame buffer. It's a lot easier than to set up a decent VBE mode.

[Klakap]

Easiest way is use bios:

Code: Select all

  mov ah, 0x00
  mov al, 13 ;300x220 mode
  int 10h

If this isn't working for you, this code (from https://files.osdev.org/mirrors/geezer/ ... cs/modes.c) can do same result:

Code: Select all

#define	VGA_AC_INDEX		0x3C0
#define	VGA_AC_WRITE		0x3C0
#define	VGA_AC_READ		0x3C1
#define	VGA_MISC_WRITE		0x3C2
#define VGA_SEQ_INDEX		0x3C4
#define VGA_SEQ_DATA		0x3C5
#define	VGA_DAC_READ_INDEX	0x3C7
#define	VGA_DAC_WRITE_INDEX	0x3C8
#define	VGA_DAC_DATA		0x3C9
#define	VGA_MISC_READ		0x3CC
#define VGA_GC_INDEX 		0x3CE
#define VGA_GC_DATA 		0x3CF
/*			COLOR emulation		MONO emulation */
#define VGA_CRTC_INDEX		0x3D4		/* 0x3B4 */
#define VGA_CRTC_DATA		0x3D5		/* 0x3B5 */
#define	VGA_INSTAT_READ		0x3DA

#define	VGA_NUM_SEQ_REGS	5
#define	VGA_NUM_CRTC_REGS	25
#define	VGA_NUM_GC_REGS		9
#define	VGA_NUM_AC_REGS		21
#define	VGA_NUM_REGS		(1 + VGA_NUM_SEQ_REGS + VGA_NUM_CRTC_REGS + \
				VGA_NUM_GC_REGS + VGA_NUM_AC_REGS)

unsigned char g_320x200x256[] =
{
/* MISC */
	0x63,
/* SEQ */
	0x03, 0x01, 0x0F, 0x00, 0x0E,
/* CRTC */
	0x5F, 0x4F, 0x50, 0x82, 0x54, 0x80, 0xBF, 0x1F,
	0x00, 0x41, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
	0x9C, 0x0E, 0x8F, 0x28,	0x40, 0x96, 0xB9, 0xA3,
	0xFF,
/* GC */
	0x00, 0x00, 0x00, 0x00, 0x00, 0x40, 0x05, 0x0F,
	0xFF,
/* AC */
	0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07,
	0x08, 0x09, 0x0A, 0x0B, 0x0C, 0x0D, 0x0E, 0x0F,
	0x41, 0x00, 0x0F, 0x00,	0x00
};

void write_regs(unsigned char *regs)
{
	unsigned i;

/* write MISCELLANEOUS reg */
	outportb(VGA_MISC_WRITE, *regs);
	regs++;
/* write SEQUENCER regs */
	for(i = 0; i < VGA_NUM_SEQ_REGS; i++)
	{
		outportb(VGA_SEQ_INDEX, i);
		outportb(VGA_SEQ_DATA, *regs);
		regs++;
	}
/* unlock CRTC registers */
	outportb(VGA_CRTC_INDEX, 0x03);
	outportb(VGA_CRTC_DATA, inportb(VGA_CRTC_DATA) | 0x80);
	outportb(VGA_CRTC_INDEX, 0x11);
	outportb(VGA_CRTC_DATA, inportb(VGA_CRTC_DATA) & ~0x80);
/* make sure they remain unlocked */
	regs[0x03] |= 0x80;
	regs[0x11] &= ~0x80;
/* write CRTC regs */
	for(i = 0; i < VGA_NUM_CRTC_REGS; i++)
	{
		outportb(VGA_CRTC_INDEX, i);
		outportb(VGA_CRTC_DATA, *regs);
		regs++;
	}
/* write GRAPHICS CONTROLLER regs */
	for(i = 0; i < VGA_NUM_GC_REGS; i++)
	{
		outportb(VGA_GC_INDEX, i);
		outportb(VGA_GC_DATA, *regs);
		regs++;
	}
/* write ATTRIBUTE CONTROLLER regs */
	for(i = 0; i < VGA_NUM_AC_REGS; i++)
	{
		(void)inportb(VGA_INSTAT_READ);
		outportb(VGA_AC_INDEX, i);
		outportb(VGA_AC_WRITE, *regs);
		regs++;
	}
/* lock 16-color palette and unblank display */
	(void)inportb(VGA_INSTAT_READ);
	outportb(VGA_AC_INDEX, 0x20);
}

void set_vga_300x220(void) {
  write_regs(g_320x200x256);
}

But I didn't work with VGA in C too long so sorry if this code includes some mistake.

[eekee]

Nobody should use the 320x200 mode anymore. Bit-plane and palette mapped modes are big no-nos. If you want to use VGA / VBE, make sure you use one that uses RGB components directly. The best alternative is probably to boot with EFI and use the EFI mode linear frame buffer. It's a lot easier than to set up a decent VBE mode.

Palettes can be nice for certain visual styles. Bit-plane is a pain if you try to tackle it with normal graphics algorithms, but I do remember a web page claiming it can be really powerful if you know how to use it. I think there was something about a barrel shifter.

[Octocontrabass]

Are you perhaps thinking of this page? VGA does have some interesting hardware for manipulating planar graphics, but modern display adapters implement VGA compatibility as a separate unit from the rest of the adapter (if they have VGA compatibility at all) so it's not especially useful.

It's a lot more useful if you're writing a driver for some ancient SVGA chip...

[eekee]

Thanks! It's not the page I remember, but it's interesting nonetheless. I might use it with the thoroughly retro Forths I'm thinking of implementing for fun.