English Amiga Board


Go Back   English Amiga Board > Coders > Coders. General

 
 
Thread Tools
Old 02 November 2015, 18:05   #41
hooverphonique
ex. demoscener "Bigmama"

 
Join Date: Jun 2012
Location: Fyn / Denmark
Posts: 1,062
MFM encoding is not quite that simple, and doesn't allow any adjacent one-bits.. the disk controller uses the encoding as a clock when reading back the data in order to detect the bit rate. Also, the problem of adjacent ones is not an Amiga problem; MFM encoding was used on many floppy and harddrive systems back then...

If JW just got lucky that the controller would read back his strange encoding, I don't know.
hooverphonique is offline  
Old 03 November 2015, 02:34   #42
dlfrsilver
CaptainM68K-SPS France
dlfrsilver's Avatar
 
Join Date: Dec 2004
Location: Melun nearby Paris/France
Age: 42
Posts: 8,388
Send a message via MSN to dlfrsilver
the fact is that he used a correspondance table. that how he is reconstituting the 8 bits of data from the 6 bits of data saved and crammed on disk.

imagine, it's like if i said in a table that 00101111 must be saved on disk as 01111.

Of course saving 01111 instead of 00101111 takes less space on disk.

It's like he used the MFM bit encoding system as a compression system directly
dlfrsilver is offline  
Old 03 November 2015, 03:22   #43
ReadOnlyCat
Code Kitten

 
Join Date: Aug 2015
Location: Montreal/Canadia
Age: 48
Posts: 1,143
Quote:
Originally Posted by dlfrsilver View Post
the fact is that he used a correspondance table. that how he is reconstituting the 8 bits of data from the 6 bits of data saved and crammed on disk.
I think that part is not in question. What NorthWay and hooverphonique probably meant is that in order to reconstitute the clock signal the stream of bits must respect some constraints such as making sure that the flux reversals happen within a certain frequency range and they assume that JW's encoding must somehow respect both the lower and upper limit of this range.

Quote:
Originally Posted by dlfrsilver View Post
Of course saving 01111 instead of 00101111 takes less space on disk.

It's like he used the MFM bit encoding system as a compression system directly
Actually he did not use MFM at all. He replaced it with his own encoding which is a bit more compact than MFM.
MFM(1,3) which is used on the Amiga is not the most compact encoding available: the wikipedia page on the topic lists RLL 1,7 which has the same information density as the one used by JW (three encoded bits for two signal bits).

Encodings used on floppy disk (and other streamed media) must satisfy two constraints:
  • Avoid too frequent flux reversals which might exceed the magnetic media resolution.
    This is why it is "forbidden" to have consecutive 1s: they might be incorrectly written.
  • Have frequently enough flux reversals that the clock drift can be detected and corrected "live".
    This is why too many zeros are bad, they prevent clock resynchronization.
In short: consecutive 1s are bad but consecutive 0s are bad too.

The many 0s puzzled NorthWay but I guess clock drift does not happen that fast so maybe it is fine to have quite a few zeros every now and then. I guess JW must have been experimenting with different combinations of bits and found out that the media density was a more constraining factor than clock drift.
ReadOnlyCat is offline  
Old 03 November 2015, 11:16   #44
hooverphonique
ex. demoscener "Bigmama"

 
Join Date: Jun 2012
Location: Fyn / Denmark
Posts: 1,062
Exactly ROC,

it's still interesting that it works, though, since the disk controller must know the encoding in use to actually know how to retrieve the clock. When you program it (i.e. the hardware), you have the options of MFM or GCR, which tells me that the controller relies on the encoding, at least to some degree.
hooverphonique is offline  
Old 03 November 2015, 14:27   #45
ReadOnlyCat
Code Kitten

 
Join Date: Aug 2015
Location: Montreal/Canadia
Age: 48
Posts: 1,143
Quote:
Originally Posted by hooverphonique View Post
Exactly ROC,

it's still interesting that it works, though, since the disk controller must know the encoding in use to actually know how to retrieve the clock. When you program it (i.e. the hardware), you have the options of MFM or GCR, which tells me that the controller relies on the encoding, at least to some degree.
Oh, interesting, I did not know that.
I guess the hardware relies on derivative properties of MFM/GCR which JW's encoding probably exhibits too, likely accidentally though.

