TweetFollow Us on Twitter

Volume Number:1
Issue Number:5


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    22 field   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

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!
   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 item.check  7 0 item.check 

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

   0 “ DiskEdit”
    “ Read;Write;Dump;Change;-(;Hex;Ascii” append.items menu.selection:
     0   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
     events on abort ; set.hex open.vol


Community Search:
MacTech Search:

Software Updates via MacUpdate

Suitcase Fusion 6 17.3.0 - Font manageme...
Suitcase Fusion 6 is the creative professional's font manager. Every professional font manager should deliver the basics: spectacular previews, powerful search tools, and efficient font organization... Read more
Nisus Writer Pro 2.1.2 - Multilingual wo...
Nisus Writer Pro is a powerful multilingual word processor, similar to its entry level products, but brings new features such as table of contents, indexing, bookmarks, widow and orphan control,... Read more
calibre 2.40.0 - Complete e-book library...
Calibre is a complete e-book library manager. Organize your collection, convert your books to multiple formats, and sync with all of your devices. Let Calibre be your multi-tasking digital librarian... Read more
Vivaldi - An advanced browser...
Vivaldi is a browser for our friends. In 1994, two programmers started working on a web browser. Our idea was to make a really fast browser, capable of running on limited hardware, keeping in mind... Read more
OmniPlan 3.0 - Robust project management...
With OmniPlan, you can create logical, manageable project plans with Gantt charts, schedules, summaries, milestones, and critical paths. Break down the tasks needed to make your project a success,... Read more
Yummy FTP 1.11 - FTP/SFTP/FTPS client fo...
Yummy FTP is an FTP + SFTP + FTPS file transfer client which focuses on speed, reliability and productivity. Whether you need to transfer a few files or a few thousand, schedule automatic backups, or... Read more
Tweetbot 2.1 - Popular Twitter client. (...
Tweetbot is a full-featured OS X Twitter client with a lot of personality. Whether it's the meticulously-crafted interface, sounds and animation, or features like multiple timelines and column views... Read more
MacPilot 8.0 - Enable over 1,200 hidden...
MacPilot gives you the power of UNIX and the simplicity of Macintosh, which means a phenomenal amount of untapped power in your hands! Use MacPilot to unlock over 1,200 features, and access them all... Read more
Typinator 6.7 - Speedy and reliable text...
Typinator turbo-charges your typing productivity. Type a little. Typinator does the rest. We've all faced projects that require repetitive typing tasks. With Typinator, you can store commonly used... Read more
Adobe Lightroom 6.2 - Import, develop, a...
Adobe Lightroom is available as part of Adobe Creative Cloud for as little as $9.99/month bundled with Photoshop CC as part of the photography package. Lightroom 6 is also available for purchase as a... Read more

Balloony Land offers a fresh twist on th...
Balloony Land by Palringo offers a fresh twist on the match three genre and is out now on iOS and Android. First-off, you'll be popping balloons instead of crushing candy and the balloons will float up and fill the empty spaces instead of dropping... | Read more »
Sago Mini Babies (Education)
Sago Mini Babies 1.0 Device: iOS Universal Category: Education Price: $2.99, Version: 1.0 (iTunes) Description: Introducing the Sago Mini babies. Boys and girls love caring for these adorable characters. Feed Robin her favorite mush... | Read more »
PAUSE - Relaxation at your fingertip (H...
PAUSE - Relaxation at your fingertip 1.1 Device: iOS iPhone Category: Healthcare & Fitness Price: $1.99, Version: 1.1 (iTunes) Description: | Read more »
Super Sharp (Games)
Super Sharp 1.1 Device: iOS Universal Category: Games Price: $1.99, Version: 1.1 (iTunes) Description: Your finger has never been so sharp! Cut with skill to complete the 120 ingenious physics levels of Super Sharp and become a cut... | Read more »
Assembly - Graphic design for everyone...
Assembly - Graphic design for everyone 1.0 Device: iOS Universal Category: Photography Price: $2.99, Version: 1.0 (iTunes) Description: Assembly is the easiest most powerful design tool on the App Store. Create anything you can... | Read more »
Dub Dash (Games)
Dub Dash 1.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0 (iTunes) Description: ARE YOU READY FOR THE ULTIMATE CHALLENGE? UNIQUE SYMBIOSIS OF MUSIC AND GRAPHICS | Read more »
Leave Me Alone (Games)
Leave Me Alone 1.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0 (iTunes) Description: 33% off launch sale!!! Somewhere between the 1980s and 1990s there exists a world that never was. A world of skatepunks,... | Read more »
YAMGUN (Games)
YAMGUN 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: The invasion has begun! Protect the walls of the citadel against waves of enemies! But watch out, you will soon run out of ammo...... | Read more »
Chesh (Games)
Chesh 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: It’s like chess, only not at all. ***40% off for a limited time to celebrate our launch!*** Chesh is a game of skill, strategy, luck,... | Read more »
Dust: An Elysian Tail (Games)
Dust: An Elysian Tail 1.051 Device: iOS Universal Category: Games Price: $5.99, Version: 1.051 (iTunes) Description: *Dust: An Elysian Tail is a visually rich experience and thus requires devices with 1GB RAM**Please note that... | Read more »

