Disks: The Alto File System

This document describes the disk formats used in the Alto  File System.
It also describes  a "disk object," a  Bcpl software construct  that is
used to interface low-level  disk drivers with packages  that implement
higher-level objects, such as streams.

The primary focus  of the description will  be for the  "standard" Alto
disks: either (1) up to 2 Diablo Model 31 disk drives or (2) one Diablo
Model 44 disk drive.  The low-level drivers for these disks  are called
"Bfs" (Basic File  System).  With minor modifications,  the description
below  applies to  the Trident  Model T80  and T300  disk  drives, when
formatted  for  Alto  file  system  conventions.   The  differences are
flagged with the string  [Trident].  Low-level drivers for  the Trident
disks are called "Tfs."

1. Distribution

Relocatable binary files for the BFS are kept in  <Alto>BFSBrs.dm.  The
sources,  command  files, and  test  program (described  later  in this
document)  are  kept  in  <AltoSource>BFSSources.dm  Relocatable binary
files  for  the TFS  are  kept  in <Alto>TFS.dm;  sources  are  kept on

2. File and Disk Structure

This section describes  the conventions of  the Alto file  system.  The
files AltoFileSys.D and Bfs.D contain Bcpl structure  declarations that
correspond to this description ([Trident]: See also "Tfs.D").

The unit  of transfer between  disk and memory,  and hence that  of the
file system, is  the disk sector.  Each  sector has three fields:  a 2-
word header, an  8-word label, and  a 256-word data  page.  ([Trident]:
The fields are a 2-word  header, a 10-word label, and a  1024-word data

A sector is identified by a  disk address; there are two kinds  of disk
addresses, real  and virtual.   The hardware  deals in  real addresses,
which have a somewhat arbitrary format.  An unfortunate  consequence is
that the real addresses for all the pages on a disk unit are  sparse in
the set of 16 bit integers.  To correct this defect,  virtual addresses
have been introduced.  They have the property that the pages of  a disk
unit  which  holds  n  pages  have  virtual  addresses  0   ...  (n-1).
Furthermore,  the ordering  of pages  by virtual  address is  such that
successive pages  in the  virtual space are  usually sequential  on the
disk.   As  a result,  assigning  a sequence  of  pages  to consecutive
virtual  addresses will  ensure that  they can  be read  in as  fast as

                   Copyright Xerox Corporation 1982

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2.1. Legal Alto Files

An  Alto  file  is  a  data  structure  that  contains  two   sorts  of
information: some is  mandatory, and is  required for all  legal files;
the  remainder  is  "hints".  Programs  that  operate  on  files should
endeavor to  keep the hints  accurate, but should  never depend  on the
accuracy of a hint.

A legal Alto file  consists of a sequence  of pages held together  by a
doubly-linked list  recorded in the  label fields. Each  label contains
the mandatory information:

   The forward and backward links, recorded as real disk addresses.

   A page  number which  gives the position  of the  page in  the file;
   pages are numbered from 0.

   A count of the number of characters of data in the  page (numchars).
   This may range from  0 (for a completely  empty page) to 512  (for a
   completely  full  page).  ([Trident]:  A  full  page  contains  2048

   A real  file id,  which is  a three-word  unique identifier  for the
   file.   The  user normally  deals  with virtual  file  ids  (see the
   discussion  of  file  pointers,  below),  which   are  automatically
   converted into real file ids when a label is needed.

Three bits in the file id deserve special mention:

   Directory: This bit  is on if the  file is itself a  directory file.
   This information is  used by the disk  Scavenger when trying  to re-
   build a damaged disk data structure.

   Random: This bit is currently unused.

   NoLog:  This bit  is no  longer used,  but many  existing  files are
   likely to have it set.

Leader Page: Page 0 of a file is called the leader page; it contains no
file data, but only a  collection of file properties, all of  which are
hints.   The structure  LD in  AltoFileSys.D declares  the format  of a
leader page, which contains the following standard items:

     The file name, a hint so that the Scavenger can enter this file in
     a directory if it is not already in one.

     The times for creation,  last read and last write,  interpreted as

          A  file's  creation  date  is  a  stamp  generated  when  the
          information in the  file is created.   When a file  is copied
          (without modification),  the creation  date should  be copied
          with it.  When a file is modified in any way (either in-place
          or  as  a  result  of  being  overwritten   by  newly-created
          information), a new creation date should be generated.

          A  file's  write  date  is  updated  whenever  that  file  is
          physically written on a given file system.

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          A  file's  read  date  is  updated  whenever  that   file  is
          physically read from within a given file system.

     A pointer  to the  directory in which  the file  is thought  to be
     entered (zeroes imply the system directory SysDir).

     A "hint" describing the last page of the file.

     A "consecutive" bit which is a hint that the pages of the file lie
     at consecutive virtual disk addresses.

     The changeSerial  field related to  version numbering:  whenever a
     new version of a file "foo" is made, the changeSerial field of all
     other files "foo" (i.e., older versions) is incremented.   Thus, a
     program that wishes  to be sure that  it is using the  most recent
     version of a  file can verify  that changeSerial=0.  If  a program
     keeps an  FP as  a hint  for a  file, and  is concerned  about the
     relative position of that file in the list of version  numbers, it
     can  also keep  and  verify the  changeSerial entry  of  the file.
     Version numbers have been deimplemented.

