Simtel MSDOS 1992 June/SIMTEL_0692.cdr/msdos/dskutl/emmcache.arc/EMC110.DOC

Text File | 1987-08-03 | 16KB | 331 lines

                            EXPANDED MEMORY DISK CACHE 1.10



  1. Introduction.

       EMC110.COM is a program designed to cache IBM-XT/AT hard disks in LIM
  (Lotus-Intel-Microsoft) expanded memory; it will not cache floppies, nor will
  it cache in DOS memory or AT extended memory.  It should work with any system
  which has a LIM-spec expanded memory board and software, a disk system which is
  IBM-XT/AT-compatible at the ROM-BIOS level, and NO OTHER STORAGE DEVICES.  If
  your system does not clearly fit this description --- in particular, if you
  have a hard disk system which exceeds the 32-meg DOS limit, or if you have any
  removable-media storage devices other than standard floppies --- you should NOT
  use this program without first reading the 'Technical Notes' section below.


  2. Installing the Cache.

       The cache can be installed by typing

                                EMC110 [max [min]]

  at the DOS prompt, where the optional parameters 'max' and 'min' are the
  maximum and minimum number of expanded memory pages you wish to use for the
  cache (each page is 16K).  If 'max' is not specified it defaults to 512.  The
  point of specifying 'max' is to prevent the cache from consuming all available
  expanded memory.  If 'min' is not specified it defaults to 1.  The point of
  specifying 'min' is to prevent the installation of a cache which is too small
  to be effective.  Note that the parameters are positional: if you want to
  specify 'min', you must specify 'max' first.

       If at least 'min' pages of expanded memory are available, EMC110 installs
  itself as a resident program which, subject to the constraint of 'max', uses as
  much expanded memory as possible and up to 64K of DOS memory.  Specifically,
  the program requires approximately 3K plus 128 bytes per expanded memory page
  used.  Thus a 1-meg cache uses about 11K of DOS memory and, if you have enough
  expanded memory, a cache of nearly 8 meg should be possible.


  3. Changing the Cache Size.

       If EMC110 is already installed, the cache size can be dynamically changed
  by typing

                                EMC110 [max [min]]

  at the DOS prompt, where 'max' and 'min' have the same significance as for the
  initial installation.  The result is that the cache is purged, all expanded
  memory being used by the cache is released, a new allocation of expanded memory
  of the appropriate size is obtained, and the cache is reinitialized.  Since no
  reallocation of DOS memory or manipulation of interrupts is involved, the cache
  can be resized under circumstances which would not permit removing it and
  reinstalling it.  However, since the cache retains its original allocation of
  DOS memory, this imposes an additional constraint on the maximum cache size in
  addition to 'max' and the amount of expanded memory available: the new size can
  never exceed the size of the original installation.




                                     Page 1 of 5




                            EXPANDED MEMORY DISK CACHE 1.10


  4. Disabling the Cache.

       If EMC110 is installed, it can be disabled by typing

                                     EMC110 X

  at the DOS prompt.  The result is that the cache is purged, all expanded memory
  being used by the cache is released, and subsequent BIOS disk interrupts are
  immediately passed on to the prior interrupt handler.  However, the cache
  maintains its allocation of DOS memory and its position in the BIOS disk
  interrupt chain, so it can be reactivated as described under 'Changing the
  Cache Size' above.


  5. Uninstalling the Cache.

       If EMC110 is installed, it may be possible to uninstall it (i.e., remove
  it from memory entirely) by typing

                                     EMC110 XX
  or
                                     EMC110 XXX

  at the DOS prompt.  The 'XX' variant will complete the uninstallation only if
  it can be done without fragmenting DOS memory.  The 'XXX' variant will complete
  the uninstallation whenever possible.  If EMC110 was not the last program to
  insert itself in the BIOS disk interrupt chain, then the uninstallation will
  not be possible.  If the uninstallation is not completed, then the cache will
  be left in the disabled state.


  6. Querying the Cache.

       The status of the cache can be determined by typing

                                     EMC110 ?

  at the DOS prompt.  The result, in addition to an identifying banner, will be
  the display of either

                                Cache not installed
  or
                             xx page cache installed.

