Contents
1. About Jemm
2. Jemm's Features
3. Commandline Options
4. EMS Implementation Notes
5. Emulation of privileged Opcodes
6. Other Tools
6.1 UMBM
6.2 JEMFBHLP
6.3 CPUSTAT
6.4 EMSSTAT
6.5 XMSSTAT
6.6 MEMSTAT
6.7 MOVEXBDA
7. Troubleshooting, Hints
8. License
1. About Jemm
Jemm is an "Expanded Memory Manager", based on the source of FreeDOS
Emm386. It should work with MS-DOS and compatible DOSes, including FreeDOS.
Like other EMMs it installs the following services:
- uses extended memory to simulate expanded memory (EMS) according to
EMS v3.2 and EMS v4.0.
- upper memory blocks (UMB) where drivers and resident programs may
be loaded, thus increasing available free DOS memory.
- mapping RAM to the video address segments A000-AFFF and B000-B7FF.
- VCPI services to allow DOS applications running in V86-mode to
switch to protected mode. VCPI also implements a simple memory management.
- VDS API to give drivers/applications some control over DMA and physical
addresses in V86-mode.
There exist 2 versions of Jemm:
- Jemm386: standard version which needs an external XMM (HimemX.exe for
example) to be loaded.
- JemmEx: extended version which has an XMM already included.
JemmEx most likely is the better choice because it will need less DOS
memory than an external XMM + Jemm386.
2. Jemm's Features
The main purpose of making Jemm was to make it use less resources than
other EMMs, without making compromises regarding speed or compatibility.
The results currently are:
- Jemm386 needs just 128 bytes DOS upper memory. JemmEx uses more, mostly
because it includes the XMS handle array which is located in DOS memory.
With 32 XMS handles JemmEx needs 496 bytes DOS memory.
- Jemm's extended memory usage is:
+ 44 kB for Jemm itself
+ 64 kB for the DMA buffer (default size)
+ xx kB for UMBs mapped in the first MB
+ 4 kB fixed amount for EMS handle management.
+ xx kB variable amount for EMS/VCPI memory management. For each 1.5 MB
VCPI memory 64 bytes are needed, for each EMS page 5 bytes are needed.
For the default values (120 MB VCPI, 2048 EMS pages) this is 16 kB.
- VCPI shared address space in extended memory is just 4 kB.
Other features are:
- Jemm can be loaded and unloaded from the command line. Disadvantage:
DOS won't care about UMBs supplied this way.
- CPUs which provide the Virtual-8086 Mode Extensions (Pentium+) are
actively supported, which increases the emulation speed.
- the FASTBOOT option shortens reboot time.
- the SPLIT option can gain additional DOS high memory.
- Exceptions in protected-mode are detected and displayed.
- a (rudimentary) API is supplied which allows to extend Jemm. [ JLoad
uses this API to add support for 32bit protected-mode extensions (JLMs). ]
3. Commandline Options
For a list of available options enter:
JEMM386/JEMMEX -?
Option Comment
----------------------------------------------------------
A20/NOA20 will set the A20 enable/disable emulation accordingly.
ALTBOOT this option is meant to select an alternate reboot handler,
if the standard handler doesn't work. Currently it does nothing.
B=xxxx specify lowest segment address for EMS banking (default=4000,
min=1000).
D=nnn this option will set the size of the DMA buffer to <nnn> kB.
The default size is 64 kB, max size is 128 kB. Will always be
rounded up to next 4 kB.
EMX option to prevent EMX DOS extender from quickly terminating with
message "out of memory (RM)" on machines with large amounts
of RAM (> 256 MB). This is optionally because it makes Jemm
behave not fully VCPI compliant, but shouldn't hurt usually.
FASTBOOT option might allow a fast reboot. There is no guarantee that it
will work, though, see hints in "Troubleshooting".
FRAME=nnn instructs Jemm to use a certain page frame. Accepted are frame
values from 8000 to E000 or NONE. The page frame should start
at the beginning of a physical EMS page, that is, the frame
address should be divisible by 0x400 without remainder. FRAME=NONE
disables the page frame, but there are quite some programs which
won't run with this setting. A page frame below A000 should
be set only on computers with 512 kB of conventional memory.
Usually it's better to let Jemm find a page frame on its own,
because choosing an address which is not free might cause troubles.
