RFC for FAT16 on the DCPU-16

FAT16-DCPU16.txt

			RFC for an FAT16-like file system on the DCPU16



General:
	This document aims to be the rfc for the FAT16 file system for the DCPU-16
	The file system is inspired by the FAT systems from other parallel universes. 
	It is divided into sectors consisting of usually 512 words. But this can be adapted, 
	if necessary. Though it can happen being not supported.
	All multi-word notations are little endian.
	The file system is case-sensitive.

Memory layout:
	+-------------------------+
	|Boot sector              |
	|reserved sector 2        |
	|...                      |
	|reserved sector n        |
	|FAT sector 1             |
	|...                      |
	|FAT sector n             |
	|1st Root directory sector|
	|free usable sectors      |
	|...                      |
	|last usable sector       |
	+-------------------------+

Boot sector:
	The boot sector contains the necessary data to identify the media and contains the boot loader code
	+------+----+--------------------------------------------------------------+
	|Offset|size|contains/purpose                                              |
	+------+----+--------------------------------------------------------------+
	|0x0   |1   |Magic Number/Jump to boot loader Code                         |
	|      |    |Boot able disk: 0b1100 1011 1000 0010 (0xCB82 - Add pc, 0x11  |
	|      |    |Non-boot able Disk: 0                                         |
	|0x1   |5   |Media Name, 7Bits per character, 11 characters in total       |
	|0x6   |1   |words per sector.          512 is standard                    |
	|0x7   |1   |sectors per cluster.         1 is standard                    |
	|0x8   |1   |Number of reserves sectors including boot sector. At least 1. |
	|0x9   |1   |Number of FAT tables.        1 is standard                    |
	|0xA   |2   |Total number of sectors      i.e. 1440 for HMU1440            |
	|0xC   |1   |Number of sectors per FAT    i.e. 3 for HMU1440               |
	|0xD   |1   |Number of Sides of media     i.e. 2 for HMU1440               |
	|0xE   |1   |Number of tracks per side    i.e. 40 for HMU1440              |
	|0xF   |1   |Number of sectors per track  i.e. 18 for HMU1440              |
	|0x10  |2   |Serial Number of media, generated on formatting               |
	|0x12  |494 |Boot loader code                                              |
	+------+----+--------------------------------------------------------------+

Reserved sectors:
	On boot-able media they contain the core code for the OS/CS/whatever.

FAT Tables:
	The FAT itself describes the entries for the specified sectors. 16 bit per sector, with no spacing.
	The 1st Root directory sector is number 0.
	The boot sector, the reserved sector and the fat sectors are not counted for the FAT. 
	FAT values:
	+---------------+------------------------------+
	|Value          |purpose                       |
	+---------------+------------------------------+
	|0x0000         |free cluster                  |
	|0x0001 - 0xFFFD|next cluster of current entry |
	|0xFFFE         |fault cluster                 |
	|0xFFFF         |last cluster of current entry |
	+---------------+------------------------------+

	Example: The Root directory contains cluster 0, 2 and ends with cluster 3, cluster 1 is empty
		Word No.    0      1      2      3
		FAT:     0x0002 0x0000 0x0003 0xFFFF ... 

Root directory:
	The root directory always starts at cluster 0.
	It is an standard directory.

Directory Entry:
	Directory entries defines, which files they do contain, including additional information
	Each entry is 16 words in size, thus 32 entries are fitting into 1 512-word cluster
	The starting date for time measurement can be adapted until the final release of this document.
	+------+----+-----------------------------------------------------------------+
	|Offset|size|contains/purpose                                                 |
	+------+----+-----------------------------------------------------------------+
	|0x0   |5   |File Name, 7Bits per character, 11 characters in total           |
	|0x5   |1   |Flags, Active Flags have their corresponding bits set            |
	|      |    |Bit 0: read only                                                 |
	|      |    |Bit 1: hidden                                                    |
	|      |    |Bit 2: system file                                               |
	|      |    |Bit 3: reserved                                                  |
	|      |    |Bit 4: subdirectory                                              |
	|      |    |Bit 5: archive                                                   |
	|      |    |Bit 6: executable                                                |
	|      |    |Bit 7-15: reserved                                               |
	|0x6   |3   |DateTime of file creation, multiplies of 100ms from 1.1.2012     |
	|0x9   |3   |DateTime of file modification, multiplies of 100ms from 1.1.2012 |
	|0xC   |1   |start cluster of File                                            |
	|0xD   |2   |file size in words                                               |
	|0xF   |1   |reserved                                                         |
	+------+----+-----------------------------------------------------------------+