PC-98 format

This document describes the format of data in Prince of Persia 1 for the PC-98.

General

Byte order: Numbers larger than a byte use little-endian (Intel) byte order, unless noted otherwise.

Disk images

Disk images can be either headerless, or have a header.

Online, this game is most commonly found in *.FDI files, these have a header of 0x1000 bytes.

• Tools: datacut.exe to extract files from a (headerless!) disk image, makeimg.exe to make a disk image from files.

FDI header

The FDI header contains the following information:

Offset in bytes	Size	Value on PoP disks	Meaning
0x00	4 bytes	0	(unknown)
0x04	4 bytes	0x90 = 144	(unknown)
0x08	4 bytes	0x1000 = 4096	Start offset of the raw disk image (i.e. the size of the FDI header).
0x0C	4 bytes	0x134000 = 1261568	Size of the raw disk image in bytes.
0x10	4 bytes	0x400 = 1024	Size of a sector (or cluster?) in bytes.
0x14	4 bytes	8	Number of sectors per track?
0x18	4 bytes	2	Number of sides?
0x1C	4 bytes	0x4D = 77	Number of tracks per side?

If you multiply the last four numbers, you get the size of the raw disk image: 0x400 * 8 * 2 * 0x4D = 0x134000.

The rest of the header is filled with zeroes.

File system

A disk consists of 0x4D0 sectors, numbered from 0 to 0x4CF. Each sector is 0x400 = 1024 bytes.

The game disk (disk A) contains the following parts:

Offset in bytes	Occupied sectors	Contents
0x0000 - 0x03FF	0	#Boot sector
0x0400 - 0x0FFF	1 - 3	#File allocation table
0x1000 - 0x1FFF	4 - 7	#Directory
0x2000 - 0x133FFF	8 - 0x4CF	Data area

The offsets in the table assume you have a headerless disk image.

Boot sector

The boot sector contains the code for starting the game.

From offset 2 is the disk label "GAME DISK" on disk A, and "USER DISK" on disk B.

Directory

This area contains 0x100 = 256 entries, of 0x10 = 16 bytes each.

Each entry describes a file in the following format:

Offset in bytes	Size	Meaning
0	8 bytes	Base name. If it's shorter than 8 characters then it's padded with spaces (0x20).
8	3 bytes	Extension. (The same padding is used here, although it does not occur in the original PoP disk image.)
0x0B = 11	3 bytes	Unknown. Might be file attributes and/or modification date/time?
0x0E = 14	2 bytes	The number of the first sector of this file.

To get the full file name, concatenate the base name (with padding removed), a dot, and the extension.

Unused entries are filled with 0xFF bytes.

File allocation table

This area contains 2-byte entries for each sector on the disk.

There is space for 0x600 entries, but only the first 0x4D0 entries are used, since the disk has only that many sectors. The unused entries are filled with zeroes.

The possible values of each entry are:

• 0xFFFF : If the sector is before the data area (sectors 0-7).
• 0x0008 - 0x04CF : If this sector is part of a file but not the last sector.
  • This whole sector is part of the file.
  • The value is the number of the next sector of the file.
• 0xFC00 - 0xFFFF : If this is the last sector of a file.
  • Subtract 0xFC00 from the value to get a size. The first size bytes of this sector are part of the file.
  • If you get 0 from the subtraction, then the whole sector is part of the file. (This does not occur in the original PoP disk image, though.)
  • The unused area of last sectors is filled with 0x00. (makeimg.exe fills it with 0xE5.)
• 0x0000 : If the sector is not used or does not exist (unused entries).
  • Unused sectors are filled with 0xE5.

How to read a file

• Find the file in the #Directory.
• Read the number of the first sector into current_sector.
• Repeat until exit:
  • current_sector should be a valid sector number pointing into the data area (between 0x0008 and 0x04CF). If it's not then it's an error.
  • Read the #File allocation table entry corresponding to current_sector into next_sector. (offset = 0x400 + current_sector * 2, length = 2)
  • Read the sector numbered current_sector into sector_data. (offset = current_sector * 0x400, length = 0x400)
  • If next_sector < 0xFC00:
    • Append sector_data to the output.
    • Set current_sector to next_sector.
    • Continue the loop.
  • else:
    • Set size to next_sector - 0xFC00.
    • If size = 0 then set size to 0x400.
    • Append the first size bytes of sector_data to the output.
    • Exit the loop.
• Done.

TODO: How to make an empty disk. How to add a file.

TODO

• Compression
• Levels
• Graphics
• Fonts
• Music and sounds
• Texts?
• User disk
• FM Towns version

External links

• Documentation and tools by zarazala, and the game itself.