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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


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