These standard  items use up  about 40 words  of the leader  page.  The
remaining space  is available for  storing other information  in blocks
which start with a one  word header containing type and  length fields.
A  zero  terminates the  list.   The structure  FPROP  in AltoFileSys.d
defines the header format.  The  only standard use of this  facility is
to record the logical shape of the disk in the leader page of SysDir.

Data: The first data byte of a file is the first byte of page 1.

In a legal file with n pages, the label field of page i must contain:

   A next link with the real disk address of page (i+1), or 0 if i=n-1.

   A previous link with  the real disk address  of page (i-1), or  0 if

   A page number between 0 and (n-1), inclusive.

   A numchars word  = 512 if  i<n-1, and <512  if i=n-1. The  last page
   must not be completely full. ([Trident]: = 2048 if i<n-1,  and <2048
   if i = n-1.)

   A real file  id which is  the same for every  page in the  file, and
   different from the real file id of any other file on the disk.

A file is addressed by an  object called a file pointer (FP),  which is
declared in AltoFileSys.D.  A file pointer contains a virtual  file id,
and also the virtual address of the leader page of the file.   The low-
level disk routines construct a real file id from the virtual  one when
they must deal with a disk label.  Since it is possible for the user to
read a label from the  disk and examine its contents, the  drivers also
provides a  routine which will  convert the real  file id in  the label
into a file pointer (of  course, the leader address will not  be filled

Note: Real disk address 0 (equal virtual disk address 0) cannot be part
of any legal Alto file because the value 0 is reserved to terminate the
forward and backward chains in sector labels.  However, disk  address 0

Disks & Bfs             April 10, 1982  5:46 PM                       4

is used for "booting"  the Alto: when the  boot key is pressed  when no
keyboard keys are down, sector 0 is read in as a bootstrap loader.  The
normal way to make  a file the "boot file"  is to first create  a legal
Alto file with  the bootstrap loader as  the first data page  (page 1),
and then to copy this page  (label and data) into disk sector  0.  Thus
the label in sector 0 points forward to the remainder of the boot file.

2.2. Legal Alto Disks

A legal disk is one on which every page is either part of a legal file,
or free, or "permanently bad." A  free page has a file id of  all ones,
and the rest of its label is indeterminate.  A permanently bad page has
a file id with each of the three words set to -2, and the  remainder of
the label indeterminate.

2.3. Alto Directory Files

A directory is  a file for associating  string names and FP's.   It has
the directory  bit set  in its file  id, and  has the  following format
(structure DV declared in AltoFileSys.D).

It is a sequence  of entries.  An entry  contains a header and  a body.
The length field of  the header tells how  many words there are  in the
entry, including the header.  The interpretation of the body depends on
the type, recorded in the header.

   dvTypeFree=0: free entry.  The body is uninterpreted.

   dvTypeFile=1:  file entry.   The body  consists of  a  file pointer,
   followed by a Bcpl string containing the name of the file.  The file
   name must  contain only  upper and lower  case letters,  digits, and
   characters in the string "+-.!$".  They must terminate with a period
   (".") and not be  longer than maxLengthFn characters.  If  there are
   an odd number of  bytes in the name,  the "garbage byte" must  be 0.
   The interpretation  of exclamation  mark (!) is  special; if  a file
   name ends with  ! followed only by  digits (and the  mandatory "."),
   the digits specify a file version number.

The main directory is  a file with its  leader page stored in  the disk
page with virtual address 1.  There is an entry for the  main directory
in the main directory, with the name SysDir.  All other directories can
be reached by starting at the main directory.

2.4. Disk Descriptor

There is  a file  called DiskDescriptor entered  in the  main directory
which contains a disk descriptor structure which describes the disk and
tells which pages  are free.  The disk  descriptor has two parts:  a 16
word header  which describes  the shape of  the disk,  and a  bit table
indexed  by  virtual  disk  address.   The  declaration  of  the header
structure is in AltoFileSys.D.

The  "defaultVersionsKept"  entry  in  the  DiskDescriptor  records the
number of old versions of files that should be retained by  the system.
If this  entry is 0,  no version accounting  is done: new  files simply
replace old ones.  Version numbers have been deimplemented.

Disks & Bfs             April 10, 1982  5:46 PM                       5

The entry in the disk descriptor named "freePages" is used  to maintain
a count of free pages on the disk.  This is a hint about a hint:  it is
computed when a disk is opened  by counting the bits in the  bit table,
and  then  incrementing  and decrementing  as  pages  are  released and
allocated.  However the bit table is itself just a collection of hints,
as explained below.

The bit table contains a "1" corresponding to each virtual disk address
that is believed to be occupied by a file, and "0" for  free addresses.
These values are, however, only hints.  Programs that assign  new pages
should check to be sure that a page thought to be free is indeed  so by
reading the label  and checking to see  that it describes a  free page.
(The WriteDiskPages  and CreateDiskFile procedures  in the  disk object
perform this checking for you.)

2.5. Oversights

If the Alto file system were to be designed again, several deficiencies
could be corrected:

   Directory entries and label entries should have the same  concept of
   file identifier.  Presently, we have filePointers and fileIds.

