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-   -   GCR Format on Amiga (http://eab.abime.net/showthread.php?t=20698)

NOB 07 November 2005 16:30

GCR Format on Amiga
Anyone out there who has (amiga)sources in assembler to read/write
tracks in GCR format?
Which games are using GCR formatted tracks for copy protection?
Can you copy GCR formatted tracks with copyprograms like XCOPY?

musashi5150 07 November 2005 20:00

The CAPS (SPS) project did find a game using GCR a while ago, but I can't remember what it was. If you search their site enough you should find out.

AFAIK X-Copy will only deal with MFM based disks. Unless anyone else knows different?

dlfrsilver 07 November 2005 22:06

yep games using atari ST tracks like Satan (retail), astro marine corps, james pond 1,
wings of death, lethal xcess ^^ and also ADI eduationnal disks that uses ST/PC data disk. amiga launcher uses special routines to read them ^^

X-copy can't copy these ^^

DrBong 08 November 2005 04:18

Yep, GCR was used on just about any dual/tri-format release that contained Amiga, PC, and/or ST games on the one set of disks. Kixx was notable for releasing dual-format Ami/PC games on budget (e.g. 3D Pool, Action Fighter, Bionic Commando etc.). HOL has detailed this info. for some of the Kixx releases.

NOB 08 November 2005 18:37

Great infos.Thank you!:)

Toni Wilen 08 November 2005 19:04

GCR? Atari ST? Atari ST uses MFM format like Amiga and most other computers.

NOB 08 November 2005 19:58

HMM, then it is like this?::confused

A: GCR was used on just about any dual/tri-format release that contained Amiga, PC, and/or ST games on the one set of disks.

B: (found on CAPS site):) :
Alternate Reality
The first Amiga game found that has an GCR encoded track.
(only one game? :confused )

C:Atari ST is using the MFM and not the GCR format.

Codetapper 08 November 2005 22:12

A. No - GCR not on any of those multi format disks.

B. Quite possibly only one game yes. Lucky for us crackers, we only had to learn MFM and not the GCR crap!

C. Correct.

Interceptor 08 November 2005 22:49

the amiga does have native support for reading/writing GCR. i've seen this mentioned in a couple of books, although i've not seen any examples myself.

a guy called simon brooke who i met at CGE-UK was brandishing a set of old technical magazines, in one of them was an article about it. unfortunately i've not heard back from him since :|

DrBong 09 November 2005 07:27


Originally Posted by Codetapper
A. No - GCR not on any of those multi format disks.

Maybe my memory is fading, but I thought the Kixx Ami/PC dual format budget releases were a MFM/GCR hybrid? The GCR stuff relating to the PC side on the disk and MFM to the Amiga side? I take it you would know as you've patched a few of the Kixx releases (3D Pool, Rick Dangerous spring to mind)! ;)

Anyway, I should probably learn to read subject headings as I gather NOB was asking about GCR in relation to Amiga games per se (rather than the multi-format stuff).

Interceptor 09 November 2005 17:15

the only 2 computers i know of using gcr are c64 and apple ][. there may be more obscure ones.

otherwise, its MFM,

PC uses it, as does amiga, st, spectrum, cpc etc etc.

NOB 09 November 2005 19:07

Thanks! that makes it clearer.

There is possibly only one (Amiga)game with a GCR coded track.

Even my floppy-book contains only two pages about the GCR format.

Interceptor 10 November 2005 12:39

that document link you posted then edited out is a scan from the exact same magazine article i mentioned earlier.

it did make me wonder about any examples of it too, which is why this thread caught my attention.

IFW 10 November 2005 15:07

All the multi-format, st etc disks are mfm.
Indeed there is only one game that is known to use gcr on the miggy.

NOB 17 November 2005 20:15

@CAPS Team
Has your dumping technique the ability to dump Alternate Reality.
(That game with the GCR encoded track)?
Or will your technique dump only legal/not legal MFM Disks?

fiath 18 November 2005 17:27

The dump might be okay, but we won't know until we have GCR support in the analyser.

Phantasm 18 December 2005 01:57

Mac floppies use GCR but not alot else as far as I know.

They also use constant angular velocity drives instead of constant linear velocity (or is it the other way around - i forget).

PC and Atari ST used MFM encoding but used the index flag rather than an mfm sync value unless im mistaken.

I've never actually tried to read / write a gcr track, but if memory serves there is a flag somewhere in one of the registers that selects between MFM and gcr. I've never actually seen it explained anywhere how the affects the actual bits written to the disk. I cant really see why you would need to set any flags if GCR is just a different way of encoding the bits compared to MFM but you do, so there must be more to it than that.

Those multi format disks were typically amiga formatted tracks on one side and st/pc formatted on the other. Fortunately side 0 was the opposite side on both machines, so they both booted up nicely from their respective boot blocks.

IFW 18 December 2005 04:15

Index flag usage is not necessary, it is being used as an error/retry counter by a hardware controller rather than actually an index apart from track commands.

The reason for that should be timing and the precompensation patterns used. Also data window is probably different.

Multi-format disks have their boots on the very same side.
The track is encoded and data is interleaved in a way that each respective os can read it properly with a few retries. This is possible because the syncing used on the blocks is different so data and header parts are embedded into another format's data part. Normally the host format is amiga, and the amiga boot track has hw mfm style headers and data blocks embedded.

Baron 18 December 2005 17:17

GCR is also a very wasteful disk format, less capacity than an MFM disk due to the way GCR is encoded.

Codetapper 19 December 2005 00:41

I was led to believe the opposite is true - GCR can hold more data but is twice as slow to read on the Amiga. Enabling GCR slows down the drive so it can read the data reliably. (However I've never used it or investigated too much so I maybe wrong)

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