File Formats - Petit Computer

This is a comprehensive list of the of the information contained in each file format used by Petit computer.


Common Heading

Total Size: 48 Bytes

Byte(s) Contained information
0-3 Petit Computer header: Always contains "PX01"
4-7 Number of bytes after MD5 hash
8-11 Possibly continuation of bytes after header: seems to always be 0x00000000.
12-19 File name
20-35 MD5 hash string. See notes
36-47 File type: Given in each category

Notes:

The hash string is made from the following information, in order:

You can use the following C++ library to generate MD5 hash strings. it is very portable and each encryption is broken into modules. The only thing you need to include to use this is the relavant header ("md5.h") and "<string>". http://create.stephan-brumme.com/hash-library/

Since writing a tutorial on MD5 hash generation would require a page of its own, and because I do not have time to break the whole algorithm down I will leave a link to how the algorithm operates if you want to make it yourself.


PRG File

File Type: PETC0300RPRG

Total Size(Without Heading): 12+ Bytes

Byte(s) Contained information
48-51 Unused(needs to be tested)
52-55 Package string, only useful if saved as package.
56-59 The number of characters in file(Including spaces, carriage returns, etc.). See notes
60-(size+59) Petit computer character data: See Character Table for reference
(size+60)-EOF All of the packaged file, in package string bit order. Only exists if saved as package.

Notes:

The absolute maximum number of characters that can be in a program is 524,228(0x07FFC4), which for whatever reason is less than the maximum value that 3 bytes can store. This is probably because the header takes up 60 bytes (0x00003C) which when added to the max character count gets 524,288, the maximum value Petit Computer allows.

Packaged programs just append the content of the files (without the PX01 header) at the end of the file.

CHR File

File Type: PETC0100RCHR

Total Size(Without Heading): 8193

Byte(s) Contained information
48-8239 Each byte makes up two pixels units which can have a value pointing to one of the 16(0-15) palette indexes.

Format:

Each character block is made up of 64 pixel elements which is stored into 32 bytes. The data for the character to the right (or first position next column) after this block. The following image will help to visualize how each block is stored

How a CHR file is stored

COL File

File Type: PETC0100RCOL

Total Size(Without Heading): 512 Bytes

Byte(s) Contained information
48-559 Palette data: See below for format

Format:

Each color consists of 2 bytes(16 bits) and the color is read in the following format.

GGGRRRRR GBBBBBGG

Each color has 5 bits -- except for green, which has 6.

The G channel must be right-rotated 2 bits in order to be parsed correctly.

MEM File

File Type: PETC0200RMEM

Total Size(Without Heading): 516 Bytes

Byte(s) Contained information
48-559 UCS-2 encoded string. Unused characters are filled with 0x00 for strings less than 256 characters.
560-563 Length of string (in characters - not bytes.)

Format:

Strings are encoded as UCS-2 characters, and all printed characters are encoded as their full width equivalents (eg, 'H' (ASCII 72) is encoded as 'Ｈ' (Unicode 65320/0xFF28))

GRP File

File type: PETC0100RGRP

Total Size(Without heading): 49152 (256x192 8bpp)

Byte(s) Contained information
48-49199 GRP data: See below, or linked page.

Format:

GRP data is actually stored essentially as blocks of character data: A 4x3 page of 8x8 blocks of 8x8 characters. The difference is that the characters have 256 colors instead of 16. See here for more information.

SCR File

File type: PETC0100RSCR

Total Size(Without heading): 8192

Byte(s) Contained information
48-8239 SCR data: See below.

Format:

Each BG tile is represented as a 16 bit (2 byte) value. The 64x64 BG layer is broken into four 32x32 chunks, which are stored in the file as tile values, going left to right and then top to bottom through each chunk. The chunks are similarly stored left to right and top to bottom.

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