   There is no reason  why the last page  of a file cannot  contain 512

   It is unfortunate that the  disk controller will not check  an entry
   of 0 in a label,  because these values often arise (numChars  of the
   last  page, page  number of  the leader  page).  Another  don't care
   value should be chosen: not  a legal disk address; with  enough high
   order bits so that it will check numChars and page number fields.

   The value used  to terminate the chain  of disk addresses  stored in
   the labels should not be a legal disk address.  (It should  also not
   be zero, so that it may  be checked.) If it is a legal  address, and
   if you try to run the disk at full speed using the trick of pointing
   page i's label at page i+1's disk address in the command  block, the
   disk will try to read the page at the legal disk address represented
   by the chain terminator.  Only when this results in an error  is end
   of file detected.  A terminator of zero has the undesirable property
   that a  seek to track  0 occurs whenever  a chain runs  into end-of-

3. The Disk Object

In order  to facilitate  the interface  between various  low-level disk
drivers and higher-level software,  we define a "disk object."  A small
data structure defines a number of generic operations on a disk  -- the
structure DSK is  defined in "Disks.D."  Each procedure takes  the disk
structure as its first argument:

   ActOnDiskPages: Used to read and  write the data fields of  pages of
   an existing file.

   WriteDiskPages: Used to read and write data fields of the pages of a
   file, and to extend the file if needed.

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   DeleteDiskPages: Used to delete pages from the end of a file.

   CreateDiskFile: Used  to create a  new disk file,  and to  build the
   leader page correctly.

   AssignDiskPage: Used to find a free disk page and return its virtual
   disk address.

   ReleaseDiskPage: Used  to release a  virtual disk address  no longer

   VirtualDiskDA:  Converts a  real disk  address into  a  virtual disk

   RealDiskDA:  Converts  a  virtual  disk  address  into  a  real disk

   InitializeDiskCBZ:  Initializes  a  Command  Buffer  Zone  (CBZ) for
   managing disk transfers.

   DoDiskCommand: Queues a Command  Buffer (CB) to initiate  a one-page

   GetDiskCb:  Obtains  another  CB, possibly  waiting  for  an earlier
   transfer to complete.

   CloseDisk: Destroys the disk object.

In addition, there are several standard data entries in the DSK object:

   fpSysDir: Pointer to  the FP for the  directory on the  disk.  (This
   always has a constant format -- see discussion above.)

   fpDiskDescriptor: Pointer to the FP for the file "DiskDescriptor" on
   the disk.

   fpWorkingDir: Pointer to the FP to use as the "working directory" on
   this disk.  This is usually the same as fpSysDir.

   nameWorkingDir: Pointer to a  Bcpl string that contains the  name of
   the working directory.

   lnPageSize: This is  the log (base  2) of the  number of words  in a
   data page on this disk.

   driveNumber: This  entry identifies the  drive number that  this DSK
   structure describes.

   retryCount: This value gives  the number of times the  disk routines
   should retry an operation before declaring it an error.

   totalErrors: This value  gives a cumulative  count of the  number of
   disk errors encountered.

   diskKd: This entry points to a copy of the DiskDescriptor in memory.
   Because the bit table can get quite large, only the header  needs to
   be in memory.  This header can be used, for example, to  compute the
   capacity of the disk.

Disks & Bfs             April 10, 1982  5:46 PM                       7

   lengthCBZ, lengthCB: The fixed overhead for a CBZ and the  number of
   additional words required per CB.

In addition to  this standard information, a  particular implementation
of a disk class may include other information in the structure.

4. Data Structures

The following  data structures  are part of  the interface  between the
user and the disk class routines:

pageNumber: as  defined in  the previous section.   The page  number is
represented by an integer.

DAs: a vector  indexed by page number  in which the ith  entry contains
the virtual disk address of page  i of the file, or one of  two special
values (which are declared as manifest constants in Disks.D):

   eofDA: this page is beyond the current end of the file;
   fillInDA: the address of this page is not known.

Note that  a particular  call on  the file  system will  only reference
certain elements of this vector,  and the others do not have  to exist.
Thus, reading page i will cause references only to DAs!i and DAs!(i+1),
so the user can have a two-word vector v to hold these  quantities, and
pass v-i to the file system as DAs.

CAs: a vector  indexed by page number  in which the ith  entry contains
the core  address to or  from which page  i should be  transfered.  The
note for DAs applies here also.

fp (or  filePtr): file  pointer, described above.   In most  cases, the
leader page address is not used.

action:  a  magic  number  which specifies  what  the  disk  should do.
Possible values are declared as manifest constants in Disks.D:

    DCreadD:         check the header and label, read the data;
    DCreadLD:        check the header, read the label and data;
    DCreadHLD:       read the header, label, and data;
    DCwriteD:        check the header and label, write the data;
    DCwriteLD:       check the header, write the label and data;
    DCwriteHLD:      write the header, label, and data;
    DCseekOnly:      just seek to the specified track

A  particular implementation  of  the disk  class may  also  make other
operations available by defining additional magic numbers.

