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From: spolsky-joel@cs.yale.edu (Joel Spolsky)
Newsgroups: comp.windows.ms
Subject: Re: Lost characters in com sessions
Message-ID: <27466@cs.yale.edu>
Date: 29 Nov 90 06:25:47 GMT
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In article <3335@ns-mx.uiowa.edu> williams@umaxc.weeg.uiowa.edu (Kent Williams) writes:
>And another thing -- when windows scrolls a dos session, why does it
>redraw the whole window from the top, rather than bitblt the top+1 to
>bottom lines up to the top and then write the bottom line?  Or is that
>what it does?

Windows doesn't scroll the DOS session, the BIOS does, and it does it
by basically "bitblt"ing a chunk of text mode video memory. (Well, it
does a REP MOV, the bytewise equivalent). Windows takes snapshots of
the text mode video memory, converts them to graphics, and displays
them. It does this a certain number of times per second. If it does
this while BIOS is in the middle of a scroll operation you get the
effect of doubled lines (which you can get in straight DOS by hitting
Pause at the right moment during a scroll).

Joel Spolsky
spolsky@cs.yale.edu                                     Silence = Death