I=mmmm-nnnn force Jemm to use a memory range for UMBs (or page frame).
<mmmm> must be >= A000. Specifying a range which is not really
free may give "unexpected" results.
I=TEST scan ROMs for unused address space. This option will regard any
4 kB page containing identical byte values as "unused".
LOAD installs Jemm from the command line. Be aware that UMBs
cannot be provided this way, since DOS will ignore them.
[MAX=]nnn limit the maximum amount of memory to be allocated for VCPI (and
EMS, if <nnn> is below 32 MB). The MAX= prefix is optional (a
number found as option will be handled as if it has a MAX=
prefix).
MIN=nnn preallocates <nnn> kB of XMS memory thus making sure this
memory is truly available for EMS/VCPI. If MIN is higher than
MAX, MAX will be adjusted accordingly. Default for <nnn> is 0.
NOCHECK disallows access via Int 15h, AH=87h to address regions which
aren't backuped by RAM.
NODYN disables XMS dynamic memory allocation.
NOEMS disables EMS support.
NOHI this option will prevent Jemm from moving its resident part
into upper memory. If no UMBs are installed by Jemm, NOHI
has no effect.
NOINVLPG disables usage of INVLPG opcode on 80486+ cpus. Might be useful
if Jemm runs in a virtual environment (see "Troubleshooting").
NOVCPI disables VCPI. Option can be set from the command line.
PGE/NOPGE options will enable/disable the Page Global Enable feature
on Pentium Pro+ cpus. This allows to mark all PTEs for the
real-mode address space 0-110000h as "global", which gives
a slight speed benefit for VCPI applications. Option is off by
default because some DOS extenders will not work with PGE
enabled. Also setting both PGE + NOINVLPG will not work.
RAM/NORAM will instruct Jemm to supply UMBs or not. RAM is the default.
NORAM is intended to be used when loading Jemm from the
cmdline, in which case adding UMBs might be less useful.
S=mmmm-nnnn add a memory region (which must be in range C800-EFFF) as UMB.
The memory region has to be filled with Shadow-RAM activated by
UMBPCI, which must have been loaded *before* Himem/JemmEx.
NOTE: for Jemm386, instead of using "S=" one might as well use
UMBM.EXE, which additionally allows to load the XMS host "high".
SPLIT if ROMs are found which don't end exactly at a 4 kB boundary
then setting this option will increase available UMB space. ROM
sizes are defined in 0.5 kB units, so there might be up to 3.5 kB
wasted in the ROM's last 4 kB page. There is a small catch: the
full 4k page will be made writeable, including the ROM part.
UNLOAD uninstalls Jemm from the command line. Uninstalling the EMM
might confuse resident programs which rely on EMS or VCPI but
didn't ensure this configuration to keep unchanged (by allocating
an EMS page) while they are running.
V86EXC0D makes Jemm route General Protection Faults (GPF) which occur in
V86-mode to Int 0Dh. Without this option they are routed to
Int 06h. Usually there's no need to set this option.
VCPI (re)enables VCPI. Option can be set from the command line.
VERBOSE talk a bit more during the load process (abbreviation: /V).
VME/NOVME options will enable/disable using the V86 Mode Extensions
on Pentium+ CPUs. These options can be set from the command line.
X=mmmm-nnnn exclude a memory range to be used as UMBs or page frame.
<mmmm> must be >= A000.
X=TEST will exclude all upper memory regions which contain byte values
other than 00 or FF.
JemmEx additionally understands:
A20METHOD:x select A20 switch method.
Possible values for <x>:
ALWAYSON Assume that A20 line is permanently ON
BIOS Use BIOS to toggle the A20 line
FAST Use port 92h, bypass INT 15h test
PS2 Use port 92h, bypass PS/2 test
KBC Use the keyboard controller
PORT92 Use port 92h always
HMAMIN=k set minimum amount in kB to get the HMA (default=0, max=63).
MAXEXT=l limit extended memory controlled by XMM to <l> kB.
NOE801 don't use int 15h, ax=E801h to get amount of extended memory.
NOE820 don't use int 15h, ax=E820h to get amount of extended memory.
X2MAX=m limit for free extended memory in kB reported by XMS V2
(default 65535). It is reported that some old applications
need a value of <m>=32767.