5. Higher-level Subroutines

There are two high-level calls on the basic file system:

Disks & Bfs             April 10, 1982  5:46 PM                       8

pageNumber = ActOnDiskPages(disk, CAs, DAs, filePtr, firstPage,
    lastPage, action, lvNumChars, lastAction, fixedCA, cleanupRoutine,
    lvErrorRoutine, returnOnCheckError, hintLastPage).

Parameters  beyond  "action"  are  optional  and  may  be  defaulted by
omitting them or making them 0.

Here firstPage and lastPage are the page numbers of the first  and last
pages  to be  acted on  (i.e. read  or written,  in normal  use).  This
routine does  the specified action  on each page  and returns  the page
number of the last page  successfully acted on.  This may be  less than
lastPage if the file turns out to have fewer pages.  DAs!firstPage must
contain  a   disk  address,  but   any  of   DAs!(firstPage+1)  through
DAs!(lastPage+1) may  be fillInDA,  in which case  it will  be replaced
with the  actual disk address,  as determined from  the chain  when the
labels are read.  Note that the routine will fill  in DAs!(lastPage+1),
so this word must exist.

The value of the numChars field in the label of the last page  acted on
will be left in rv  lvNumChars.  If lastAction is supplied, it  will be
used  as the  action for  lastPage  instead of  action.  If  CAs  eq 0,
fixedCA is  used as the  core address for  all the data  transfers.  If
cleanupRoutine  is  supplied,   it  is  called  after   the  successful
completion of each disk command, as described below  under "Lower-level
disk access".  (Note: providing a cleanup routine defeats the automatic
filling in of disk addresses in DAs).

Disk transfers that generate errors are retried several times  and then
the error routine is called with
        rv lvErrorRoutine(lvErrorRoutine, cb, errorCode)

In other words, lvErrorRoutine is the address of a word  which contains
the (address of the) routine to be called when there is an  error.  The
errorCode tells what kind of error it was; the standard error codes are
tabulated in a later section.  The cb is the control block which caused
the error; its format  depends on the particular implementation  of the
drivers (Bfs: the structure CB in Bfs.D).

The intended use of lvErrorRoutine  is this.  A disk stream  contains a
cell A, in  a known place in  the stream structure, which  contains the
address of  a routine which  fields disk errors.   The address of  A is
passed as lvErrorRoutine.   When the error  routine is called,  it gets
the address of A as a parameter, and by subtracting the  known position
of A in  the disk stream  structure, it can  obtain the address  of the
stream structure, and thus determine which stream caused the error.

The    default   value    of   returnOnCheckError    is    false.    If
returnOnCheckError is true and an error is  encountered, ActOnDiskPages
will not  retry a check  error and then  report an error.   Instead, it
will return  -(#100+i), where  i is the  page number  of the  last page
successfully  transferred.  This  feature allows  ActOnDiskPages  to be
used when  the user  it not  sure whether  the disk  address he  has is
correct.  It is  used by the disk  stream and directory  routines which
take hints; they try to read  from the page addressed by the  hint with
returnOnCheckError true, and if they get a normal return they know that
the hint was good.   On the other hand, if  it was not good,  they will
get the abnormal return just described, and can proceed to try again in
a more conservative way.

Disks & Bfs             April 10, 1982  5:46 PM                       9

The hintLastPage  argument, if supplied,  indicates the page  number of
what the caller  believes to be the  last page of the  file (presumably
obtained from the  hint in the leader  page).  If the hint  is correct,
ActOnDiskPages will ensure that the disk controller does not chain past
the end  of the file  and seek to  cylinder zero (as  described earlier
under  "Oversights").  If  the hint  is incorrect,  the  operation will
still be performed correctly,  but perhaps with a loss  in performance.
Note that the label is not rewritten by DCwriteD, so that the number of
characters per page will not change.  If you need to change  the label,
you should use WriteDiskPages unless you know what you are doing.

ActOnDiskPages can be used to both read and write a file as long as the
length of the file does not  have to change.  If it does, you  must use

pageNumber = WriteDiskPages(disk, CAs, DAs, filePtr, firstPage,
    lastPage, lastAction, lvNumChars, lastNumChars, fixedCA, nil,
    lvErrorRoutine, nil, hintLastPage).

Arguments beyond lastPage are optional and may be defaulted by omitting
them or making them 0 (but lastNumChars is not defaulted if it is 0).

This routine writes  the specified pages from  CAs (or from  fixedCA if
CAs is 0, as for ActOnDiskPages).  It fills in DAs entries in  the same
way as ActOnDiskPages, and also allocates enough new pages  to complete
the specified write.  The numChars field in the label of the  last page
will be set  to lastNumChars (which  defaults to 512  [Trident]: 2048).
It is  generally necessary that  DAs!firstPage contain a  disk address.
The  only situation  in which  it is  permissible for  DAs!firstPage to
contain fillInDA is when firstPage is zero and no pages of the file yet
exist on the disk (i.e., when creating page zero of a new file).

In most cases, DAs!(firstPage-1)  should have the value which  you want
written into the backward chain pointer for firstPage, since this value
is needed whenever the label for firstPage needs to be  rewritten.  The
only case in which it doesn't need to be rewritten is when the  page is
already  allocated,  the next  page  is not  being  allocated,  and the
numChars field is not changing.