Update: actually, I think I understand why the encoding is needed by the controller. The section about densities of the Wikipedia RLL page seems to give a hint about it: since encoding can differ in compactness, that influences how the clock should be deduced. An encoding which guarantees that each 1/reversal is followed by two 0s/non-reversal can effectively store three encoded bits on the same surface as an encoding which guarantees only one 0/non-reversal in-between. Thus, the former encoding's clock will tick three times when the latter ticks two times. The controller must be informed of it otherwise it will misinterpret the stream of flux reversals.

Update2: the book "Amiga disk drives inside and out" by Abacus/Data-Becker actually explains that when GCR is set the reading/writing speed is halved. This makes sense since GCR allows consecutive 1s to be written which would exceed the resolution of the magnetic medium. MFM 1-sparseness on the other hand probably allows it use the maximum read/write speed.
Quote:
The problem is that several one bits can follow each other in this format and the controller cannot process these. This has the result of changing the data recording density. To work with the GCR format without error, a switch is made to half (sic) the recording speed (from 2ms to 4ms).
So the controller probably does not need to know the encoding, it probably re-clocks using the same technique all the time.

Update3: I noticed that there is also a GCR dedicated bit in
ADKCON
in addition to the speed selection one so maybe the controller does need to know about the encoding after all. So mysterious.

Also, could you just refer to me as "Cat"? I am not too sure why but "ROC" feels horribly impersonal and cold.

Last edited by ReadOnlyCat; 03 November 2015 at 16:07. Reason: Added update paragraph. And update2 paragraph. And update3. :D
ReadOnlyCat is offline  
Old 20 January 2020, 09:41   #46
kaffer
Registered User
 
Join Date: May 2011
Location: Cambridge
Posts: 593
Quote:
Originally Posted by dlfrsilver View Post
basically, when you have an alternate version, the whdload slave doesn't work with the game.

Yeah Gremlin name is a bad idea, because Gremlin, us gold, and even psygnosis used the protoscan longtrack protection.
One thing I'm missing in my Disk-Utilities is proper/accepted names for the various protections, where they have such a name. It took a while to work out what Protec was (some game had a fragment of the Protec source on disk, which gave that one away). I had no idea about Protoscan... And it's not like this info is easy to look up Sometimes WHDLoad notes give it away...

I will fix this one at least
kaffer is offline  
Old 21 January 2020, 13:33   #47
BarryB
Amigaholic

 
Join Date: Dec 2009
Location: UK
Posts: 3,186
Sometimes the Softpres WIP pages give a particular protection/format a name, it's just trawling through all the WIP pages to find them!
BarryB is offline  
Old 21 January 2020, 21:41   #48
BarryB
Amigaholic

 
Join Date: Dec 2009
Location: UK
Posts: 3,186
Two examples:

B.A.T. is identifed in Disk-Utilities as bat_longtrack, Softpres WIP: Protec variant on BAT, named Prolance by the game.

Batman The Caped Crusader and The Untouchables are identified in Disk-Utilities as batman, Softpres WIP: SFX, SpecialFX: variants on Batman The Caped Crusader, Head Over Heels, Midnight Resistance, The Untouchables
BarryB is offline  
Old 21 January 2020, 21:57   #49
dlfrsilver
CaptainM68K-SPS France
dlfrsilver's Avatar
 
Join Date: Dec 2004
Location: Melun nearby Paris/France
Age: 42
Posts: 8,388
Send a message via MSN to dlfrsilver
Quote:
Originally Posted by BarryB View Post
Two examples:

B.A.T. is identifed in Disk-Utilities as bat_longtrack, Softpres WIP: Protec variant on BAT, named Prolance by the game.