XMSHANDLES=n set number of XMS handles (default=32, min=8, max=128).
4. EMS Implementation Notes
- The number of EMS pages is limited to 2048 (= 32 MB). It can be increased
up to 32768 pages (= 512 MB) by setting MIN=<nnn> to a value higher than
32 MB. However, this memory will then be preallocated, and not be available
as XMS memory. Some applications will not work if EMS > 32 MB.
- There is one memory pool, which is shared by EMS and VCPI.
- The following EMS 4.0 functions aren't implemented. Calling these
functions will return error code 84h in register AH:
+ Int 67h, AH=5Ch, prepare expanded memory manager hardware for warm boot
+ Int 67h, AH=5Dh, enable/disable OS/E
5. Emulation of privileged Opcodes
To provide Expanded Memory an EMM Emulator like Jemm runs the cpu in
so-called v86-mode. This mode does not allow to run privileged opcodes.
Some of these opcodes which might be useful for application programs
are emulated by Jemm, however. These are:
- mov <special_reg>, <reg> ;special_reg = CRn, DRn, TRn
- mov <reg>, <special_reg> ;reg = eax, ebx, ecx, edx, esi, edi, ebp
- WBINVD
- INVD
- WRMSR
- RDMSR
- RDTSC
6. Other Tools
6.1. UMBM
UMBM is a small tool only useful if Jemm386 is used in conjunction with
Uwe Sieber's UMBPCI. The purpose is to be able to load the XMM into
upper memory. This driver must be loaded before Jemm386 and therefore
cannot use UMBs provided by Jemm386.
For JemmEx, UMBM is not needed, since it can be fully replaced by the
S=XXXX option.
How does UMBM work? It expects to find a "shadow" RAM region activated by
UMBPCI. Then it installs inself as a temporary XMS host which just provides
support for allocating UMBs. This is enough for most DOSes to grab the
memory. After the UMBs have been allocated, UMBM will be removed from
memory automatically.
UMBs based on "shadow" RAM, as it is supplied by UMBPCI+UMBM, may have
limitations depending on the motherboard's chipset. Very often the memory
is inaccessible for DMA. Read the documentation coming with UMBPCI for more
details.
Enter "UMBM" on the command line and read the example how to add UMBM
to CONFIG.SYS. UMBM has been tested to run with MS-DOS 6/7 and FreeDOS.
6.2. JEMFBHLP
JEMFBHLP is a tiny device driver only needed if both FreeDOS and Jemm's
FASTBOOT option are used. FreeDOS v1.0 does not provide the information
that Jemm needs for FASTBOOT to work, so this driver tries to cure FreeDOS'
incapability. It saves the values for interrupt vectors 15h and 19h at
0070h:0100h, which is the MS-DOS compatible way to do it.
I was told that in FreeDOS v1.1 this problem has been fixed.
6.3. CPUSTAT
CPUSTAT can be used to display the current status of the CPU.
6.4. EMSSTAT
EMSSTAT can be used to display the current status of the installed EMM.
It works with any EMM, not just Jemm.
6.5. XMSSTAT
XMSSTAT can be used to display the current status of the installed XMM.
It allows to control current values of JemmEx options X2MAX, MAXEXT and
XMSHANDLES.
6.6. MEMSTAT
MEMSTAT may be used to display the machine's memory layout, as it is
returned by the BIOS. The most interesting infos are:
- address region reserved for the ROM-BIOS
- total amount of free memory
6.7. MOVEXBDA
MOVEXBDA is a device driver supposed to move the Extended BIOS Data
Area ( XBDA or EBDA ) to low DOS memory. If an XBDA exists, it is usually
located just below the A000 video segment, with a size of 1, 2 up to 8 kB.
Moving the XBDA to low memory allows Jemm to increase DOS low memory up to
736 kB with commandline parameter I=A000-B7FF ( one should be aware of a
few limitations of this setting - see below for more details ).
Some DOS versions will move the XBDA on their own - at least if its size
doesn't exceed 1 kB; then MOVEXBDA is not needed and will do nothing if
launched.
MOVEXBDA should work with other EMMs as well. Brief tests with MS Emm386
didn't reveal any problems. However, it looks like MOVEXBDA won't work
reliably with FreeDOS!