If lastPage already exists:

   1) the old value  of the numChars field of  its label is left  in rv

   2) if lastAction is supplied,  it is applied to lastPage  instead of
   DCwriteD.  It defaults to DCwriteD.

WriteDiskPages handles  one special case  to help in  "renaming" files,
i.e. in changing the FP (usually the serial number) of all the pages of
a file.  To do  this, use ActOnDiskPages to  read a number of  pages of
the file into memory and to build a DAs array of valid  disk addresses.
Then a call to WriteDiskPages with lastAction=-1 will write  labels and
data  for  pages  firstPage  through  lastPage  (DAs!(firstPage-1)  and
DAs!(lastPage+1)  are of  course used  in this  writing  process).  The
numChars field of  the label on the  last page is set  to lastNumChars.
To  use this  facility, the  entire DAs  array must  be valid,  i.e. no
entries may be fillInDA.

Disks & Bfs             April 10, 1982  5:46 PM                      10

In addition to these two  routines, there are two others  which provide
more specialized services:

CreateDiskFile(disk, name, filePtr, dirFilePtr, word1 [0], useOldFp
    [false], pageBuf[0])

Creates a  new disk file  and writes its  leader page.  It  returns the
serial number and leader disk  address in the FP structure  filePtr.  A
newly created file has one data page (page 1) with numChars eq 0.

The  arguments  beyond filePtr  are  optional, and  have  the following

   If  dirFilePtr is  supplied,  it should  be  a file  pointer  to the
   directory which owns  the file.  This  file pointer is  written into
   the leader page, and is used  by the disk Scavenger to put  the file
   back into the directory if it becomes lost.  It defaults to the root
   directory, SysDir.

   The value of word1 is "or"ed into the filePtr>>FP.serialNumber.word1
   portion of the file  pointer.  This allows the directory  and random
   bits to be set in the file id.

   If useOldFp is  true, then filePtr already  points to a  legal file;
   the purpose of calling CreateDiskFile is to re-write all  the labels
   of  the  existing  file  with the  new  serial  number,  and  to re-
   initialize the leader page.  The data contents of the  original file
   are  lost. Note  that this  process effectively  "deletes"  the file
   described by filePtr when CreateDiskFile is called, and makes  a new
   file; the FP for the new file is returned in filePtr.

   If pageBuf is supplied, it is written on the leader page of  the new
   file  after setting  the  creation date  and directory  FP  hint (if
   supplied).  If pageBuf is omitted, a minimal leader page is created.

DeleteDiskPages(disk, CA, firstDA, filePtr, firstPage, newFp,

Arguments beyond firstPage are optional.  Deletes the pages of  a file,
starting with the page whose number is firstPage and whose disk address
is  firstDA.  CA  is  a page-sized  buffer  which is  clobbered  by the
routine.  hintLastPage is as described under ActOnDiskPages.

If  newFp  is supplied  and  nonzero, it  (rather  than  freePageFp) is
installed as the FP of the file, and the pages are not deallocated.

6. Allocating Disk Space

The  disk class  also contains  routines for  allocating space  and for
converting between  virtual and  real disk  addresses.  In  most cases,
users need not call these routines directly, as the four routines given
above (ActOnDiskPages, WriteDiskPages, DeleteDiskPages, CreateDiskFile)
manage disk addresses and disk space internally.

AssignDiskPage(disk, virtualDA, nil)  returns the virtual  disk address
of the first free page following virtualDA, according to the bit table,

Disks & Bfs             April 10, 1982  5:46 PM                      11

and sets the corresponding bit.   It does not do any checking  that the
page is actually free (but WriteDiskPages does).  If there are  no free
pages it returns -1.  If it is called with three arguments,  it returns
true if (virtualDA+1) is available without assigning it.

If virtualDA is  eofDA, AssignDiskPage makes a  free-choice assignment.
The disk object remembers the virtual DA of the last page  assigned and
uses it as the first page to attempt to assign next time AssignDiskPage
is called with a virtualDA of  eofDA.  This means that you can  force a
file to be created starting at a particular virtual address by means of
the following strategy:

        ReleaseDiskPage(disk, AssignDiskPage(disk, desiredVDA-1))
        CreateDiskFile(disk, ...)  // or whatever (e.g., OpenFile)

ReleaseDiskPage(disk,  virtualDA) marks  the page  as free  in  the bit
table.  It  does not  write anything on  the disk  (but DeleteDiskPages

VirtualDiskDA(disk, lvRealDA) returns the virtual disk address, given a
real disk address  in rv lvRealDA.   (The address, lvRealDA,  is passed
because  a  real  disk  address may  occupy  more  than  1  word.) This
procedure returns eofDA if the real disk address is zero (end-of-file),
and fillInDA if  the real disk address  does not correspond to  a legal
virtual disk address in this file system.

RealDiskDA(disk, virtualDA,  lvRealDA) computes  the real  disk address
and stores it in rv lvRealDA.  The function returns true if the virtual
disk address is legal, i.e. within the bounds of disk addresses for the
given "disk." Otherwise, it returns false.

