NetNews Offline 2/NetNews Offline Volume 2.iso/news/comp/sys/cbm/5855

Text File | 1996-08-05 | 3.2 KB | 79 lines

Path: sdcc12.ucsd.edu!cs!galvarez
From: [e-mail] (Guillermo Alvarez)
Newsgroups: comp.sys.cbm
Subject: Re: Unlock Disk, 1541
Date: 19 Apr 1996 19:09:20 GMT
Organization: CSE Dept., UC San Diego
Distribution: world
Message-ID: <4l8od0$[e-mail]>
References: <[e-mail]> <3177c00b.1948181@199.181.141.3>
NNTP-Posting-Host: beowulf.ucsd.edu


What Tony suggests is true; however, I did it much more easily in my
1571 several years ago.  I had a ROM disassembly of the drive, and there
I found the couple of instructions that checked for this special value
on the disk.  I simply copied the whole ROM to RAM, altered this couple
of bytes so that the result of the test would be irrelevant, and changed
the memory map of the drive to use the modified routines in RAM.  I then
used the DOS interface as usual to overwrite the sector, and got the job
done.

Guillermo

In article <3177c00b.1948181@199.181.141.3>, Tony Postmayer ([e-mail]) writes:
|> On Thu, 18 Apr 1996 19:24:59 -0700, Ejner <[e-mail]> wrote:
|>
|> >Does anyone know how to unlock a locked disk (1541 formattet) ?
|> >It has been locked by changing the $41 value in position 2 of track 18,
|> >sector 00... Instead of $41, it now reads $42... When I try changing it
|> >back, it fails, ┤coz it thinks the disk is another format than 1541...
|> >How can I change it back? I use MC...
|> >
|> >Notice: I have programs for it, but I want to know how!! :-)
|> >
|> Essentially, you have to read the sector into memory, make the desired
|> alterations, and write it back out.  The trick is you have to write it
|> out without using the 1541 DOS in order to avoid the error detection
|> that is stopping you.
|>
|> No doubt there are shortcut ways to do this, perhaps by doing a little
|> preliminary setup and then calling some 1541 ROM routines directly.  A
|> lot of that stuff has been written up in magazines over the years.  I
|> prefer to do it the hard way which is to control the entire process
|> though machine code routines which execute in the drive.  I only call
|> a minimum of ROM routines, such as the GCR conversion ones.
|>
|> This is by no means a simple feat.  You have to be a top notch ML
|> programmer and have good information on the architecture of the drive.
|> (for example, a copy of the book Inside Commodore DOS)
|>
|> My advice to you, if you want to take up the challenge, is to get a
|> copy of one of the many fast format routines and disassemble it.  When
|> you've studied that and learned all the steps needed to format a 1541
|> disk you'll be able to solve your present problem as well as many
|> more.  I recommend the format code from the "Fast Hack Em" disk
|> utility - it's the most direct and concise one I've seen.
|>
|> Just to give you an idea, here's a rough flowchart of what has to be
|> done:
|>
|>     Turn the motor on and give it time to come up to speed
|>     Figure out what track you are on
|>     Step the head to the target track
|>     Seek the target sector
|>     Read in the target data block
|>     Convert from GCR to Petscii
|>
|>     Make your alterations
|>
|>     Convert from Petscii to GCR
|>     Seek the target sector
|>     Write out the data
|>     Shut everything down
|>
|> Good luck.
|>
|>   Tony -
|>