TweetFollow Us on Twitter

Disks
Volume Number:1
Issue Number:5
Column Tag:FORTH FORUM

Disks

By Jörg Langowski

This month we are going to look at the organization of the 400 K bytes on the standard Macintosh disk. The operating system does a very good job of hiding this organization from you, but for patching disks, changing file attributes, and looking at files of unknown structure it is very convenient to know a little more about the ‘deep structure’ of the Macintosh disk.

Fortunately, it is very easy to read any byte at any position on the disk. The toolbox routines READ and WRITE do not make any distinction between files and whole disks. Let’s recall how a read or write through a toolbox call is done. The toolbox traps are A002 for read and A003 for write. You have to set up a file control block, pass its address in register A0, and execute the trap. File I/O, direct disk I/O, serial I/O and even the sound generation are all handled through this mechanism. The only difference is in the file control block (FCB). It has the following structure:

Bytes 0 11 : Header; IM tells us nothing about it

Bytes 12 15 : Address of the I/O completion routine

Bytes 16 17 : I/O result code ( also returned in D0 )

Bytes 18 21 : Pointer to filename string ( for files )

Bytes 22 23 : Drive number ( for direct disk I/O )

Bytes 24 25 : Reference number ( explained later )

Bytes 32 35 : Pointer to data buffer

Bytes 36 39 : Requested byte count for I/O operation

Bytes 40 43 : Actual # of bytes read/written

Bytes 44 45 : Positioning mode, 0 : relative, 1 : absolute

Bytes 46 49 : Position offset ( in bytes )

If you want to do file I/O, you have to open the file first. This is done by setting up the FCB with a valid file name (with optional volume prefix) and calling the trap OPEN ( A000 ). This will return a reference number (positive 16-bit integer) in the FCB, through which all read/write calls are made from now on.

The only difference between doing a file I/O and direct device I/O operation is this reference number. The predefined Macintosh device drivers have negative reference numbers. They are listed on pages 22 and 23 of the Device Manager Programmer’s Guide in IM, and the important one for us is the disk reference number, -5. If you set up the FCB like above, with the reference number -5, and then do a READ or WRITE call, the disk will be read/written directly. That is, the operating system treats the whole disk as one large file 400K bytes long. The position from which I/O starts is given by the offset in bytes 46 49, and the number of bytes to I/O is in 36 39. If 44 45 contain a 0, the offset is counted from the last byte read/written; if it is one, is is counted from the start of the disk. After the I/O is completed, a result code will be returned in the FCB and in register D0. Zero means that everything went OK; a negative return code means that something was wrong. For instance, if you try to read or write to a non-existing position on the disk, -67 is returned; -50 is returned if the number of bytes actually read into the buffer is greater than the number requested. This happens if you don’t read an integer multiple of 512 bytes; the number is then rounded up to the next 512 bytes.

In FORTH we call the traps through the defining word OS.TRAP. The FCB address is then passed on the stack, and the result code is stored into the variable IO-RESULT.

This is about all the information you need to understand the simple disk editor program that is listed at the end. It is menu-oriented and reads, writes, dumps to screen and modifies any 1024 byte block on the (internal) disk. Using that program, you can verify very easily what I am telling you in the rest of this article.

Macintosh disks are read and written by the operating system in 512 byte blocks (‘logical blocks’). However, the operating system refers to 1K blocks as the smallest unit (‘allocation block’). Therefore, the program reads 2 logical blocks at a time and the block number that the program asks for is INT(logical block number / 2).

A Directory Entry

With the described program it is now quite easy to figure out some facts about Mac disk organization; the IM manual helps, too. Starting with block O as the first block on the disk, the directory resides in logical blocks $4 to $B (allocation blocks $2 to $5); easily recognizable because all the file names are there. The map in Fig. 1 shows the structure of a directory entry.

The first part of the entry tells the system several parameters it needs to know about the file. ‘Attributes’ contains 8 bits of file attributes. For instance, Bit 7 set means that the file is open, bit 0 set means it is software locked, . Bit 6 is the copy protect bit. If you reset this one to zero, you will be able to copy a ‘protected’ file by dragging the icon. Bytes $2-$5 give the file type, such as APPL (application), ZSYS (system file) or TEXT (text file) in ASCII format, bytes $6-$9 give the creator. The four Finder words contain information that is used by the Finder internally. All directory entries are numbered sequentially ($14-$15).

Bytes $16 and $17 (16 bit integer) give the starting block of the data fork, bytes $18 to $1B (32 bit integer) its length in bytes and bytes $1C to $1F this length rounded up to the next 512 byte boundary. (The blocks referred to in the directory entry are allocation blocks; block number 2 starts right after the last directory block). The resource fork is referenced in the same way by the next 10 bytes in the directory entry. The creation and modification dates of the file are kept in the next 8 bytes.