Batman The Caped Crusader and The Untouchables are identified in Disk-Utilities as batman, Softpres WIP: SFX, SpecialFX: variants on Batman The Caped Crusader, Head Over Heels, Midnight Resistance, The Untouchables
indeed. Prolance format on B.A.T., and SpecialFX MFM longtrack format.
dlfrsilver is offline  
Old 21 January 2020, 22:35   #50
BarryB
Amigaholic

 
Join Date: Dec 2009
Location: UK
Posts: 3,186
Plus the multitude of early Rob Northern protections that have no 'official' name!
BarryB is offline  
Old 22 January 2020, 09:14   #51
kaffer
Registered User
 
Join Date: May 2011
Location: Cambridge
Posts: 593
Quote:
Originally Posted by BarryB View Post
Two examples:

B.A.T. is identifed in Disk-Utilities as bat_longtrack, Softpres WIP: Protec variant on BAT, named Prolance by the game.

Batman The Caped Crusader and The Untouchables are identified in Disk-Utilities as batman, Softpres WIP: SFX, SpecialFX: variants on Batman The Caped Crusader, Head Over Heels, Midnight Resistance, The Untouchables
Thanks for those. Fixed.
kaffer is offline  
Old 22 January 2020, 12:41   #52
BarryB
Amigaholic

 
Join Date: Dec 2009
Location: UK
Posts: 3,186
Quote:
Originally Posted by kaffer View Post
Thanks for those. Fixed.
Super

I'll see if I can find some more!
BarryB is offline  
Old 22 January 2020, 16:32   #53
BarryB
Amigaholic

 
Join Date: Dec 2009
Location: UK
Posts: 3,186
Looking at formats supported in Disk-Utilities and identifying 'variants' so they can all be grouped together under one umbrella:

Seems DMA Design formats were dubbed DMA DOS: DMA DOS: various DMA Design titles, found on Beast 3 and Lemmings so far.
DMA DOS, a different one: Menace, Blood Money (encrypted). Alternate DMA DOS format found on Lemmings Demo

archipelgos format seems to be called LGO or Logotron: LGO, Logotron: variants on Archipelagos, Kid Gloves, Gauntlet 2, Golden Axe.

typhoon format on Typhoon Thompson, Prince of Persia is named Broderbund: Prince of Persia renamed to Broderbund as it is used on the US version of Typhoon Thompson.

My dumps of Puffy's Saga/Iron Lord (not supported) identified as Ubi Soft: Ubi Soft protection found on RanX, Iron Lord, Puffy’s Saga and possibly all of their own original releases.

barbarian_ultimate_warrior protection identified on my dumps of Buggy Boy, Football Manager 2 and Three Stooges (although Three Stooges takes ages to pass the protection checks!). Seems to be a Rob Northen 'flakey bit' protection: Flakey bit protection on the original European releases of Barbarian (Palace), Armageddon Man, Buggy Boy, Three Stooges, Wizball, and an alternate version on ECO (Ocean). Likely to be other games too.

A few games with the apprentice format can be renamed CHW (My dumps of Gazza II, Flip-It & Magnose, Star Goose and 9 Lives also use the same format): Apprentice: Another CHW variant. CHW (alternate), on Conqueror.

scooby_doo, hi_tec_b appears to be a Gary Antcliffe format: GA, Gary Antcliffe: Scooby-Doo & Scrappy-Doo, Universe. Yogi’s Great Escape: A Gary Antcliffe format without a checksum.

Also, some formats were either named after the author, game or company if no other name was identified!

