SCUMM Disk Format

by Brian Bagnall

In order to construct a proper SCUMM editor, it must be able to read room data (scripts, etc...) from the original Zak or MM disk, modify it, then rewrite it to a 1541 disk. The actual file the SCUMM editor will use is a .d64 file, which is 174,848 bytes in size. (This file can then be written to an actual 5 1/4" floppy disk using an XE1541 cable, or the file can be used directly in a C64 emulator.)

The original C64 Maniac Mansion came on one disk, with the program/data on side one and more data on side two. Zak McKracken came on two disks, and used three sides of those disks. There is a boot disk, which only has a single executable file, and the data disk. All we are interested in is the data disk - side one and two.

Standard Disk Format

(taken from the Commodore user manual)

Here is the format of a regular 1541 disk:

  • 256 bytes per sector (sector = block)
  • 683 sectors/blocks per disk
  • There are 35 tracks on a disk.
  • Each track has between 17 and 21 sectors. ie Outer tracks have more sectors than inner because of difference in circumferance of round disk.
  • Track 1-17 has 21 sectors
  • 18-24 has 19
  • 25-30 has 18
  • 31-35 has 17
  • This comes to 174,848 bytes total

    Here is the format of the directory information of a standard 1541 disk:

  • Normally 1541 disks store directory info on track 18 (19 blocks used - ie 4,864 bytes)
  • In hex editor, the data located at bytes 91,392 through 96,255
  • 91,392 to 91,535 stores the BAM
  • 91,536 to 96,255 stores the file indexes
  • It can store a maximum of 144 files

    The Lucasarts data disks use a different, unknown format that does not store the directory information on track 18. The Zak McKracken data disks are not in the standard File Format! Here is what we know about it so far:

  • SCUMM may use track 25 for file info because in the Vice emulator it seems to check track 25 when accessing a new room.

    Possible Strategies

  • Maybe there is no index. Maybe you just tell it which side of the disk it is on, and each room is allocated the same amount of disk space (highly unlikely!)
  • On the PC, all this data is in the 00.lfl file. On the C64, the data is VERY similar but different (see screenshot). Try to understand the 00.lfl format, then see what it means on the C64.
  • if the file pointer data is written on the same track and sector on disk 1 and 2, then a program to compare the data and find identical data will point out where this data is located.
  • Possibly this data is on the boot disk and stored in memory at all times?
  • Look at all the file space used by rooms. The leftover file space must have the file pointer data. Search this space.

    How to manually pull a room file

    Here's how you figure out exactly where the room starts:

    1. Look at disk 1, at location 0x500. The data reads:

      2. 0x500: D1 0A 00 9D 28 11 00 0E ... The first two pair of bytes are, I think, the offset to the init or main script of the room. The second two pairs = unknown?
    2. But here's the beef: 28 11 <- room width, room height
      3. 40x17 tiles
    3. all rooms have 11 as height, while most rooms have 40 (except for the ones that scroll) but they're either 0x28, 0x50 or 0xA0 (iirc)
      4. so they're pretty easy to spot
    4. Maniac Mansion has 52 .lfl files (rooms) are on both sides. On disk one it lists which side it's on. 14 on side 1, 38 on side 2.
    5. Zak has 58 rooms. The disk even lists them all at the start. It reads exactly:
      2222222222222221222222222222212112222221112211111211
    6. Right after this is a table of numbers, starting at about 316. These have the track number of the lfl file, followed by the sector number that it starts at. The first word of the lfl file is the length of the file.

    Is the Zak McKracken SCUMM Engine Backward Compatible with Maniac Mansion?

    Since the scripting codes are the same, I thought I'd test to see if the Zak core engine could run the MM data. When Zak loads it says to insert disk 1, so I tried inserting the MM disk 1 instead. The program started loading data, then it asked me to insert disk __ [unknown characters]. Interesting...

    The Zak McKracken Rooms

    For the heck of it, I started listing the rooms and where they are found on the Zak McKracken disks.

    PC File: Room: Track: 01.lfl Front of House 1 02.lfl Bottom of Pool 03.lfl Living Room 3 04.lfl Dungeon 05.lfl Library 06.lfl Pool 07.lfl Kitchen 9 08.lfl 09.lfl 10.lfl 11.lfl 12.lfl 13.lfl 14.lfl 15.lfl 16.lfl 17.lfl 18.lfl 19.lfl 20.lfl 21.lfl 22.lfl 23.lfl 24.lfl 25.lfl 26.lfl 27.lfl 28.lfl 29.lfl 30.lfl 31.lfl 32.lfl 33.lfl 34.lfl 35.lfl 36.lfl 37.lfl 38.lfl 39.lfl 40.lfl 41.lfl 42.lfl 43.lfl 44.lfl 45.lfl 46.lfl 47.lfl 48.lfl 49.lfl 50.lfl 51.lfl 52.lfl