Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!uwm.edu!zaphod.mps.ohio-state.edu!usc!apple!dlyons From: dlyons@Apple.COM (David A. Lyons) Newsgroups: comp.sys.apple Subject: Re: Really small question (a really long explanation) Message-ID: <37569@apple.Apple.COM> Date: 2 Jan 90 22:58:29 GMT References: <9542@microsoft.UUCP> <1989Dec15.200302.8233@ncsuvx.ncsu.edu> <10041@microsoft.UUCP> Organization: Apple Computer Inc, Cupertino, CA Lines: 30 In article <10041@microsoft.UUCP> brianw@microsoft.UUCP (Brian Willoughby) writes: >[...] Writes (but not reads) to banks $00 or $01 occur >at the same speed as writes to $E0/$E1 as long as shadowing is on (provided >that you are accessing the addresses set aside for video). The "Apple guys" >only allowed shadowing so that ][+ and //e programs would still function, even >though these programs are unaware that video memory has been moved to $E0/$E1. >Thus, it was a compatibility issue, not a speed issue. I don't think that >there is a case (for a GS-specific program) where shadowing allows faster >execution times. For a non-GS program it just wouldn't work without shadowing. The first part of that is the key: reads from 0 and 1 are fast. Consider scrolling the text screen, for example: the reads and writes are all to banks 0 and 1, so the scrolling is *faster* than if the reads and writes were to banks $E0 and $E1--so shadowing *is* partially a speed issue. >Fortunately, shadowing doesn't cause writes OUTSIDE of the video areas to be >slowed. I read that wrong on my first try--to clarify, access to banks $E0 and $E1 is always slow, but access to non-shadowed areas of banks 0 and 1 is fast, and all reads from 0 and 1 are fast. -- --David A. Lyons, Apple Computer, Inc. | DAL Systems Apple II Developer Technical Support | P.O. Box 875 America Online: Dave Lyons | Cupertino, CA 95015-0875 GEnie: D.LYONS2 or DAVE.LYONS CompuServe: 72177,3233 Internet/BITNET: dlyons@apple.com UUCP: ...!ames!apple!dlyons My opinions are my own, not Apple's.