  If the cache is installed, but has been disabled or resized to less than its
  original installed size, then either

                                 Cache is disabled
  or
                              xx page cache is active

  will also be displayed.  In addition, if any disk read, disk write, or expanded
  memory errors have occurred, further lines will be displayed detailing these
  errors.  Note that the error counts are not reset by resizing or disabling the
  cache.  Moreover, only errors which arise from disk or expanded memory requests
  issued by the cache itself are counted.


                                     Page 2 of 5




                            EXPANDED MEMORY DISK CACHE 1.10



       The display described above is also produced by all other calls to EMC110
  which don't result in removing the cache from memory entirely.  The difference
  is that, whereas the other calls result in the cache's being reinitialized, the
  '?' variant leaves the cache intact.


  7. Compatibility with Resident Program Managers.

       EMC110 should not be used with any program which claims to be able to
  suspend and later reactivate resident programs.  All disk caches work by
  keeping copies of disk sectors in memory.  If a cache is suspended and later
  reactivated by another program, the cache has no way of knowing that its
  contents may no longer be correct.  The results could be disastrous.

       EMC110 may also be incompatible with programs which merely uninstall
  resident programs.  Such programs presumably work by recording the computer's
  state before a resident program is installed, and restore that state when
  uninstalling it.  If you use such a program to uninstall EMC110, or if you
  never use EMC110 to resize, disable or uninstall itself, this technique should
  work.  But consider the following scenario: first you install EMC110, then you
  install resident program 'A', then you disable EMC110, then you install
  resident program 'B' which allocates some expanded memory, then you reactivate
  EMC110.  At this point the expanded memory owned by EMC110 is probably not the
  same as it was when 'A' was installed, so simply removing 'A' and 'B' from
  memory and restoring the computer's state to what it was when 'A' was installed
  will not work.  Admittedly this scenario is unlikely, and some resident program
  managers may even be clever enough to cope with it, but the integrity of your
  hard disk is at stake.  If want to use both EMC110 and a resident program
  manager, the safe way is never to use EMC110 to resize, disable or uninstall
  itself.


  8. Compatibility with Multitasking Programs.

       EMC110 doesn't work with DoubleDOS 4.0.  It does appear to work with both
  Microsoft Windows 1.03 and Desqview 1.3 and 2.0.  In fact the resize and
  disable features work in a Desqview window.  But note that EMC110 must be
  installed BEFORE you invoke Windows or Desqview; you should NEVER install
  EMC110 in a window.


  9. Operational Notes.

       EMC110 is a pure write-through cache.  Both reads and writes are cached in
  the sense that, whenever an actual disk read or write occurs, copies of the
  sectors involved are placed in the cache.  But whereas read requests are
  serviced from the cache contents rather than the disk whenever possible, write
  requests are always passed on to the prior interrupt handler.  Unlike most
  commercial caches, EMC110 doesn't try to avoid writing to disk by checking if
  the data to be written agrees with the cache contents.

       A cache performs like an automatic RAM-disk.  A cache is more convenient
  than a RAM-disk in that you don't have to copy files to it, you don't have to
  delete old files to make room for new ones, and you don't have to remember to



                                     Page 3 of 5




                            EXPANDED MEMORY DISK CACHE 1.10


  copy updated files back to disk.  On the other hand, a cache is slower than a
  RAM-disk, and writing temporary files to disk is a waste of time.

       Both caches and RAM-disks provide increased performance by decreasing the
  time required to locate data (the seek time) and the time required to transfer
  it.  Whether there is a significant improvement in transfer time is highly
  system dependent; on a system with a slow processor and a fast disk system
  there may be no improvement at all.  But RAM-disks eliminate seek time
  entirely, and caches usually reduce it dramatically.  Whether there is a
  significant improvement in overall system performance depends on what you're
  doing.  If your primary activity is word-processing, all you'll get is a
  psychological lift from seeing the program load faster.  But for disk-intensive
  activities like sorting a huge fragmented database, the improvement should be
  significant and, if the files involved are too large to fit in a RAM-disk, a
  cache is your only choice.

       One factor in the performance of a cache is its size.  The smaller the
  cache the less likely it is that a read request can be serviced from the cache
  contents.  If the cache is too small, then the time spent copying each newly
  accessed sector into the cache (replacing previously accessed sectors) may
  exceed the time saved by occasionally finding the desired sector in the cache.
  The result will be a degradation rather than an improvement in performance.  On
  the other hand, the larger the cache the longer it may take to find a sector in
  it.  Ideally the cache should be just large enough to contain all the sectors
  that are currently being accessed, but using an unnecessarily large cache is
  preferable to using one that is too small.

