Q91088: HIMEM.SYS Memory Allocation Scheme
Article: Q91088
Product(s): Microsoft Windows 95.x Retail Product
Version(s): WINDOWS:3.1,3.11
Operating System(s):
Keyword(s): win31
Last Modified: 20-SEP-1999
3.10 3.11
WINDOWS
kbother
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The information in this article applies to:
- Microsoft Windows versions 3.1, 3.11
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SUMMARY
=======
When you start your computer, HIMEM.SYS allocates all available extended memory
by default. It then manages the use of this memory by other applications with
the extended memory specification (XMS).
How HIMEM.SYS Allocates Memory for Itself
-----------------------------------------
HIMEM.SYS uses IBM ROM BIOS interrupt 15h to determine how much extended memory
is available. It then uses the same interrupt to set the available extended
memory to zero. This prevents other applications from allocating extended memory
once HIMEM.SYS has started managing memory. (If you run other programs that
allocate extended memory by using interrupt 15h, use the /INT15= switch. For
more information, refer to the "User's Guide" for Windows 3.1.)
After setting the available extended memory to zero, HIMEM.SYS assigns handles to
each free memory block. On a machine with 4 megabytes (MB) of random-access
memory (RAM), the memory starting at 1 MB up to the 4-MB boundary is usually
contiguous. In this situation, HIMEM.SYS assigns only one handle to this memory
block. More handles are assigned on machines with multiple memory blocks, such
as Compaq machines that have a small amount of memory starting at the 16-MB
boundary.
Note: HIMEM.SYS does not allocate any memory until another program makes an XMS
function call. SMARTDrive and RAMDrive make XMS function calls when they are
loaded. The "DOS=HIGH" statement in the CONFIG.SYS file also makes an XMS call.
This call requests the use of the high memory area (HMA) and forces HIMEM.SYS to
allocate extended memory.
How HIMEM.SYS Allocates Memory for Programs
-------------------------------------------
HIMEM.SYS gives memory to programs that request it by making a call to an XMS
function. It first looks for a large enough block to serve the memory request.
Because some machines have more than one block, there is no way to determine
which block will be used first.
When HIMEM.SYS finds a large enough block of memory, it divides the memory block
into two, which allows the program to use the memory it requested and leave the
remainder as a new smaller memory block. The memory allocated to the program
starts from the bottom of the block and extends until the memory request is
fulfilled. The new block, created from the extra memory, starts where the memory
request stopped.
Interrupt 15h Memory
--------------------
Interrupt (or function) 15h always allocates memory starting from the top of
memory moving downwards. Before HIMEM.SYS starts, interrupt 15h sets the
starting location or address of extended memory at the top or highest extended
memory address. When the amount of available extended memory lowers, the
starting address also lowers. After HIMEM.SYS starts, if no interrupt 15h memory
was allocated, HIMEM.SYS sets the starting address to the 1-MB boundary (the
bottom of extended memory).
Interrupt 15h does not manage memory the same way that HIMEM.SYS does. Interrupt
15h is a large group of several functions for accessing extended memory.
Programs actually take control of interrupt 15h by inserting a small piece of
code into a table that tells the system where interrupt 15h is located. This is
known as creating a hook. Each program that wants extended memory creates a hook
and adds some code to report how much memory is available. Programs report the
initial available memory minus the amount they decided to use. HIMEM.SYS, when
it starts, is no different. It adds a hook that tells all other programs that no
extended memory is left. HIMEM.SYS then doles out memory to those programs that
know how to use XMS functions, such as SMARTDrive, RAMDrive, and Windows.
Isolating Memory Areas with SMARTDRV.SYS and RAMDRIVE.SYS
---------------------------------------------------------
By using SMARTDrive from Microsoft MS-DOS 5.0 and RAMDrive from either Windows or
MS-DOS, you can roughly determine whether or not a block of RAM is causing
problems. This can be very useful for tracking down problems such as memory
parity errors.
Although the HIMEM.SYS memory allocation scheme does not allow you to determine
which memory it allocates at any given time, you can use features of SMARTDrive
and RAMDrive to perform a rough memory test. SMARTDrive allows you to create a
cache limited only by the amount of extended memory available. It also lets you
set two sizes for the cache, one for MS-DOS and one for Windows. RAMDrive also
allows you to allocate a large amount of extended memory, but it does not have
the dual size capabilities of SMARTDrive.
By using SMARTDrive from MS-DOS 5.0, you can load SMARTDrive before RAMDrive in
the CONFIG.SYS file. You can then tell SMARTDrive to create an MS-DOS cache size
of some large amount of memory and a Windows cache size of zero. When you start
Windows, SMARTDrive reduces its cache size to zero. Because RAMDrive installs
after SMARTDrive, this leaves a gap of unused memory at the bottom, which
enables you to test the lower half of extended memory. You can check the top
half by simply installing RAMDrive and nothing else.
To clarify this process, review the following step-by-step example, which
explains what happens in extended memory as Windows and the various device
drivers load. This example is for a machine with 4 MB of memory that is running
MS-DOS 5.0 and Windows 3.1 and has frequent memory parity errors:
1. Add the following two lines, in the order given, to your CONFIG.SYS file:
DEVICE=C:\DOS\SMARTDRV.SYS 2048 0
DEVICE=C:\WINDOWS\RAMDRIVE.SYS 2048 /e
Check your AUTOEXEC.BAT file to ensure SMARTDrive is not being loaded from
AUTOEXEC.BAT.
2. Restart your computer. At this point, SMARTDrive occupies the lower two
megabytes of extended memory, and RAMDrive occupies the upper two.
3. Start Windows by typing "win" (without the quotation marks). SMARTDrive
reduces its cache size to zero, leaving two megabytes free for Windows to
use. RAMDrive still occupies the upper two megabytes of extended memory.
4. Perform the steps necessary to reproduce the memory parity error. If you can
reproduce the error, you can be fairly certain that the problem is in the
lower two megabytes.
5. Exit Windows and edit your CONFIG.SYS file again. This time, remove the
SMARTDrive statement.
6. Restart your computer. Now RAMDrive occupies the lower two megabytes of
extended memory, and Windows can use the remainder.
7. Again, go through the steps to reproduce the memory parity error. If you
cannot reproduce the error, the problem is probably in the lower two
megabytes of extended memory.
To narrow down the memory range, you can manipulate the SMARTDrive settings so
that you can determine a more accurate memory range and repeat these steps. In
the case of a memory parity error, you can then simply replace the memory
causing the error.
Additional query words: 3.10 3.11
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Keywords : win31
Technology : kbWin3xSearch kbZNotKeyword3 kbWin310 kbWin311
Version : WINDOWS:3.1,3.11
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