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

GamerWolf548 · **Joined:** Fri Jul 31, 2020 9:53 am **Posts:** 1

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 · **Posted:** Mon Oct 12, 2020 3:15 am

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 · **Joined:** Wed Oct 01, 2008 1:55 pm **Posts:** 3192

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 · **Joined:** Sat Mar 10, 2018 10:16 am **Posts:** 296

Easiest way is use bios:

Code:

  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/osd/graphics/modes.c) can do same result:

Code:

#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 · **Posted:** Tue Oct 20, 2020 1:22 pm

rdos wrote:

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 · **Joined:** Mon Mar 25, 2013 7:01 pm **Posts:** 5134

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 · **Posted:** Fri Oct 23, 2020 6:59 am

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.

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How do i switch from 8025 vga text mode to 320200 mode

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