7. Lower-level Disk Access

The transfer routines described  previously have the property  that all
disk activity occurs  during calls to  the routines; the  routines wait
for  the  requested  disk  transfers  to  complete   before  returning.
Consequently,  disk transfers  cannot conveniently  be  overlapped with
computation, and the number of pages transferred consecutively  at full
disk speed is generally limited by the number of buffers that  a caller
is able to supply in a single call.

It is also possible to use the disk routines at a lower level  in order
to overlap transfers with computation and to transfer pages at the full
speed of the disk (assuming the file is consecutively allocated  on the
disk and the amount of computation per page is kept  relatively small).
The  necessary  generic  disk  operations  and  other  information  are
available to permit callers to operate the low-level disk routines in a
device-independent fashion for most applications.

This level  makes used  of a Command  Block Zone  (CBZ), part  of whose
structure is public  and defined in Disks.d,  and the rest of  which is
private to the implementation.  The  general idea is that a CBZ  is set
up with empty disk command blocks in it.  A free block is obtained from
the CBZ with GetDiskCb and  sent to the disk with  DoDiskCommand.  When
it is sent  to the disk,  it is also put  on the queue  which GetDiskCb
uses,  but GetDiskCb  waits until  the disk  is done  with  the command
before returning it, and also checks for errors.

Disks & Bfs             April 10, 1982  5:46 PM                      12

If you plan to use these routines, read the code for  ActOnDiskPages to
find out  how they are  intended to  be called.  An  example of  use of
these  routines in  a disk-independent  fashion (i.e.,  using  only the
public  definitions in  Disks.d) may  be found  in  the DiskStreamsScan
module of the Operating System.  Only in unusual applications should it
be necessary to make use of the implementation-dependent information in
Bfs.d or Tfs.d.

InitializeDiskCBZ(disk, cbz, firstPage, length, retry, lvErrorRoutine).
CBZ is  the address of  a block of  length words which  can be  used to
store CBs.  It takes at least three CBs to run the disk at  full speed;
the disk object contains the values DSK.lengthCBZ (fixed  overhead) and
DSK.lengthCB (size of each command block) which may be used  to compute
the   required   length   (that  is,   length   should   be   at  least
lengthCBZ+3*lengthCB).  FirstPage is used to initialize the currentPage
field  of the  cbz.  Retry  is a  label used  for an  error  return, as
described below.  lvErrorRoutine is an error routine  for unrecoverable
errors, described below; it  defaults to a routine that  simply invokes
SysErr.  The arguments  after firstPage can  be omitted if  an existing
CBZ is  being reinitialized,  and they will  remain unchanged  from the
previous initialization.

cb  =   GetDiskCb(disk,  cbz,   dontClear[false],  returnIfNoCB[false])
returns the next CB for the CBZ.  If the next CB is empty (i.e., it has
never been  passed to  DoDiskCommand), GetDiskCb  simply zeroes  it and
returns  it.  However,  if the  next CB  is still  on the  disk command
queue, GetDiskCb  waits until  the disk has  finished with  it.  Before
returning  a  CB, GetDiskCb  checks  for errors,  and  handles  them as
described below.  If  there is no  error, GetDiskCb updates  the nextDA
and    currentNumChars    cells     in    the    CBZ,     then    calls
cbz>>CBZ.cleanupRoutine(disk,  cb,  cbz).   Next,  unless  dontClear is
true, the CB  is zeroed. Finally,  the CB is  returned as the  value of
GetDiskCb.  If  returnIfNoCB is true,  GetDiskCb returns zero  if there
are no  CBs in  the CBZ or  the next  CB is still  on the  disk command

If the next CB has suffered an error, then GetDiskCb instead  takes the
following actions.  First  it increments cbz>>CBZ.errorCount.   If this
number is ge the value disk>>DSK.retryCount, GetDiskCb calls  the error
routine which was passed to InitializeDiskCBZ; the way this is  done is
explained in  the description of  ActOnDiskPages above.  (If  the error
routine  returns,  GetDiskCb  will  proceed  as  if  an   error  hadn't
occurred.) Otherwise, after doing  a restore on the disk  if errorCount
ge  disk>>DSK.retryCount/2,  it reinitializes  the  CBZ  with firstPage
equal to the page with the error, and returns to  cbz>>CBZ.retry (which
was initialized  by InitializeDiskCBZ)  instead of  returning normally.
The idea is that the code following the retry label will retry  all the
incomplete  commands,  starting  with  the  one  whose  page  number is
cbz>>CBZ.currentPage and whose disk address is cbz>>CBZ.errorDA.

DoDiskCommand(disk, cb,  CA, DA,  filePtr, pageNumber,  action, nextCb)
Constructs a  disk command  in cb  with data  address CA,  virtual disk
address DA, serial and version number taken from the virtual file id in
filePtr, page number taken from pageNumber, and disk  command specified
by action.  The nextCb  argument is optional; if supplied  and nonzero,
DoDiskCommand will "chain" the current CB's label address to nextCb, in
such a way that the DL.next word will fall into nextCb>>CB.diskAddress.