Price Scanner via

TP-LINK Next-Gen Routers Support a Large Numb...
TP-LINK, specialists in consumer and business networking products, have announced the availability of Archer C2600, the company’s next-generation router featuring wireless AC, multi-user MIMO, and 4-... Read more
Apple’s Education discount saves up to $300 o...
Purchase a new Mac or iPad using Apple’s Education Store and take up to $300 off MSRP. All teachers, students, and staff of any educational institution qualify for the discount. Shipping is free, and... Read more
Save up to $350 with Apple refurbished iMacs
Apple has Certified Refurbished iMacs available for up to $350 off the cost of new models. Apple’s one-year warranty is standard, and shipping is free: - 27″ 3.5GHz 5K iMac – $1949 $350 off MSRP - 27... Read more
Mac Pros on sale for up to $300 off MSRP
B&H Photo has Mac Pros on sale for up to $300 off MSRP. Shipping is free, and B&H charges sales tax in NY only: - 3.7GHz 4-core Mac Pro: $2818.99, $181 off MSRP - 3.5GHz 6-core Mac Pro: $3699... Read more
5K iMacs on sale for up to $150 off MSRP, fre...
B&H Photo has the 27″ 3.3GHz 5K iMac on sale for $1899.99 including free shipping plus NY tax only. Their price is $100 off MSRP. They have the 27″ 3.5GHz 5K iMac on sale for $2149, $150 off MSRP... Read more
Twelve South Redesigns BookArc For Today’s Sm...
Twelve South has announced a redesigned version of their very first product, BookArc for MacBook. Tailored specifically for the newest generation of MacBooks, BookArc holds the new, smaller Apple... Read more
Phone 6s Tips & Tricks – Tips Book For iP...
Poole, United Kingdom based Tap Guides Ltd. has announced the release and immediate availability of iPhone 6s Tips & Tricks, an in-depth eBook available in the iBookstore that’s priced just $2.99... Read more
Apple refurbished 2014 13-inch Retina MacBook...
Apple has Certified Refurbished 2014 13″ Retina MacBook Pros available for up to $400 off original MSRP, starting at $979. An Apple one-year warranty is included with each model, and shipping is free... Read more
13-inch 2.5GHz MacBook Pro on sale for $994,...
Best Buy has the 13″ 2.5GHz MacBook Pro available for $994.99 on their online store. Choose free shipping or free instant local store pickup (if available). Their price is $105 off MSRP. Price valid... Read more
Is The iPad Pro Really A Serious Laptop Repla...
Probably not, at least for productive professionals and other power users. Steve Jobs declared that we’d entered the “post-PC Era” with the advent of the original iPad in 2010, a phrase we don’t hear... Read more

Jobs Board

*Apple* Desktop Analyst - KDS Staffing (Unit...
…work with some of the brightest engineers. Job Description Title: Apple Desktop AnalystPosition Type: Full-time PermanentLocation: White Plains, NYHot Points:Depth of Read more
*Apple* Retail Online, Customer Experience R...
**Job Summary** Apple Retail's Online Store sells Apple products to customers around the world. In addition to selling Apple products with unique services such Read more
Frameworks Engineer, *Apple* Watch - Apple...
**Job Summary** Join the team that is shaping the future of software development for Apple Watch! As a software engineer on the Apple Watch Frameworks team you will Read more
Senior Manager, Global Direct Marketing *App...
**Job Summary** Apple Retail is looking for an experienced Direct Marketing Leader to join its Marketing team. This position will take a leadership role in creating Read more
*Apple* Online Store Expansion - Apple (Unit...
**Job Summary** The Online Apple Store is seeking a person to lead its expansion into new countries. Based in Cupertino, CA, this person will develop and maintain an Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.