The last part of the entry gives the file name; remarkable here is that the directory entries are not all the same length. Since file names may be up to 255 characters long, reserving the maximum space for every file name would be inefficient; therefore the name is stored as a standard string starting at byte $32 (Hex) with a length byte and the name thereafter.

The volume information table

Logical blocks $2 and $3 (Fig. 2) on the Macintosh disk contain information about the disk itself and a block allocation table that tells the system which blocks are in use.

The first two bytes are always $D2D7; if they are not, the disk will not be recognized as a Macintosh disk. Following that are two 4-byte words that give the time and date of initialization and last backup. The 16-bit word Volume Attributes will have bit 7 set if the write protect latch is set on the diskette and bit 15 set if the disk is locked by software. The volume copy protection bit is also located here, it is bit 14 and if you reset it, the disk will be copyable with the Disk Copy routine on the System Disk, regardless of whether individual files are ‘protected’ or not. The next entries give the total number of files in the directory, the first logical block of the file directory and the number of logical blocks in the directory.

Following are the total number of allocation blocks on the volume and the size of the allocation block in bytes ($0400 on a standard Macintosh disk). The meaning of the remaining parameters should be clear from the diagram.

IM describes how the volume allocation block map is organized; I’ll quickly repeat that here. Every allocation block (1024 bytes) is represented by a 12-bit entry. If this entry is zero, the block is unused. If it is used in a file, it contains the number of the next block in the file. The last block in the file is indicated by a 1.

: disk.editor ;
18 field +fcb.name    22 field +fcb.drive   24 field +fcb.vrefnum 
32 field +fcb.buf     36 field +fcb.request 40 field +fcb.actual
44 field +fcb.posmode 46 field +fcb.position
12 constant dsk.menu

variable vol.fcb  variable vol.fnumber  variable hex.asc
create this.fcb 50 allot     create vol.buffer 1024 allot
hex a002 os.trap read  a003 os.trap write decimal

: open.vol  this.fcb dup vol.fcb ! dup +fcb.vrefnum -5 swap w!
            +fcb.drive 1 swap w!  ;

: input 0 0 >in ! query 32 word convert drop ;

: dump.fcb .” Header    : “ 3 0 do dup i 4* + @ . .”  “ loop cr
   .” completion: “ dup 12 + @ . cr  .” ioresult  : “ dup 16 + w@ . cr
   .” filename  : “ dup 18 + @ . cr  .” drive     : “ dup 22 + w@ . cr
   .” refnum    : “ dup 24 + w@ . cr .” buffer    : “ dup 32 + @ . cr
   .” request   : “ dup 36 + @ . cr  .” actual    : “ dup 40 + @ . cr
   .” posmode   : “ dup 44 + w@ . cr .” offset    : “ dup 46 + @ . cr 
;

: setup.fcb ( buffer \ block# \ fcb -- fcb )
 dup +fcb.posmode 1 swap w!  dup +fcb.position rot 1024 * swap !
 dup +fcb.buf rot swap !     dup +fcb.request 1024 swap ! ;

: read.pb ( buffer \ block# \ fcb -- )    setup.fcb  read ;
: read.disk ( block# -- ) vol.buffer swap vol.fcb @ read.pb ;

: write.pb ( buffer \ block# \ fcb -- )    setup.fcb  write ;
: write.disk ( block# -- )  vol.buffer swap vol.fcb @ write.pb ;

: dump.32 ( start address -- )
  32 0 do dup i + c@ hex.asc @ if
                   dup 16 < if .” 0" then . else
                   dup 32 < if .” .” drop else emit then then loop ;

: dump.buffer ( buffer address -- )
 9 textsize 9 line.height condensed textstyle cr
 32 0 do dup i 32 * dup 16 < if .” 00" else dup 256 < if .” 0" then then
         dup . +  dump.32 drop cr loop ;

: read.block 12 textsize 15 line.height plain textstyle cr
   .” Read block #: “ input dup 0< if error” Negative Block #” then
       read.disk io-result @ 0= not if cr .” OS Error “ io-result @ . 
cr abort
                                     else .” block read” cr  then ;

: write.block 12 textsize 15 line.height plain textstyle cr
   .” Write to block #: “ input dup 0< if error” Negative Block #” then
     write.disk io-result @ 0= not if cr .” OS Error “ io-result @ . 
cr abort
                                     else .” block written” cr then ;

: dump.block hex vol.buffer dump.buffer decimal ;

: patch.block 12 textsize 15 line.height plain textstyle cr
  .” change byte#: “ hex input decimal dup 1023 >
  if .” too large” cr abort then
             vol.buffer + .” to: “ hex input decimal swap c! ;

