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

Pinegrow Web Designer 2.94 - Mockup and...
Pinegrow Web Designer is desktop app that lets you mockup and design webpages faster with multi-page editing, CSS and LESS styling, and smart components for Bootstrap, Foundation, Angular JS, and... Read more
ExpanDrive 5.4.1 - Access cloud storage...
ExpanDrive builds cloud storage in every application, acts just like a USB drive plugged into your Mac. With ExpanDrive, you can securely access any remote file server directly from the Finder or... Read more
MacUpdate Desktop 6.1.3 - Search and ins...
MacUpdate Desktop 6 brings seamless 1-click app installs and version updates to your Mac. With a free MacUpdate account and MacUpdate Desktop 6, Mac users can now install almost any Mac app on... Read more
VOX 2.8.6 - Music player that supports m...
VOX just sounds better! The beauty is in its simplicity, yet behind the minimal exterior lies a powerful music player with a ton of features and support for all audio formats you should ever need.... Read more
Espionage 3.6.6 - Simple, state-of-the-a...
Espionage offers state-of-the-art encryption and plausible deniability for your confidential data. Sometimes, encrypting your data isn't enough to protect it. That's why Espionage 3 goes beyond data... Read more
VOX 2.8.6 - Music player that supports m...
VOX just sounds better! The beauty is in its simplicity, yet behind the minimal exterior lies a powerful music player with a ton of features and support for all audio formats you should ever need.... Read more
MacUpdate Desktop 6.1.3 - Search and ins...
MacUpdate Desktop 6 brings seamless 1-click app installs and version updates to your Mac. With a free MacUpdate account and MacUpdate Desktop 6, Mac users can now install almost any Mac app on... Read more
ExpanDrive 5.4.1 - Access cloud storage...
ExpanDrive builds cloud storage in every application, acts just like a USB drive plugged into your Mac. With ExpanDrive, you can securely access any remote file server directly from the Finder or... Read more
Espionage 3.6.6 - Simple, state-of-the-a...
Espionage offers state-of-the-art encryption and plausible deniability for your confidential data. Sometimes, encrypting your data isn't enough to protect it. That's why Espionage 3 goes beyond data... Read more
Pinegrow Web Designer 2.94 - Mockup and...
Pinegrow Web Designer is desktop app that lets you mockup and design webpages faster with multi-page editing, CSS and LESS styling, and smart components for Bootstrap, Foundation, Angular JS, and... Read more

Infinite Tanks (Games)
Infinite Tanks 1.0.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0.0 (iTunes) Description: | Read more »
Agatha Christie - The ABC Murders (FULL)...
Agatha Christie - The ABC Murders (FULL) 1.0 Device: iOS Universal Category: Games Price: $6.99, Version: 1.0 (iTunes) Description: Agatha Christie: The ABC Murders Your weapon is your knowledge. Your wits will be put to the ultimate... | Read more »
HeadlessD (Games)
HeadlessD 1.0 Device: iOS Universal Category: Games Price: $.99, Version: 1.0 (iTunes) Description: HeadlessD is hand-painted dungeon crawler with intuitive touch controls and NO in-app purchases. | Read more »
Leaf for Twitter (Social Networking)
Leaf for Twitter 1.0.1 Device: iOS iPhone Category: Social Networking Price: $4.99, Version: 1.0.1 (iTunes) Description: | Read more »
Banner Saga 2 (Games)
Banner Saga 2 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: The epic award winning story-based role-playing game continues its emotional journey across a breaking world. Lead your Viking... | Read more »
Concrete Jungle (Games)
Concrete Jungle 1.16 Device: iOS Universal Category: Games Price: $4.99, Version: 1.16 (iTunes) Description: A follow up to the puzzle hit 'MegaCity'! Concrete Jungle is a new take on the city building genre that swaps micro-... | Read more »
5 great apps for the budget traveller
Travelling abroad, or even within your home country, has never been easier thanks to our handy smartphone companions. There are hundreds of apps on the market that promise to make your world journeys hassle-free, but we've selected five of the... | Read more »
Zip—Zap (Games)
Zip—Zap 1.01 Device: iOS Universal Category: Games Price: $1.99, Version: 1.01 (iTunes) Description: Touch to contract.Release to let go.Bring the clumsy mechanical beings home. · · · over 100 levelsno adsno in-app-purchases Zip—... | Read more »
Paperback: The Game (Games)
Paperback: The Game 1.0 Device: iOS Universal Category: Games Price: $3.99, Version: 1.0 (iTunes) Description: You are an author trying to finish kitschy paperback novels. Complete Westerns, Science Fiction, Romance or even a Crime... | Read more »
How to Rule With a Firm Hand in My Majes...
My Majesty is a kingdom management sim not unlike August’s magisterial hit, Reigns. It’s essentially a reskin of developer Tigrido’s previous management sim, Dictator. As supreme ruler of the land, you must consult with a number of subjects to... | Read more »