       As you use EMC110, you may find that the cache doesn't always contain
  everything that you think it should.  There are two reasons why such behavior
  may be consistent with correct operation of the program.  First, the cache may
  be too small.  Second, EMC110 organizes cached sectors into groups, and its
  response to any error or unexpected condition is to purge either the group
  involved or the entire cache.  So a disk error may result in seemingly
  unrelated sectors being purged from the cache, even though the sector causing
  the error may get into the cache on a retry.  Similarly, since EMC110 treats
  any request except read, write or status as an "unexpected condition", any such
  request will cause the entire cache to be purged.


  10. Technical Notes.

       EMC110 works by intercepting calls to interrupt 13h.  It assumes that the
  calls it intercepts and passes on can be interpreted as calls to the IBM-XT
  hard disk BIOS, i.e., that the contents of the processor registers have the
  significance described in the IBM-XT Technical Reference Manual.  In particular
  it assumes that if the high bit of register DL is 1 then the call is directed
  to a hard disk.  It also makes three assumptions that are not explicit in the
  Technical Reference Manual description, namely that all hard disks have non-
  removable media, that all hard disks have 512-byte sectors, and that multi-
  sector requests are serviced by incrementing the sector number, the head number
  and the cylinder number, in that order.  If your system contains storage
  devices other than standard XT/AT hard disks and floppies, you must determine
  whether their operation involves interrupt 13h and, if so, whether the above
  assumptions are valid.  If they're not, the use of EMC110 will produce
  disastrous results.



                                     Page 4 of 5




                            EXPANDED MEMORY DISK CACHE 1.10


       EMC110 also uses the drive-parameters subfunction (AH = 8) of interrupt
  13h for two purposes of its own.  First, it uses this subfunction at
  installation time to determine how many sectors per track and how many heads
  each hard disk has; it will not cache hard disks which are not reported by this
  subfunction.  Second, communication with an installed copy of EMC110 is
  implemented in part by requesting parameters for a non-existent hard disk.  If
  EMC110 is not installed the result should be an error return.  If your hard-
  disk BIOS is willing to return parameters for a non-existent disk, no harm
  should be done; EMC110 will simply refuse to install itself.

       Except for this non-standard use of the drive-parameters subfunction,
  EMC110 expects requests for only three subfunctions of interrupt 13h: status
  request (AH = 1), read sectors (AH = 2), and write sectors (AH = 3).  A request
  for any other subfunction is handled by purging the cache and passing the
  request on.

       EMC110 is not re-entrant.  Its correct operation depends upon the fact
  that, since the IBM disk BIOS is not re-entrant, no operating system, multi-
  tasking program, or interrupt-driven program dare permit two concurrent calls
  to interrupt 13h.  This fact is also the reason for implementing communication
  with an installed copy of EMC110 by making non-standard use of interrupt 13h
  rather than by using a more respectable method.


  11. Copyright, Disclaimer, etc.

       EMC110 is copyrighted by the author.  However, the author hereby grants
  permission for anyone to use this program, and to distribute it freely in
  unmodified form along with this documentation for no more than a nominal media
  charge.

       The author makes no claim about this program except that he has done
  everything he could think of to ensure that it is error-free, and that he's
  using it himself.  If you use EMC110, you do so with the understanding that
  even the most carefully written program may contain subtle errors that appear
  only after prolonged use, that any resident program is subject to corruption by
  other ill-behaved programs, and that either of these eventualities could result
  in loss of data stored on your disks.

       I'm interested in any comments and suggestions you have to make.  Even if
  you don't have any comments or suggestions, it's encouraging to hear from
  people who find EMC110 useful.  I'm also interested in reports of systems and
  software with which EMC110 doesn't work, but it's unlikely that I'll be able to
  help.  While you might be willing to send me copies of the software,
  duplicating your hardware would almost certainly be impossible.  I don't return
  long distance calls, but I do make an effort to answer all letters that require
  a reply.  Please address any correspondence to:


                         Frank Lozier
                         Department of Mathematics
                         The Cleveland State University
                         Euclid Avenue at East 24th Street
                         Cleveland, Ohio 44115




                                     Page 5 of 5