: set.hex 1 hex.asc ! 6 -1 dsk.menu item.check  7 0 dsk.menu item.check 
;

: set.ascii 0 hex.asc !  6 0 dsk.menu item.check  7 -1 dsk.menu item.check 
;

: disk.menu
   0 “ DiskEdit” dsk.menu new.menu
    “ Read;Write;Dump;Change;-(;Hex;Ascii” dsk.menu append.items
     draw.menu.bar dsk.menu menu.selection:
     0 hilite.menu   case
     1 of  read.block      endof  2 of  write.block     endof
     3 of  dump.block      endof  4 of  patch.block     endof
     6 of  set.hex         endof  7 of  set.ascii       endof
                     endcase
     events on do.events abort ;
  
disk.menu set.hex open.vol

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Logic Pro X 10.1.1 - Music creation and...
Apple Logic Pro X is the most advanced version of Logic ever. Sophisticated new tools for professional songwriting, editing, and mixing are built around a modern interface that's designed to get... Read more
VLC Media Player 2.2.0 - Popular multime...
VLC Media Player is a highly portable multimedia player for various audio and video formats (MPEG-1, MPEG-2, MPEG-4, DivX, MP3, OGG, ...) as well as DVDs, VCDs, and various streaming protocols. It... Read more
Sound Studio 4.7.8 - Robust audio record...
Sound Studio lets you easily record and professionally edit audio on your Mac. Easily rip vinyls and digitize cassette tapes, or record lectures and voice memos. Prepare for live shows with live... Read more
LibreOffice 4.4.1.2 - Free, open-source...
LibreOffice is an office suite (word processor, spreadsheet, presentations, drawing tool) compatible with other major office suites. The Document Foundation is coordinating development and... Read more
Freeway Pro 7.0.3 - Drag-and-drop Web de...
Freeway Pro lets you build websites with speed and precision... without writing a line of code! With its user-oriented drag-and-drop interface, Freeway Pro helps you piece together the website of... Read more
Cloud 3.3.0 - File sharing from your men...
Cloud is simple file sharing for the Mac. Drag a file from your Mac to the CloudApp icon in the menubar and we take care of the rest. A link to the file will automatically be copied to your clipboard... Read more
Cyberduck 4.6.5 - FTP and SFTP browser....
Cyberduck is a robust FTP/FTP-TLS/SFTP browser for the Mac whose lack of visual clutter and cleverly intuitive features make it easy to use. Support for external editors and system technologies such... Read more
Firefox 36.0 - Fast, safe Web browser. (...
Firefox for Mac offers a fast, safe Web browsing experience. Browse quickly, securely, and effortlessly. With its industry-leading features, Firefox is the choice of Web development professionals and... Read more
Thunderbird 31.5.0 - Email client from M...
As of July 2012, Thunderbird has transitioned to a new governance model, with new features being developed by the broader free software and open source community, and security fixes and improvements... Read more
VOX 2.4 - Music player that supports man...
VoxIt just sounds better! The beauty is in its simplicity, yet behind the minimal exterior lies a powerful music player with a ton of features & support for all audio formats you should ever need... Read more

Get The Whole Story – Lone Wolf Complete...
Get The Whole Story – Lone Wolf Complete is Now Available and On Sale Posted by Jessica Fisher on February 27th, 2015 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
Check Out the Trailer for the Upcoming F...
Check Out the Trailer for the Upcoming FINAL FANTASY: Record Keeper Posted by Jessica Fisher on February 26th, 2015 [ permalink ] DeNA and Square Enix have announced that | Read more »
Legacy Quest is an Upcoming Rouge-like T...
Legacy Quest is an Upcoming Rouge-like That’ll Kill the Whole Family Posted by Jessica Fisher on February 26th, 2015 [ permalink ] Nexon Co. | Read more »
Grudgeball: Enter the Chaosphere Review
Grudgeball: Enter the Chaosphere Review By Jordan Minor on February 26th, 2015 Our Rating: :: MUSCLE MENUniversal App - Designed for iPhone and iPad Regular Show gets an above average game.   | Read more »
Action RPG League of Angels – Fire Raide...
Gaia is being invaded by the Devil Prince and the demonic Devil Army at his disposal, and it’s up to you and your avatar to defeat him in League of Angels – Fire Raiders. Raise a mighty army from hundreds of recruitable angel heroes and take the... | Read more »
Burn Rubber on the Ice With a New Cars:...
Burn Rubber on the Ice With a New Cars: Fast as Lightning Update Posted by Jessica Fisher on February 26th, 2015 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
AdVenture Capitalist Review
AdVenture Capitalist Review By Jordan Minor on February 26th, 2015 Our Rating: :: DAS KAPITALUniversal App - Designed for iPhone and iPad An inadvertent Marxist manifesto.   | Read more »
Monster vs Sheep Review
Monster vs Sheep Review By Jennifer Allen on February 25th, 2015 Our Rating: :: SAMEY FUNUniversal App - Designed for iPhone and iPad What Monster vs Sheep lacks in variety it makes up for with stress relieving fun. At least, for a... | Read more »
Is Your Face Ready for the New Outwitter...
Is Your Face Ready for the New Outwitters 2.0 Trailer? Posted by Jessica Fisher on February 25th, 2015 [ permalink ] One Man Left Studios has announced that their turn-based strategy game, | Read more »
HowToFormat Review
HowToFormat Review By Jennifer Allen on February 25th, 2015 Our Rating: :: USEFUL TIPSiPhone App - Designed for the iPhone, compatible with the iPad Making a presentation and want to get it just right? HowToFormat teaches you how... | Read more »