DoDiskCommand expects the  cb to be  zeroed, except that  the following

Disks & Bfs             April 10, 1982  5:46 PM                      13

fields  may  be preset;  if  they  are zero  the  indicated  default is

       labelAddress      lv cb>>CB.label
       numChars          0

If DA eq fillInDA, the real disk address in the command is not set (the
caller should  have either set  it explicitly or  passed the CB  as the
nextCb  argument  for a  previous  command).  Actions  are  checked for

The public  cells in  the CBZ  most likely  to be  of interest  are the

   client: information of the  caller's choosing (e.g., a pointer  to a
   related higher-level data structure such as a stream.)

   cleanupRoutine: the cleanup  routine called by  GetDiskCb (defaulted
   to Noop by InitializeDiskCBZ).

   currentPage: set to the firstPage argument of  InitializeDiskCBZ and
   not touched by the  other routines.  (Note, however,  that GetDiskCb
   calls  InitializeDiskCBZ when  a retry  is about  to occur,  so when
   control arrives at the retry  label, currentPage will be set  to the
   page number of the command that suffered the error.)

   errorDA: set by GetDiskCb to the virtual disk address of the command
   that suffered an error.

   nextDA: set  by GetDiskCb to  the virtual disk  address of  the page
   following the one whose CB is being returned.  (This  information is
   obtained from the  next pointer in  the current page's  label.  Note
   that errorDA  and nextDA are  actually the same  cell, but  they are
   used in non-conflicting circumstances.)

   currentNumChars: set by GetDiskCb to the numChars of the  page whose
   CB is being returned.

   head: points to the first CB on GetDiskCb's queue; contains  zero if
   the queue is empty.

8. Error Codes

The  following errors  are generated  by the  BFS.  Similar  errors are
generated by other instances of a disk object.

        1101    unrecoverable disk error
        1102    disk full
        1103    bad disk action
        1104    control block queues fouled up
        1105    attempt to create a file without creation ability
        1106    can't create an essential file during NewDisk
        1107    bit table problem during NewDisk
        1108    attempt to access nonexistant bit table page

Disks & Bfs             April 10, 1982  5:46 PM                      14

9. Implementation -- Bfs

The  implementation expects  a  structure BFSDSK  to be  passed  as the
"disk" argument to the routines.  The initial portion of this structure
is the standard DSK structure followed by a copy of  the DiskDescriptor
header and  finally some  private instance  data for  the disk  in use.
(Note: The Alto operating system maintains a static sysDisk that points
to such a structure for disk drive 0.)

Bfs ("Basic File  System") is the name  for a package of  routines that
implement the  disk class  for the standard  Alto disks  (either Diablo
Model 31 drives  or a single Diablo  Model 44 drive).   The definitions
(in addition to  those in AltoFileSys.D  and Disks.D) are  contained in
Bfs.D.   The code  comes  in two  "levels:"  a "base"  for  reading and
writing  existing  files  (implements  ActOnDiskPages,  RealDiskDA  and
VirtualDiskDA  only);  and  a  "write"  level  for  creating, deleting,
lengthening   and   shortening   files    (implements   WriteDiskPages,
CreateDiskFile, DeleteDiskPages, AssignDiskPage, ReleaseDiskPage).  The
source files  BfsBase.Bcpl, Dvec.Bcpl and  BfsMl.Asm comprise  the base
level;   files   BfsWrite.Bcpl   BfsCreate.bcpl,   BfsClose.bcpl,   and
BfsDDMgr.bcpl implement the write level.

BfsMakeFpFromLabel(fp, la)  constructs a  virtual file  id in  the file
pointer fp from the real file id in the label la.

disk  =  BFSInit(diskZone,  allocate[false],  driveNumber[0], ddMgr[0],
freshDisk[false],  tempZone[diskZone])   returns  a  disk   object  for
driveNumber or zero.   The permanent data  structures for the  disk are
allocated  from  diskZone;  temporary free  storage  needed  during the
initialization  process is  allocated  from tempZone.   If  allocate is
true,  the  machinery for  allocating  and deallocating  disk  space is
enabled.  If it is enabled, a small DDMgr object and a 256  word buffer
will be extracted  from diskZone in order  to buffer the bit  table.  A
single  DDMgr,  created  by calling  'ddMgr  =  CreateDDMgr(zone)', can
manage both disks.  If freshDisk  is true, BFSInit does not  attempt to
open and read the DiskDescriptor file.  This operation is essential for
creating a virgin file system.

success  =   BFSNewDisk(zone,  driveNum[0],   nDisks[number  spinning],
nTracks[physical size], dirLen[3000], nSectors[physical  size]) creates
a virgin Alto  file system on the  specified drive and returns  true if
successful.  The zone must be capable of supplying about 1000  words of
storage.  The logical size of the file system may be different from the
physical size of driveNum: it may span both disks (a  'double-disk file
system'), or it  may occupy fewer  tracks (a model  44 used as  a model
31).  The length in words of SysDir, the master directory, is specified
by dirLen.  Some machines  that emulate Altos implement 14  sectors per

BFSExtendDisk(zone, disk, nDisks, nTracks) extends (i.e. adds pages to)
the filesystem on 'disk'.  Presumably 'nDisks' or 'nTracks' or  both is
bigger than the corresponding  parameters currently in disk.   A single
model 31 may be extended to a double model 31 or a single model 44 or a
double model  44, and a  single model  44 may be  extended to  a double
model 44.   The zone must  be capable of  supplying about 750  words of

0  = BFSClose(disk,  dontFree[false]) destroys  the disk  object  in an

Disks & Bfs             April 10, 1982  5:46 PM                      15

orderly  way.  If  dontFree is  true,  the ddMgr  for the  disk  is not
destroyed; presumably it is still in use by the other disk.  (Note that
this procedure is the one invoked by the CloseDisk generic operation.)