Price Scanner via MacPrices.net

Apple refurbished Mac minis available startin...
Apple has Certified Refurbished Mac minis available starting at $419. Apple’s one-year warranty is included with each mini, and shipping is free: - 1.4GHz Mac mini: $419 $80 off MSRP - 2.6GHz Mac... Read more
13-inch 2.5GHz MacBook Pro available for $928...
Overstock has the 13″ 2.5GHz MacBook Pro available for $927.99 including free shipping. Their price is $171 off MSRP. Read more
Buying McLaren Would Give Apple Instant Car C...
Apple “iCar” rumors have waxed and waned over the years, piquing interest and speculation as to whether Apple is seriously interested in getting into the automotobile business, either in a joint... Read more
Aetna to Transform Members’ Consumer Health E...
Health care benefits company Aetna, which has an estimated 46.3 million clients, today announced a new initiative to revolutionize members consumer health experience by combining the power of iOS... Read more
USB-IF Announces USB Audio Device Class 3.0 S...
USB Implementers Forum (USB-IF), the support organization for the advancement and adoption of USB technology, today announced the USB Audio Device Class 3.0 specification to establish USB Audio over... Read more
Clearance 12-inch 1.2GHz Retina MacBooks, App...
Apple has Certified Refurbished 2015 12″ 1.2GHz Retina MacBooks available for $1189, or $410 off original MSRP. Apple will include a standard one-year warranty with each MacBook, and shipping is free... Read more
Logitech SmartDock and Skype For Business Com...
Logitech has announced Logitech SmartDock, an AV meeting room solution designed in collaboration with Microsoft. Logitech SmartDock works with Skype for Business and qualified devices, including... Read more
27-inch iMacs on sale for up to $220 off MSRP
B&H Photo has 27″ Apple iMacs on sale for up to $200 off MSRP including free shipping plus NY sales tax only: - 27″ 3.3GHz iMac 5K: $2099 $200 off MSRP - 27″ 3.2GHz/1TB Fusion iMac 5K: $1899.99 $... Read more
Apple Macs and iPads available for up to $300...
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 $600 with Apple refurbished Mac Pr...
Apple has Certified Refurbished Mac Pros available for up to $600 off the cost of new models. An Apple one-year warranty is included with each Mac Pro, and shipping is free. The following... Read more

Jobs Board

*Apple* Retail - Multiple Positions- Chicago...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
*Apple* Retail - Multiple Positions- Raleigh...
Job Description:SalesSpecialist - Retail Customer Service and SalesTransform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
User Support Specialist *Apple* Product Spe...
…Description:Ciber, Inc. is seeking a User Support Specialist - Apple Product Support in Nashville, TN!Responsibilities:Support, implementation, and upgrade of Read more
Restaurant Manager (Neighborhood Captain) - A...
…in every aspect of daily operation. WHY YOU'LL LIKE IT: You'll be the Big Apple . You'll solve problems. You'll get to show your ability to handle the stress and Read more
US- *Apple* Store Leader Program - Apple (Un...
…Summary Learn and grow as you explore the art of leadership at the Apple Store. You'll master our retail business inside and out through training, hands-on Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.