I've almost fixed my VGA emulation to work with all video modes. Only the mode 13h display goes a bit wrong: it's displaying 640 pixels horizontally from a framebuffer of 640 pixels instead of 320 pixels. I'm using the Dosbox-X int10_modes.cpp to set the modes for my VGA emulator, character input is from the most recent Dosbox SVN. Is there anything that this mode sets that makes 320 pixels appear as 640 pixels on-screen without setting the DCR (Dot Clock Rate) in the Sequencer Clocking mode register? Anyone can tell me what's going wrong?

You want bit 6 (Clock select) of attribute register 0x10. This causes two 4-bit pixels to be interpreted as one 8-bit pixel.

I've gotten the attributes in 8-bit color mode working, I'm seeing all the pixels correctly on-screen. But according to my logs it tries to render 640 horizontal pels 1:1. The VGA memory only has 320 pels wide rows. So I get the second row (for each row, the left half is the row itself, the right half is 320 pixels of the next row, for a total of 640 pixels active display) on the right half of active display. So atm it's taking the first pixel, immediately reading the second pixel because of the 8-bit mode. It merges the values to 1 8-bit DAC lookup and outputs it on active display.

Atm the Dot Clock Rate takes care of the reloading of displayed pixels (every 8 pixels when off, every 16 pixels when set).

1void VGA_ActiveDisplay(SEQ_DATA *Sequencer, VGA_Type *VGA)
2{
3	byte loadcharacterplanes;
4	//Render our active display here! Start with text mode!		
5	static VGA_AttributeInfo attributeinfo; //Our collected attribute info!
6	static VGA_Sequencer_Mode activemode[2] = {VGA_Sequencer_TextMode,VGA_Sequencer_GraphicsMode}; //Our display modes!
7	static VGA_Sequencer_Mode activedisplayhandlers[2] = {VGA_ActiveDisplay_noblanking,VGA_Blank}; //For giving the correct output sub-level!
8	word tempx = Sequencer->tempx; //Load tempx!
9
10	othernibble: //Retrieve the current DAC index!
11	Sequencer->activex = tempx++; //Active X!
12	activemode[VGA->precalcs.graphicsmode](VGA,Sequencer,&attributeinfo); //Get the color to render!
13	if (VGA_AttributeController(&attributeinfo,VGA,Sequencer)) goto othernibble; //Apply the attribute through the attribute controller!
14	//activedisplayhandlers[blanking](VGA,Sequencer,&attributeinfo); //Blank or active display! Blanking doesn't work yet?
15	VGA_ActiveDisplay_noblanking(VGA, Sequencer, &attributeinfo); //Always active display!
16
17	if (++Sequencer->active_pixelrate > VGA->registers->SequencerRegisters.REGISTERS.CLOCKINGMODEREGISTER.DCR) //To write back the pixel clock every or every other pixel?
18	{
19		Sequencer->active_pixelrate = 0; //Reset for the new block!
20		if (VGA->precalcs.graphicsmode) //Graphics mode?
21		{
22			loadcharacterplanes = ((tempx & 7)==0); //Load the character planes?
23		}
24		else //Text mode?
25		{
26			loadcharacterplanes = (VGA->CRTC.charcolstatus[(Sequencer->tempx<<1)] != VGA->CRTC.charcolstatus[(tempx<<1)]); //Load the character planes?
27		}
28		if (loadcharacterplanes) //First of a new block? Reload our pixel buffer!
29		{
30			VGA_loadcharacterplanes(VGA, Sequencer, tempx); //Load data from the graphics planes!
31		}
32		Sequencer->tempx = tempx; //Write back tempx!
33	}
34}

Tempx is reset every row.

This works in all other modes (640 pels with 320 displayed pixels). But for some reason mode 13h doesn't set the bit, making it a 640x200 pixel mode? Anyone knows how the VGA actually makes this 320 instead of 640 pixels from VRAM (double pixels) without setting this bit?

I don't think you understood SarahWalker. It is not the Dot Clock Rate bit that is used here. You really need the attribute controller register 0x10, bit 6 to be in 256-color mode, and maybe some other registers too.

Mode 13h is really a 640x400 mode if you look at pixels and lines. Thus no DCR bit is used to divide the clock to 320 pixels.

Instead, in mode 13h you basically still have 640 4-bit pixels, but the sequencer (shift register) output is latched every 2 pixels, so instead of 640 4-bit pixels being fed to DAC, the DAC sees only 320 8-bit pixels and because they are updated every other pixel clock, the 8-bit pixels are double as wide.

http://www.osdever.net/FreeVGA/vga/vgaseq.htm

So if I understand it correctly, the 8-bit color mode actually behaves the same as the DCR when being set? What happens when the DCR bit is set and 8-bit mode is enabled? Do they stack? So every pixel is quadrupled (2 times 2, since DCR doubles pixels and 8-bit mode doubles pixels)?

My VGA emulation is now fully working with all video modes set by Dosbox-X (modes 0 to 13h).

This is my current version:

Anyone can tell me if anything is missing that needs to be emulated? Anyone knows what effect the misc output register has on the CPU's VRAM window (O/E Page Select)? Does it only have effect when using a 128K memory aperture? Does it only have effect on the high 64K of the 128K window?

This is my latest version:

All video modes are running fine now. Almost all horizontal CRTC timing works and all vertical CRTC timing works. Also fixed some bugs in the CPU Host Memory Aperture @vga_mmu.c (Misc. Graphics register according to the Intel® OpenSource HD Graphics Programmer’s Reference Manual) and the Shift Register Interleave Mode has been fixed.

For some reason, mode 0/1, 4/5 and D don't have a left empty space (I'm using Dosbox-X's int10_modes.cpp). Does Dosbox-X have an error programming the horizontal CRTC registers during modes 0,1,4,5 and D? Or is there an error in my CRTC emulation? (The emulation's fully in VGA_Sequencer.c (execution part), with timings gotten from vga_crtcontroller.c and vga_precalcs.c, which is drawn according to signals by VGA_CRTController.c).

I also notice all modes are not vertically centered on the rendered screen? All modes have some space at the top of the screen, but not at the bottom? Is this an error in my emulation or an error with the Dosbox-X VGA programming?

Anyone knows how to fix these 'border'/blanked rows/column timing?