davido@lucifer.UUCP (David Ornstein) (11/05/86)
In article <442@uwmacc.UUCP>, curtis@uwmacc.UUCP writes: > I have been perusing through some assembler code written who knows where, > and I noticed that when a particular routine writes to screen memory, > it first reads the video controller scan status and makes sure it is > low, then after it goes low it waits until it goes high before writing > to the screen buffer in memory. The associated comment remarks that > it must go high before it is safe to write directly to screen buffer > memory. The code here is checking to make sure that the CRT is doing a retrace so that no snow will be created on the screen during update. If you write directly to screen memory on the IBM CGA (Color Graphics Adaptor) during a non-retrace period, you get snow. david {seismo!ihnp4}!mirror!lucifer!davido (David Ornstein @ Lucifer @ The Ninth Plane - voice: 617-655-9191)