Content deleted Content added
m →History: Added a request for clarification. Added a request for specification. |
m Dated {{Specify}}{{Clarify}}. (Build p613) |
||
Line 6:
OFS is very good for repairing the filesystem in the event of a problem, although the so called DiskDoctor provided by Commodore quickly earned the name DiskDestroyer, because it could not repair No-DOS type autostart disks provided by third-party software manufacturers as bootable disks for games. Interestingly, the idea to create non-standard autobootable disks was born in a primitive attempt to prevent copy of such disks and to avoid the loading and launch of Amiga DOS, in order to directly access the amiga graphic, audio and memory chipsets. DiskDoctor in fact changed autostart disks bootblocks into standard AmigaDOS-based ones, renaming a disk with "Lazarus" namedisk, and made the autostart disk unusable.
Metacomco, BSTRINGS, BPOINTERS, and family contained in the Amiga FS entered the scene{{
== Characteristics ==
Line 22:
The standard Amiga filesystem has an internal 32 bit wide offset parameter (unsigned). It tells where to start the read/write operation. The biggest size for an Amiga disk is therefore 2<sup>32</sup> = 4 [[gibibyte|GiB]]. This limit was changed with Amiga OS 3.5 and 3.9 and now is 2<sup>64</sup> = 4 [[exbibyte|EiB]].
An OFS datablock stores block size BSIZE-24 bytes (i.e. normally 488 bytes at most frequently used BSIZE of 512 bytes), FFS stores BSIZE bytes (512 bytes per block).
FFS supports directory caching, links and international mode. The FFS is also faster than OFS.
The rootblock is located at the physical middle of the media: block number 880 for DD disks, block 1760 for HDs.
The exact calculation for where it is stored is as follows:
<pre>
numCyls = highCyl - lowCyl + 1
Line 38:
The rootblock contains information about the disk: its name, its formatting date, etc. It also contains information on accessing the files/directories/links located at the uppermost (root) directory.
The characters '/' and ':' are forbidden in file and volume names, but *!@#$%|^+&_()=\-[]{}';",<>.? and letters with diacritical marks like âè are allowed.
The date fields in the root block (and other blocks) are structured in the form of DAYS, MINS and TICKS. The DAYS field contains the number of days since January 1. 1978. MINS is the number of minutes that have passed since midnight and TICKS are expressed in 1/50s of a second. A day value of zero is considered illegal by most programs. Since the DAYS value is stored as a 32-bit number, the Amiga filesystem does not have an inherent [[Year 2000 problem]] or [[Year 2038 problem]].
To reach a file, directory or link in a removable media, Amiga has to compute its hash value with an algorithm often called a [[hash function]]. After having computed the hash value, this is then used to access HashTable ('ht' field in Rootblock/Directory block). The HashTable[ HashValue ] contains the number of the first block of the object (File header block, Directory block or Link block).
It happens that different names can result in the same HashValue. If more than one name has the same HashValue, the other blocks (for files and directory only) are stored in a chained list. This linked list starts at the 'next_hash' field of the file header or directory block.
For example: '''file_1a''', '''file_24''' and '''file_5u''' have the same hash value.
There was a bug in the old AmigaDOS versions when applying the hash function to international characters (ASCII codes > 128). Consequently, FFS was then created with the 'international mode' (INTL). Filename characters can be lowercase and uppercase, but are not case sensitive when accessed. That is to say, "MyFile" and "myfile" in the same directory refer to the same file.
| |||