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Aptiva - How to free up different types of memory
Applicable to: United States
This section explains how memory is divided and accessed. If you do not need to know about this, but just want to get right into
freeing up confentional memory, skip to the next section.
WHAT IS UPPER MEMORY, EXTENDED MEMORY, AND EXPANDED MEMORY?
Back in the stone ages, Intel came out with their first wonder, the 8086 CPU. It operated in what Intel called Real Mode. Due to the
addressing scheme used, a CPU (central processor unit) in the PC (personal computer) operating in Real Mode can only
address up to 1MB (megabyte) of memory. Of that 1MB, 384KB (kilobytes) is reserved for hardware in your system. That left
640KB for programs to run and thus began the infamous 640KB memory limitation of DOS.
Next, Intel came out with the 80286 processor. With it came a new CPU operating mode called Protected 16 Mode. Protected
16 Mode allows for 24 bit segmented memory addressing or addressability to 16MB. Then came the 32 bit chips in the 386DX
and the 486 family of processors. Protected 32 mode allows full 32 bit memory addressing or addressability to 4 gigabytes (GB).
The 386's and 486's also introduced another mode of operation called Virtual 86 Mode. A processor that supports Virtual 86
Mode allows multiple protected virtual 8086 machines to be run concurrently each with it's own 1MB memory space. Virtual 86
Mode is a specially managed variation of Real Mode that allows for multiple 1MB 8086 machines to coexist within the 4GB
address space. This mode continues to have the 1MB address limitation.
DOS runs in a Real Mode environment. When you run a DOS application, you are running in Real Mode with the 640KB
limitation. Why don't programs run in Protected Mode? Software designers are finally doing just that. As a result, the days of
seeing 'Not enough free conventional memory' error messages are hopefully numbered. For now, we must continue to deal
with the 640KB DOS limitation. that first 640KB is named 'conventional memory'. The 384KB of memory that is reserved for
system hardware is named the UMA (Upper Memory Area). The UMA is divided into 24 blocks of 16KB each. Anything beyond
1MB is called XMS (extended memory) except for the first 64KB of XMS beginning at 1024KB which is called the HMA (High
Memory Area).
What about EMS (expanded memory) you ask? Back in the old days of the 286 processor, you added more memory via a
plug in board like a sound card. To access that memory, you needed an Expanded Memory manager such as the
EMM386.EXE program. However, the memory installed in 386 and 486 based machines is extended memory, not expanded
memory, so EMM386.EXE has the additional job of cenverting XMS to EMS for programs that need EMS. Adding the RAM
parameter to the EMM386 line in the CONFIG.SYS file, e.g., DEVICE =C:\DOS\EMM386.EXE RAM, etc., enables EMS support.
When EMM386 sees the RAM switch, it creates a 64KB memory Page Frame in the UMA. Remember, upper memory is
addressable by the processor (386 based processors and above) because it is within the 1MB Real Mode addressing
boundary. EMM386 uses the Page Frame as a 'map' of expanded memory. That is, EMM386 takes memory locations
beyond the 1MB boundary and maps them into the Page Frame 64KB at a time (64KB is the size limit of the Page Frame).
When a program accesses expanded memory, it's actually accessing the memory withing the Page Frame. EMM386 then
re-directs the information to the requested memory location. When a program needs to access a different memory location.
When a program needs to access a different memory location outside the 64KB boundary. EMM386 remaps the Page
Frame with the appropriate expanded memory locations. The downside of using EMS memory is that is can only be
accessed in 64KB blocks before having to reload new memory locations. This makes accessing expanded memory much
slower than accessing extended memory.
Why use expanded memory? If software designers wrote programs that used extended memory only, then the people that
owned 286's would not be able to use their applications. To maintain compatibility, programs were written to utilize EMS so
everybody could use them, even if it meant that the program ran slower.
That explains how EMS is accessed, but what about extended memory? Managing extended memory is the job of
HIMEM.SYS, the first driver loaded in the CONFIG.SYS file. HIMEM.SYS acts like a traffic cop directing traffic. The traffic
in this case is data sent to and from extended memory. When a program sends data to or requests data from extended
memory, HIMEM.SYS will switch the processor into Protected Mode, take cate of addressing the memory and direct the
data to the memory location. There is more to it than that, but this is just meant to give you a general idea of what is going
on.
HOW TO INCREASE CONVENTIONAL MEMORY
"All of this is fine and dandy, but how do I go about freeing up memory so I can play my favorite game?"
EMM386 performs one other important function. It provides access to the upper memory area. Although most of upper
memory is reserved for hardware adapters, etc., there are areas that are free. EMM386 finds these areas and allocates
them to be used by system programs, device drivers, e.g., SmartDrive, the mouse driver, sound card drivers, etc. and
TSR (Terminate-Stay-Resident) programs.
To load a device driver into Upper Memory, the DOS command LOADHIGH or DEVICEHIGH is used. These commands
tell EMM386.EXE to load the specified device driver into one of the free areas of upper memory. If a driver cannot be loaded
into upper memory, then it is loaded into conventional memory where it competes for space with your programs and games.
