Browse Prior Art Database

Support of Memory above the 16-Megabyte Address Limit in OS/2

IP.com Disclosure Number: IPCOM000107794D
Original Publication Date: 1992-Mar-01
Included in the Prior Art Database: 2005-Mar-22
Document File: 8 page(s) / 290K

Publishing Venue

IBM

Related People

Gittins, RS: AUTHOR

Abstract

Systems using the IBM OS/2* 1.31 product and before are limited to sixteen megabytes of memory. Although this is a lot of memory, there are times when more memory is needed to a task. Additional memory often improves performance greatly by reducing the amount segment swapping OS/2 has to do in order to complete a given task. Examples of tasks that often require more than sixteen megabytes of memory are database servers and file servers.

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Support of Memory above the 16-Megabyte Address Limit in OS/2

       Systems using the IBM OS/2* 1.31 product and before are
limited to sixteen megabytes of memory.  Although this is a lot of
memory, there are times when more memory is needed to a task.
Additional memory often improves performance greatly by reducing the
amount segment swapping OS/2 has to do in order to complete a given
task.  Examples of tasks that often require more than sixteen
megabytes of memory are database servers and file servers.

      Often these special applications manage their own memory more
efficiently than the operating system can.  This is because the
application can determine when to throw data away, write data to disk
for permanent storage, or write data out as temporary storage.

      A good example of this is the OS/2 1.30 HPFS file system.  This
file system maintains a cache (specified by the /cache parameter in
config.sys) of disk sectors, some of these are to be written, or will
be used for read ahead at a later time, or may be discarded.  The
file system is the place that this is best managed.  The cache is
taken out of system memory.  Since memory in the system is limited,
there is a two-megabyte restriction placed on the amount of cache
space.  If the cache were placed above the sixteen-megabyte mark,
then no system memory would be needed for the file system cache.

      The following discusses how to use more than sixteen megabytes
of memory on Intel 80386-based IBM PS/2* machines.
DESCRIPTION

      IBM PS/2 computers based on an Intel 80386 can be used with
memory above sixteen megabytes.  This includes all the model 70s,
80s, 90 and 95s.  (Machines that use the Intel 80386sx cannot be used
with memory above sixteen megabytes since the microprocessor does not
access physical memory above sixteen megabytes.)

      The hardware required is different depending on whether the
machine is a model 70/80 or a model 90/95.  Models 70/80 use memory
on the MICRO CHANNEL*.  This means that it is possible to configure
the machine to use an arbitrary amount of system memory.  This is
done by using any combination of 2-8 megabyte and 2-6 megabyte memory
adapters.  The 2-16 megabyte memory adapter(s) is used for the memory
above 16 megabyte.

      For Models 90 and 95 there must be more than 16 megabytes of
memory on the planar.  These machines do not allow MICRO CHANNEL
memory, so all memory must be on the planar.

      The resulting address space is 0 to 16 megabytes are used by
OS/2 and applications,  and address above 16 megabytes are used as
buffers by special-purpose applications.  See the figure below.
Setup

      The configuration for the two types (models 70/80 and 90/95) of
hardware is slightly different.
Models 70 and 80

      For the models 70 and 80 the 2-16 Megabyte memory card must be
used.  To set up this adapter correctly a special file is used on the
set-up diskette.  This fi...