Dec 18, 20223 min read

VGA for my 80286

Updated: Dec 21, 2022

As a follow-up to Brainstorming... Video Card for 286 System, I am now working to get my VGA card working in my 80286 system.

Initial fire up is looking promising. First... no magic smoke. Second, the '286 system still boots with the VGA card installed. Third, the monitor is syncing to the proper resolution and frequency, and the content matches what I would expect for uninitialized video RAM.

I now need to add some assembly code to write to the video RAM and see if I can get the desired output (e.g., clearing the screen, or a simple test pattern).

I built a handful of simple VGA initialization routines, to test the primary colors. Here's an example:

vga_init_red:
	; Video RAM  (128 KB) = 0xA0000-0xBFFFF
	push	ax
	push	bp
	push	si

	mov		bp, 0xfffe			; offset within segment
								; (i.e., 0xffff down to 0x0000)
	mov		si, 0xb000			; segment start 
								; (i.e., 0xb000 as top)
	mov		es, si

	.offset:
		mov		word es:[bp],	0xe0e0	; write color
								; rrrgggbb, 0xe0=full red
		sub		bp,			2				
		cmp		bp,			0xfffe			
		jnz		.offset

		sub		si,	0x1000						
		mov		es, si

		cmp		si,	0x9000			; if equal, finished 128 KB 
									; of video RAM
		jnz		.offset

	.out:

		mov		ax, 0xf000			; Read-only data in ROM at 
									; 0x30000 (0xf0000 in address 
									; space  0xc0000+0c30000). 
									; Move es to this by default 		
									; easy access to constants.
		mov		es,	ax				; extra segment

		pop		si
		pop		bp
		pop		ax
		ret

After posting the above video, I added bus resistors to the video card. I used a 3.3K pull-down on the video card's internal bus and a 1K pull-up on the video card's external bus (the system external bus). I also adjusted the CLK and PCLK inline resistors, moving the PCLK resistor down to 33 and the CLK up to 100. With these changes, the system is running (with the video card) at 18 MHz (9 MHz effective processor internal clock).

I have completely removed all vertical banding by swapping out the 74HC161 ICs with 74LS161 ICs. My original schematic had LS, but I thought HC would be fine; they were not.

Schematic Issue

While working on font rendering assembly code, I was running into what I thought was a coding issue. It ends up that it's a hardware issue. In the image below, I am rendering out a pair of contiguous vertical pixels, then moving to the right and rendering another pair of contiguous vertical pixels. I do this for both the upper VRAM and lower VRAM. In the image, the lower set of pixels is how the upper set should look. Unfortunately, I am getting an extra, slightly lower pair of pixels in the upper set.

So, looking at the schematic for the VRAM, below, I noticed that I missed updating one of the address lines. Instead of VA11, I have a second VA10. This affects the upper portion of the screen (lower 64K of VRAM), and only for odd addresses.

Unfortunately, the traces connecting to the pin are on the top side of the PCB, but under the PLCC socket. I plan an attempt to modify the existing card by desoldering the PLCC socket, cutting the traces accordingly, and running bodge wires as needed.