As more programs are loaded into conventional memory, less space is available to run your favorite application.
There are areas of upper memory that can be allocated that EMM386 will not normally use. EMM386 can be forced to use
them through the INCLUDE switch: I=xxxx. The INCLUDE switch tells EMM386 that the memory range specified after the 'I' is
available in upper memory. We will take a look at a couple of those locations and how we can use them.
****WARNING****
BEFORE MAKING CHANGES TO YOUR CONFIG.SYS OR AUTOEXEC.BAT FILES, COPY THEM TO A SYSTEM DISK
(BOOT DISK) AND MODIFY ONLY THE SYSTEM DISK. SYSTEM LOCKUPS AND OTHER BOOT ERRORS CAN AND
DO OCCUR. TO RECOVER, YOU HAVE THE ORIGINAL, UNMODIFIED CONFIG.SYS AND AUTOEXEC.BAT FILES ON
YOUR HARD DRIVE TO GET YOUR SYSTEM BACK UP AND RUNNING.
****END OF WARNING****
Let's begin with the upper memory region from 'C000-C5FF'. 'C000-C5FF' is a hexadecimal number. Your computer counts
in hexadecimal because it's easier to convert from hex to binary (binary is represented by 1's and 0's and is what the computer
actually uses to execute programs) than it is to convert from decimal to binary. EMM386 doesn't normally include this region of
memory because on some computers it is used by the BIOS. On IBM machines, this memory location is free. We can use it to
store drivers by adding I=C000-C5FF to the EMM386.EXE line in the CONFIG.SYS file. This will provide an additional 24KB of
upper memory.
The next available location in the upper memory area is 'B000-B7FF'. In the days of MONOCHROME monitors, this region
was used for the monochrome video buffer. If you use a color monitor, you can use this Upper Memory region to add another
24KB of free Upper memory. If you use this region, you will also need to add the following line to the [386 Enh] section of your
Windows SYSTEM.INI file:
DEVICE=C:\DOS\MONOUMB.386
One other region of Upper Memory that is available on IBM personal computers is 'E800-EFFF'.
Another option is to disable EMS support by adding the NOEMS switch to EMM386 in the CONFIG.SYS file. As you will recall
from our earlier discussion on expanded memory, EMM386 takes 64KB of upper memory to create an EMS page frame. If
EMS support is disabled, you will be able to use that 64KB for storing drivers. Some programs require EMS, so this may or
may not be an option for you. The documentation that came with your software will tell you whether it uses EMS or XMS.
Putting it all together, here is what your EMM386 entry in the CONFIG.SYS file should look like using the above information.
* No EMS support - maximum free conventional memory *
DEVICE=C:\DOS\EMM386.EXE NOEMS HIGHSCAN I=C000-C5FF I=B000-B7FF I=E800-EFFF
* EMS support *
DEVICE=C:\DOS\EMM386.EXE RAM HIGHSCAN I=C000-C5FF I=B000-B7FF I=E800-EFFF
OK, now we have created all of this upper memory, but how do we use it? First, we need to find our device drivers. Device
drivers are programs like sound card drivers, the mouse driver, SmartDrive, CD-ROM drivers, etc. These programs can
all be loaded into the upper memory area that is now available. To do that, simply add DEVICEHIGH before the appropriate
commands in the CONFIG.SYS file and LOADHIGH (or LH) before the commands in the AUTOEXEC.BAT file.
EXAMPLE:
DEVICEHIGH=C:\{your sound card driver} or
LOADHIGH C:\DOS\MOUSE
EMM386 will load the drivers into upper memory in the order in which they appear. Drivers specified in the CONFIG.SYS
file are processed into upper memory before drivers specified in the AUTOEXEC.BAT file. After you have loaded the
drivers high, reboot your computer. At the DOS prompt, type:
MEM /C /P
This will give you a detailed breakdown of programs that are loaded, their size, whether or not they are loaded into
upper memory, and the largest executable program size (free conventional memory). Hopefully, after you have made
the changes recommend herein, the amount of conventional memory available to you has increased. Next, enter the
following at the DOS prompt:
MEM /F
This command will show you the amount of free upper memory you now have available for loading device drivers and TSRs.
Unfortunately, when programs are loaded into upper memory, they must be loaded into contiguous sections of memory. In
other words, a program can not be segmented into different memory locations. Upper memory is divided into REGIONS. In
these regions, EMM386 tries to find contiguous, unfragmented blocks of memory to store programs.
Now is where the tricky part comes. It may not always be desirable to load a program into upper memory! For example, say
that you type the command: MEM /C /P and you see all of your programs loaded high except for SmartDrive which is 34KB
in size and is loaded into conventional memory. Yet, the mouse driver, which is 27KB in size, was loaded into upper memory.
