Screen update speed of 8088 PC

Richard wrote: If you don't care about synchronizing your updates with the display cycle, you can do more. Assuming a 4.77MHz 8088 IBM PC, REP MOVSW from regular RAM to display RAM (which is slower) is about 160KB/s. For 80x25 (4000bytes) that's roughly 40fps. REP STOSW is much faster (about 240KB/s) but that's not useful/practical in a textmode scenario so I won't go with those numbers. On monochrome, you can do this. On true CGA, you can't, because the ram isn't truly dual-ported. As a result, when you read or write display memory in an 80-column text mode, the adapter draws garbage wherever the beam is instead of the real ram contents for the duration of the user-initiated access. This is more commonly knows as "CGA snow". What especially blows is that it will still draw "snow" even if you're only reading -- even if your access is to an off-screen page! To get around this, you write to CGA RAM while the beam is *not* drawing, but retracing horizontally or vertically back to the next scanline or display cycle. You can transfer about 250 words during a vertical retrace and exactly one word during horizontal retrace, so that's 450 words per frame, so a snow-less full-screen update would take 3 frames (20fps). The obvious goal is to not be in the position of having to update the *entire* screen. 40-column modes don't have a "snow" problem (not sure why), which is why I chose 40x25 for my video program (that, and the lower data requirements). -- Jim Leonard (trixter at oldskool.org) http://www.oldskool.org/ Help our electronic games project: http://www.mobygames.com/ Or check out some trippy MindCandy at http://www.mindcandydvd.com/ A child borne of the home computer wars: http://trixter.wordpress.com/