Commodore 64: TENGEN/DOMARK and IMAGE WORKS tape loader

@DrWatson

Published in 

C64 tape formats

 · 20 May 2021

Found on...

• 3D construction kit (F1)
• Cisco Heat (F1)
• Badlands (F1)
• Rugby the World Cup (F1)
• The Spy who loved me (F1)

• Skull and Crossbones A (F1)
• Skull and Crossbones B (17 F2 files)

• Pit Fighter A (1 F1 file + 6 F2 files)
• Pit Fighter B (17 F2 files)

• Rampart A (F1)
• Rampart B...

This one is weird, i think it could be a custom version of F2.
there appears to be file numbering (File ID) in the headers.

--------------------------------------------------------------------------------

Bit 0: $44
Bit 1: $65
Endianess: LSbF

Pilot: $A0 (*255)
Sync: $0A down to $01

2 File Formats are known...

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Format 1...

• 2 bytes : Data Size (LSBF).
• 1 byte : File ID. (Starting at $01 for the first file and incrementing)
• n bytes : Data (encrypted).
• 1 byte : Checksum (0 XOR all decrypted data bytes).

Notes:

Where is load address?. (not essential for FT)

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Format 2 (used for multi-load parts, if any are present)...

• 2 bytes : Data Size (LSBF).
• 2 bytes : Load address (LSBF).
• n bytes : File name (size is variable per tape).
• d bytes : Data (encrypted).
• 1 byte : Checksum (0 XOR all decrypted data bytes).

Notes:

n=1 in "Skull and Crossbones".
n=2 in "Cisco Heat", "Hydra" and "Pit Fighter".
n=4 in "Rampart".

Where is filename length?. (important)

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To decrypt data...

 a= 0  
 do 
   b = readbyte 
   b= b XOR a  
   a= (a+1) AND $FF 
 while not end of data. 

------------------------------------------------
Notes:

There are 2 sets of CBM files, the second one contains the loader.

Autostarting ROM cartridges are disabled.

File type "could" be identified by calculating the XOR checksum.
(not a method ft likes at all)

Load address is sometimes ignored ("Cisco Heat", "Pit Fighter", "Rampart").

File load address low-byte for Format 2 files seems to always be 00.
this could be used to determine the filetype as Format 1 files would have a value of at least 01 (file ID) in the same position.