Furthermore, say that you type: MEM /F and you see that there is 10KB of space left in Region 1 which is where the mouse
driver is loaded. In this situation, it would be better to remove the LOADHIGH command from the mouse driver in your
AUTOEXEC.BAT and let the mouse load into conventional memory. The reason is that if the mouse is loaded low, there
will now be 37KB available in Region 1, which is enough to load SmartDrive, a 34KB program, into upper memory. As a
result, you freed up 7KB of space in conventional memory! SmartDrive was taking 34KB, but now the mouse is taking 27KB
of conventional memory and SmartDrive is now loaded into upper memory taking up no space in conventional memory.
The process of figuring which drivers to load high and which ones to load into conventional memory can be tedious, but it
can make a difference between getting an error message of not enough memory and successfully playing your game.
There is yet another option to help with the memory crunch. With MS-DOS 6.2 came the ability to write multi-configuration
CONFIG.SYS and AUTOEXEC.BAT files. With this option, you can go to a menu at boot up and select the configuration
you want to use for that boot session. For example, you have an exciting game to play that runs from your hard drive, yet
you don't have enough conventional memory because your CD-ROM drivers are taking up too much space. The solution
is to create a configuration that will not load the CD-ROM drivers and then select that configuration at boot up. You won't be
able to use your CD-ROM for that session, but you don't care because your game doesn't need the CD-ROM and now you
can play it. Then, when you need your CD-ROM, you simply reboot and select the configuration that loads your CD-ROM
drivers. I currently have 5 configurations on my system. (1 for windows, 4 for DOS (1 with CD and EMS, one CD and no EMS,
one for no CD and EMS, and one for no CD and no EMS.) Using the memory management techniques we talked about
above, I am able to get 619KB of conventional memory with SmartDrive, the mouse and CD-ROM drivers loaded!
One final note: It is NOT always necessary to load sound-card drivers (these drivers are loaded in your CONFIG.SYS file).
By not loading them, you will not be able to control volume levels via software, but you will be able to control volume using
the controls on your speakers. Most DOS programs come with sound card drivers included, so yours are not needed.
Note that this is NOT true for all DOS applications and all sound cards. If you run across one that does not give you any
sound, then you will have to load the drivers. I place a REM in front of my sound drivers in my CONFIG.SYS file. If I run
across a program whose sound does not work, I remove the REM and let the drivers load. By REM'ing out sound drivers,
you can free up quite a bit of memory (REM means REMARK and tells DOS that the line is a comment line and should not
be executed by DOS.)
One last thing you can do is to change the STACKS size in your CONFIG.SYS file. If you look in your CONFIG.SYS, you
will see a command that says STACKS=x,yyy where x is the number of stacks and y is the size of each stack. A stack is
a temporary storage area. When your computer receives a hardware interrupt, such as a request from a program to
access the disk drive, it stops what it's doing and stores the data it was processing on the stack along with some other
things. When you're running your favorite program, it sets up its own stack. However, sometimes this stack isn't big
enough to handle everything and your system crashes. To fix this, Microsoft included the STACKS command. When a
non-zero value is included in the STACKS command, programs use the DOS stack instead of their own. The downside
of all this is that the DOS stack takes up space in conventional memory. For example, STACKS 9,256 takes up 2,304
bytes (2KB). It may not seem like much, but sometimes 2KB can mean the difference between a program running and
not running. Set the STACKS to 0,0 (STACKS=0,0). This will disable the DOS stack and let programs use their own stack.
This will not always work for all systems. It all depends on your hardware and the software you're running. STACKS=0,0
will work with most PC's. If, after setting the STACKS to 0,0, you get STACK OVERFLOW errors or your computer
locks up, you will have to reset the stacks to what it was before.
SUMMARY
Freeing up conventional memory can be a tedious, frustrating, and time consuming process. By including areas of
upper memory that aren't being used and loading TSRs into them, conventional memory can be freed rather easily.
Coupling that with the flexibility of using the multi-configuration option (IBM PC DOS 6.1 or MS-DOS 6.2 and later) and
a little experimenting using the techniques we talked about, i.e., setting the STACKS to 0, you can set up your system
to run any application with a minimum of trouble.
I hope this has helped you to beat the 640KB DOS barrier and taught you a little about what goes on in your computer.
The preceding discussion is by no means a concise, detailed explanation of how memory is accessed. The processes
I talked about in the first section barely scratched the surface of what is really going on. I left out many details as beyond
the scope of this text. This discussion is only intended to give a general idea of how things work and explain how to get
around some of the barriers that we encounter as personal computer users.
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Search Keywords |
Miscellaneous | |
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Hint Category |
Memory | |
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Date Created |
04-04-97 | |
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Last Updated |
15-03-99 | |
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Revision Date |
15-03-2000 | |
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Brand |
IBM Aptiva | |
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Product Family |
Aptiva, PS/1 | |
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Machine Type |
2011, 2134, 2144, 2159, 2168, 2176, 2121, 2123, 2133, 2155 | |
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Model |
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