martijn@dnlunx.pttrnl.nl (Reinalda M.) (09/19/90)
I was reading Amiga Intern about the disk controller. It said that is capable of doing MFM or GCR coding. I know about MFM, but I've never heard of GCR encoding. What does GCR stand for ? Is it of any use ? Anyone ?
peterk@cbmger.UUCP (Peter Kittel GERMANY) (09/20/90)
In article <1411@dnlunx.pttrnl.nl> martijn@dnlunx.pttrnl.nl (Reinalda M.) writes: >I was reading Amiga Intern about the disk controller. It said that is >capable of doing MFM or GCR coding. >I know about MFM, but I've never heard of GCR encoding. You know when you want to record to a magnetic media you must make sure that there occur enough magnetization changes per second, even when the data to be recorded are constant. One way to solve this is to treat every bit and thus produce an overhead of nearly the double data size. One way to optimize this a bit is GCR, or Group Code Recording. There you take your data and cut it into 4-bit pieces (that seems to be the referred "group") and translate them into 5-bit groups. In these 5-bit groups you are indeed able to make sure that no more than 3 identical bits follow in queue for any possible 4-bit input. So this method is a bit more efficient than MFM.in that it has only 25 % overhead. But it takes extra hardware (ROM in the old CBM floppies) or software (slow, like in C64 devices) to decode this. Provision of GCR on the Amiga is for historical reasons. It was used in the old CBM floppies from 2020 to 8250 (did you know that Commodore offered already 1983 floppies with 1 MB capacity each on simple DD disks?) and on all the C64 compatible floppies. When I'm told correctly, also Apple used GCR, perhaps in a slightly different way. In the Amiga, there is one caveat with GCR. For hardware reasons it must be cut in speed by half and is thus less effective than MFM on the Amiga drives. So to my knowledge NOBODY uses it on the Amiga, sad. -- Best regards, Dr. Peter Kittel // E-Mail to \\ Only my personal opinions... Commodore Frankfurt, Germany \X/ {uunet|pyramid|rutgers}!cbmvax!cbmger!peterk
daveh@cbmvax.commodore.com (Dave Haynie) (09/21/90)
In article <429@cbmger.UUCP> peterk@cbmger.UUCP (Peter Kittel GERMANY) writes: >Provision of GCR on the Amiga is for historical reasons. It was used >in the old CBM floppies from 2020 to 8250 (did you know that Commodore >offered already 1983 floppies with 1 MB capacity each on simple DD disks?) >and on all the C64 compatible floppies. When I'm told correctly, also >Apple used GCR, perhaps in a slightly different way. The Apple II used a GCR encoding, which was adapted to the Macintosh at a higher density. The GCR capability in the Amiga was originally to permit the use of Apple II floppy disks. Which tells you just how long ago work on the Amiga chips began. This was in the chips before Commodore bought Amiga. But by that time, Apple II compatibility wasn't much of an issue. Being that the MFM encoding used is quite a bit simpler to encode/decode, Amiga used that format for the standard Amiga disk, even though many different formats are possible. >In the Amiga, there is one caveat with GCR. For hardware reasons it >must be cut in speed by half and is thus less effective than MFM on >the Amiga drives. So to my knowledge NOBODY uses it on the Amiga, sad. Except perhaps "Fruit Friend", or whatever that called that program which reads Apple II disks. >Best regards, Dr. Peter Kittel // E-Mail to \\ Only my personal opinions... >Commodore Frankfurt, Germany \X/ {uunet|pyramid|rutgers}!cbmvax!cbmger!peterk -- Dave Haynie Commodore-Amiga (Amiga 3000) "The Crew That Never Rests" {uunet|pyramid|rutgers}!cbmvax!daveh PLINK: hazy BIX: hazy Standing on the shoulders of giants leaves me cold -REM
martijn@dnlunx.pttrnl.nl (Reinalda M.) (10/05/90)
thanks !!! Anyone outthere writing a 64 crossdosser ?