Also, maybe these 'format/protection' discussions should be moved to kaffers original thread: Protection format names

Last edited by BarryB; 22 January 2020 at 22:55. Reason: Updated info!
BarryB is offline  
Old 22 January 2020, 18:56   #54
dlfrsilver
CaptainM68K-SPS France
dlfrsilver's Avatar
 
Join Date: Dec 2004
Location: Melun nearby Paris/France
Age: 42
Posts: 8,388
Send a message via MSN to dlfrsilver
Quote:
Originally Posted by BarryB View Post
Looking at 'variants' so they can be grouped together under one umbrella:

Seems DMA Design formats were dubbed DMA DOS: DMA DOS: various DMA Design titles, found on Beast 3 and Lemmings so far.
DMA DOS, a different one: Menace, Blood Money (encrypted). Alternate DMA DOS format found on Lemmings Demo

Archipelgos format seems to be called LGO for Logotron: LGO, Logotron: variants on Archipelagos, Kid Gloves, Gauntlet 2, Golden Axe.

'typhoon' format on Typhoon Thompson, Prince of Persia is named Broderbund: Prince of Persia renamed to Broderbund as it is used on the US version of Typhoon Thompson.

A few formats can be renamed CHW (Gazza II and Star Goose also use the same format): Apprentice: Another CHW variant. CHW (alternate), on Conqueror.

Also, some formats were either named after the author, game or company if no other name was identified!

Also, maybe these 'format/protection' discussions should be moved to kaffers original thread: Protection format names
CHW = Christian Weber from SCA
dlfrsilver is offline  
Old 22 January 2020, 19:45   #55
BarryB
Amigaholic

 
Join Date: Dec 2009
Location: UK
Posts: 3,186
Another case where no name is known for the format so it's named after the author!
BarryB is offline  
Old 22 January 2020, 19:48   #56
DamienD
Global Moderator

DamienD's Avatar
 
Join Date: Aug 2005
Location: London / Sydney
Age: 43
Posts: 16,893
Quote:
Originally Posted by BarryB View Post
Also, maybe these 'format/protection' discussions should be moved to kaffers original thread: Protection format names
Done
DamienD is offline  
Old 22 January 2020, 20:02   #57
BarryB
Amigaholic

 
Join Date: Dec 2009
Location: UK
Posts: 3,186
Quote:
Originally Posted by DamienD View Post
Done
You are soooooo helpful Damien
BarryB is offline  
Old 22 January 2020, 21:55   #58
jotd
This cat is no more
jotd's Avatar
 
Join Date: Dec 2004
Location: FRANCE
Age: 48
Posts: 3,661
IIRC, the MFM format of Agony & Unreal disks is the same (disks start by "ur00"). I don't know if the format has a name though.
jotd is online now  
Old 22 January 2020, 22:13   #59
BarryB
Amigaholic

 
Join Date: Dec 2009
Location: UK
Posts: 3,186
Quote:
Originally Posted by jotd View Post
IIRC, the MFM format of Agony & Unreal disks is the same (disks start by "ur00"). I don't know if the format has a name though.
From SPS WIP 18 July 2002: Ordilogic: Found on Unreal, Agony.
BarryB is offline  
Old 23 January 2020, 04:57   #60
Hewitson
Registered User
Hewitson's Avatar
 
Join Date: Feb 2007
Location: Melbourne, Australia
Age: 37
Posts: 3,575
Quote:
Originally Posted by BarryB View Post
Plus the multitude of early Rob Northern protections that have no 'official' name!
Rob Northen. Rob Northern was a member of Classic
Hewitson is offline  
 


Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools

Similar Threads
Thread Thread Starter Forum Replies Last Post
What the hell were those names? nemazoty request.Demos 4 26 September 2013 14:50
Where did your users names come from? Freakyweakywoo Nostalgia & memories 300 22 December 2012 05:54
Partition Names? Tempest 2084 support.Hardware 7 15 May 2009 08:16
Screenmode names disappeared ancalimon support.WinUAE 3 19 July 2007 23:31
Computers and their names Akira Nostalgia & memories 55 03 October 2001 00:40

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT +2. The time now is 21:51.


Powered by vBulletin® Version 3.8.11
Copyright ©2000 - 2020, vBulletin Solutions Inc.
Page generated in 0.09815 seconds with 15 queries