Price Scanner via MacPrices.net

Apple CEO Tim Cook to Deliver 2015 George Was...
Apple CEO Tim Cook will deliver the George Washington University’s Commencement address to GWU grads on May 17, at which time he will also be awarded an honorary doctorate of public service from the... Read more
Apple restocks refurbished Mac minis for up t...
The Apple Store has restocked Apple Certified Refurbished 2014 Mac minis, with models available starting at $419. Apple’s one-year warranty is included with each mini, and shipping is free: - 1.4GHz... Read more
Save up to $50 on iPad Air 2s, NY tax only, f...
 B&H Photo has iPad Air 2s on sale for $50 off MSRP including free shipping plus NY sales tax only: - 16GB iPad Air 2 WiFi: $469.99 $30 off - 64GB iPad Air 2 WiFi: $549 $50 off - 128GB iPad Air 2... Read more
16GB iPad Air 2 on sale for $447, save $52
Walmart has the 16GB iPad Air 2 WiFi on sale for $446.99 on their online store for a limited time. Choose free shipping or free local store pickup (if available). Sale price for online orders only,... Read more
iMacs on sale for up to $205 off MSRP
B&H Photo has 21″ and 27″ iMacs on sale for up to $205 off MSRP including free shipping plus NY sales tax only: - 21″ 1.4GHz iMac: $1029 $70 off - 21″ 2.7GHz iMac: $1199 $100 off - 21″ 2.9GHz... Read more
Apple Takes 89 Percent Share of Global Smartp...
According to the latest research from Strategy Analytics, global smartphone operating profit reached US$21 billion in Q4 2014. The Android operating system captured a record-low 11 percent global... Read more
New Travel Health App “My Travel Health” iOS...
Rochester, Minnesota based Travel Health and Wellness LLC has announced that its new iOS app help safeguard the user’s health when traveling abroad — “My Travel Health” is now available on the Apple... Read more
Sale! MacBook Airs for up to $115 off MSRP
B&H Photo has MacBook Airs on sale for up to $100 off MSRP. Shipping is free, and B&H charges NY sales tax only: - 11″ 128GB MacBook Air: $799 100 off MSRP - 11″ 256GB MacBook Air: $999 $100... Read more
15-inch 2.0GHz Retina MacBook Pro (refurbishe...
The Apple Store has Apple Certified Refurbished previous-generation 15″ 2.0GHz Retina MacBook Pros available for $1489 including free shipping plus Apple’s standard one-year warranty. Their price is... Read more
Wither The iPad mini? End Of The Road Imminen...
AppleDailyReport’s Dennis Sellers predicts that the iPad mini is going to be left to wither on the vine, as it were, and then just allowed to fade away — a casualty of the IPhone 6 Plus and other... Read more

Jobs Board

Sr. Technical Services Consultant, *Apple*...
**Job Summary** Apple Professional Services (APS) has an opening for a senior technical position that contributes to Apple 's efforts for strategic and transactional Read more
Event Director, *Apple* Retail Marketing -...
…This senior level position is responsible for leading and imagining the Apple Retail Team's global engagement strategy and team. Delivering an overarching brand Read more
*Apple* Pay - Site Reliability Engineer - Ap...
**Job Summary** Imagine what you could do here. At Apple , great ideas have a way of becoming great products, services, and customer experiences very quickly. Bring Read more
*Apple* Solutions Consultant - Retail Sales...
**Job Summary** The ASC is an Apple employee who serves as an Apple brand ambassador and influencer in a Reseller's store. The ASC's role is to grow Apple Read more
*Apple* Solutions Consultant - Retail Sales...
**Job Summary** As an Apple Solutions Consultant (ASC) you are the link between our customers and our products. Your role is to drive the Apple business in a retail Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.