Date: 19961210 From: John R Pierce Subject: Re: SVGA cards on PCI To: Mailing List Recipients Eric Rehm asks... > > the actual value > > on any given graphics system can be estimated by [width * height * > > vertical_refresh * bytes_per_pixel * 1.3]). > > Why the factor of 1.3? Well, thats a ballpark fudge factor. Thats to allow for the overscan and retrace (i.e. blanking time), which is generally around 30% of the total. The actual video data fetched per second will be [width * height * vertical_refresh * bytes_per_pixel] but during active video the fetch rate will generally have to sustain the somewhat higher dotclock rate as very few if any video boards have sufficient buffering to spread the 'overhead' over a full line. Video refresh happens via two distinctly different methods, depending on whether one is dealing with a DRAM or a VRAM board. On a VRAM board, the [often 64 bit wide] outputs of the VRAM's are connected directly up to the RAMDAC, and pixel data gets clocked out of the VRAM into the RAMDAC in real time. Since most all VRAM's can clock at speeds up to 50MHz or higher, 50MHz * 8 bytes provides 400MBytes/sec or more of video refresh, enough to handle most any display mode. Since the VRAM's do this with a second 'serial' port, this display refresh activity has no impact on drawing performance. On a DRAM board its quite a bit more complex. Taking the example of a 64 bit modern EDO style DRAM card, EDO bursts may be able to fetch somewheres around 250-300MBytes/sec. This bandwidth however must be shared between display refresh, host access, and graphics controller memory operations (bitblt, etc). The display refresh controller will typically have a FIFO of a few dozen 8byte 'words'... Basically, the display controller will fill this fifo at full burst speed, then release the memory subsystem to other uses until it needs to reload the fifo... The higher the MByte/sec display refresh requirements are, the less time left over for host access and graphics controller operations. Since I've explained all this... here's a little kicker. At lower resolution/color depth combos, EDO DRAM is FASTER than VRAM. This is because EDO DRAM tends to have faster memory accesses... If the display refresh requirements are modest, this still leaves MORE bandwidth left over for host and graphics accesses. Only when you get into fairly high resolution / color depth / refresh rates does the inherently slower VRAM catch up due to its dualported nature. My general rule of thumb... if you have a 14" or 15" monitor, stick with DRAM. If you have a 17" and are going to run at 1024x768 16million colors at refresh rates like 75Hz, you'll probably want VRAM. If you have a 21" you definately want VRAM (although in fact, newer EDO DRAM cards do quite nicely at 1280x1024x64k colors). [footnote... I know this has not that much to do with PCI etc, but once I was on a roll...] -jrp