Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!mnetor!uunet!cbmvax!grr From: grr@cbmvax.UUCP (George Robbins) Newsgroups: comp.sys.amiga Subject: Re: eXtra-Half-Brite: The Full Story Please! Message-ID: <2886@cbmvax.UUCP> Date: Wed, 2-Dec-87 19:51:16 EST Article-I.D.: cbmvax.2886 Posted: Wed Dec 2 19:51:16 1987 Date-Received: Sun, 6-Dec-87 07:38:36 EST References: <6924@sunybcs.UUCP> Reply-To: grr@cbmvax.UUCP (George Robbins) Distribution: na Organization: Commodore Technology, West Chester, PA Lines: 34 In article <6924@sunybcs.UUCP> ugfeldmn@joey.UUCP (Jon Feldman) writes: > > Can anyone tell me the story behind Extra-Half-Brite, and why some > machines have it and some don't? Which Amigas do? Why? How? Where? When? > What? I don't understand! Where's the tea? Extra-Half-Brite is a display mode where five bit planes are used to lookup the color for a pixel in the color lookup table, and the sixth bitplane is used to modify the resulting color value by shifting the resulting RGB values right on position. It is kind of a kludge which takes advantage of the fact that the logic for handling six bitplanes was already present to make HAM work, however changing the color lookup table from 32 to 64 entries would have caused an undesirable increase in the size of the Denise chip. Chip cost bears an exponential relationship to area, so you try to avoid major size increases unless better technology is available to compensate. The change was incorporated in Denise Rev 6 which was used in production for about half of the A1000's. However since this half includes almost all the European A1000's, the percentage of US machines with half-brite is somewhat lower than expected. The mode is also present in Rev 8 chips and all A500's and A2000's use either the Rev 6 or Rev 8 chip. There are no functional differences between the two revisions. Commodore didn't make a big fuss about it since it wasn't really a major improvement and there were massive numbers of perfectly usable older chips in the production pipeline. If you don't have one in your machine, you can order a replacement Denise chip for $65.40 list, or whatever deal you can get through your dealer's service department. -- George Robbins - now working for, uucp: {uunet|ihnp4|rutgers}!cbmvax!grr but no way officially representing arpa: cbmvax!grr@uunet.uu.net Commodore, Engineering Department fone: 215-431-9255 (only by moonlite)