BFSWriteDiskDescriptor(disk) insures that any important state  saved in
memory is correctly written on the disk.

virtualDA  = BFSFindHole(disk,  nPages) attempts  to find  a contiguous
hole nPages long in disk.   It returns the virtual disk address  of the
first page of a hole if successful, else -1.

BFSTryDisk(drive, track, sector[0]) returns  true if a seek  command to
the specified track  on the specified  drive is successful.   Note that
the drive argument can contain an imbedded partition number.   Seeks to
track zero  will fail  if the  drive is  not on  line.  Seeks  to track
BFS31NTracks+1 will fail if the drive is a model 31.

10. Implementation -- Tfs

Operation and implementation of  the Trident T80 disks is  described in
separate documentation under  the heading "TFS/TFU" in  Alto Subsystems

11. BFSTest

BFSTest is the test and utility program for the Basic File  System.  It
performs many of the same functions as TFU (in fact much of its code is
lifted from  TFU), except  that commands which  are better  provided by
other subsystems such as the Executive (copy, rename, delete, etc.) are
omitted.   It has  a conventional  command scanner  and  implements the
following commands.

     ERASE formats one or more  disks as an Alto file system.   It asks
     you to specify  the number of  disks, cylinders and  sectors.  Any
     sectors marked "incorrigable" (by the CERTIFY command,  below) are
     not included in the file system and will never again cause trouble
     unless the disk  is erased with DiEx.   The OS erase  command also
     preserves this bad spot information.

EXERCISE creates, deletes, reads,  writes and positions files  the same
     way that normal programs  do, and checks the results  which normal
     programs do not do.  These high-level operations cause patterns of
     disk commands  which are quite  different from those  generated by
     lower-level tests such as DiEx.

     Exercise assumes  that a  file system exists  on DP0  (and perhaps
     extends  on  to  DP1).   It  creates  as  many  test  files (named
     Test.001, Test.002, ...) as  will fit in the file  system, filling
     each file with a carefully chosen test pattern.  When it  is done,
     it  deletes all  of its  files.  One  'pass' consists  of stepping
     through the test files, performing a randomly chosen  operation on
     the file, and checking the results.  The duration  and throughness
     of  a pass  depends on  the  amount of  free space  on  the disks.

Disks & Bfs             April 10, 1982  5:46 PM                      16

     Ideally, the  disk(s) under  test have just  been erased  with the
     ERASE command, below.

     While running, exercise looks for commands from the keyboard.  The
     current commands are:
          Q Quit             Delete all test files and stop.
          S StopOnError      Wait until a character is typed.

     All test files are  100 pages long.  Each  page of a file  has the
     page number in its first and last words and a data pattern  in the
     middle 254 words.  The data pattern is constant throughout a file,
     consisting of  a single  one-bit in a  word of  zeros or  a single
     zero-bit  in a  word of  ones.  Files  are read  and  written with
     ReadBlock  and  WriteBlock  using buffers  whose  lengths  are not
     multiples of the page size.  The operations are:

          Write         Write the entire file with the data pattern.

          Read          Read the entire file checking the data pattern.

          Delete        Delete the file, create it again and then write

          Copy          Copy  the file  to some  other  randomly chosen
                        file.   If  both disks  are  being  tested, one
                        third of  the time pick  a destination  file on
                        the other disk.

          Position      Position to twenty randomly chosen pages in the
                        file.  Check that the first word of the page is
                        indeed the page number.  One third of  the time
                        dirty the stream by writing the page  number in
                        the last word of the page.

CERTIFY tests  one or  more disks for  bad spots  and marks  such pages
     "incorrigable" so  that they  will not  be included  in subsequent
     file systems.   Ideally, a disk  should be certified  before being
     used.  Certify can be stopped at any time by hitting any key.

     A  bad spot  is  any sector  which  gives three  or  more checksum
     errors.  If the Scavenger encounters a bad spot, it will also mark
     the sector incorrigable.  Alto and Dorado disks almost  never have
     bad spots; but Dolphin  disks typically have a few,  so CERTIFYing
     is  a  must  for  trouble-free  operation.   Note  that  bad  spot
     information  will  be  lost  if a  certified  pack  is  written by

CREATEFILE attempts to  create a contiguous  file of a  specified size.
     If it can't, it creates a file with a minimum number of page runs.

PARTITION allows you  to set the disk  partition on which  BFSTest will
     operate.  This command is only available on Dolphins and Dorados.

A thorough, high-level "acceptance  test" for the disk subsystem  of an
Alto or Alto-emulating machine  (i.e. a Dolphin or Dorado)  consists of
at least  100 passes of  CERTIFY with  less than 5  bad spots  (any bad
spots on cylinder 0  are unacceptable), followed by ERASE,  followed by
at least 10 passes of EXERCISE with no errors.