DOS to access non-standard disks

[+DjayBee]

When playing around with disk images from MECC and Sunburst I found out that both companies used plain DOS with a modified disk layout. The VTOC and directory are moved to non-standard locations (MECC $16A and Sunburst $2C1 and $2C8).

 

I wanted to access these disks and also to be able to copy files between these disks and standard disks.

Inside the disk image you can find:

• Atari DOS 2.5
• Four sample patchers for MECC, Sunburst and back to standard DOS
• A patcher to limit the modified VTOC to only drives D1 and D2
• The MADS source code for both patchers
• A BASIC program to create patchers for arbitrary VTOC-locations

Usage of the patchers:

• Load the patcher (i.e. MECC.16A) to access modified disks. If you load DOS25.168 the patches are reverted back to the standard locations.
  It patches only three bytes of DOS code and changes DOS' copyright text to identify itself.
  
• Load D3TOD8.168 before the patcher to change only accesses to D1 and D2.
  This way you can copy disks from and to standard disks in D3 to D8 - even the 130XE ramdisk.
• D3toD8 also enables VTOCs between sector $xF8 and $xFF
  Bbecause the way DOS is programmed it cannot handle a $100-boundary between VTOC and the last sector of the directory.
  This tool loads additional code to RAM at $100.
  
• If you write DOS files through DUP's menu then the patched DOS will be written to disk.
  Modifications done by D3toD8 will be automatically unloaded. Otherwise DOS would crash during boot because at this time the code of D3toD8 is not yet loaded.

Usage MKPATCH.BAS:

• Enter a name for the copy right text to identify it
• Enter filename for it. The extension will automatically be the VTOC-location in hex.
• Enter the VTOC location in dec or hex.
• It writes the patcher to disk

I hope this tool is useful to somebody else.

Dos25Patch.atr