7. Troubleshooting, Hints
� If Jemm halts or reboots the machine, the following combinations
of parameters may help to find the reason. Generally, Jemm386 should be
loaded immediately after the XMM (HIMEMX.EXE, HIMEM.SYS), and the XMM
itself should be the first device driver to be loaded. For testing, it
might also help to prevent DOS from loading in the HMA and/or not to
use UMBs at all.
- X=A000-FFFF NOHI NOVME NOINVLPG
This is the safest combination. If this doesn't work, Jemm most
likely isn't compatible with the current DOS/BIOS.
- X=TEST NOHI NOVME NOINVLPG
This is slightly less safe, since Jemm will scan the upper memory
region to find "unused" address ranges usable for UMBs. If this
doesn't work, one has to manually set X=xxxx-yyyy to finally
find the region which causes the troubles. Tool MEMSTAT may be used
to find the address region which is reserved for the ROM-BIOS.
� Jemm has been verified to run on the following virtual environments:
Qemu, VMware, VirtualPC, Bochs, VirtualBox
However, it might be necessary to set options NOINVLPG and/or NOVME.
� Some DOS programs will not work if EMS is enabled without a page frame.
� The JEMM ;-) DOS Extender (used for "Strike Commander" and "Privateer")
requires the NOVME option to be set. This requirement is a strong sign
that this extender switches to V86 mode on its own, which is a bad idea
for a VCPI client.
� The FASTBOOT option can work with many versions of DOS. However,
additional settings might be needed:
- FreeDOS may require to set STACKS=0,0 in CONFIG.SYS.
Also the XBDA most likely must not be moved.
- FreeDOS additionally needs JEMFBHLP.EXE to be installed prior to the
XMM (Himem). Possibly this might also be needed for MS-DOS versions < 5.
� If Jemm is installed from the commandline, loading the CTMOUSE driver
v1.9x and v2.0x might cause an exception. Adding option NOHI or NORAM
when installing Jemm should avoid that. In CTMOUSE v2.1 the bug has
been fixed.
� FreeDOS regrettably accepts just one UMB provider. This makes it impossible
for example to use UMBM.EXE to supply UMB D000-DFFF and then tell Jemm to
additionally supply B000-B7FF as UMB. MS-DOS has no such problems.
� Although including the region B000-B7FF might work in most cases, one
should be aware that this region is not really free, it's used by the
VGA "monochrome" video modes.
� To make Jemm behave (almost) like MS Emm386 regarding UMBs one should
set both options X=TEST and I=TEST. For better MS Emm386 compatibility
one might also consider to restrict VCPI memory to 32 MB by adding
option MAX=32M. If NODYN is used, one should also set at least MIN=256K.
� The NOVCPI option may be used to setup an environment similiar to Windows
DOS boxes:
- install Jemm with VCPI enabled
- install a DPMI host residently (HDPMI, DPMIONE, ...)
- disable VCPI with the NOVCPI option
This forces any DOS extended application to either use DPMI or abort.
� If Jemm displays warning
System memory found at XXXX-XXXX, region might be in use
then that region is not used by Jemm. If you are sure that the region
is ok to be used, include it with 'I=XXXX-XXXX'.
� The I=XXXX commandline option may be used to include the VGA "graphics"
segment A000h. It might be possible to increase DOS conventional memory
up to 736 kB by option I=A000-B7FF. However, there are quite a few
hurdles that may cause unexpected results:
- conventional memory is increased only if the region to include is
adjacent to current memory. On newer machines, there's very often a
"hole", caused by the Extended BIOS Data Area (XBDA or EBDA); thus
the included region just becomes an UMB and won't increase lower memory.
Use tool MOVEXBDA to fix this issue!
- Once address space A000h is remapped, any attempts to run programs that
use VGA graphics most likely with cause a crash.
- Depending on the VGA-BIOS it may happen that some non-graphics functions
won't work anymore and may also cause a crash. Not unusual is that the
current text font becomes corrupted ( text font bitmaps must be copied
from ROM to VGA memory when a video text mode is set ).
8. License
- JEMM386/JEMMEX: partly Artistic License (see ARTISTIC.TXT for details)
- UMBM: Public Domain
- JEMFBHLP: Public Domain
- CPUSTAT: Public Domain
- EMSSTAT: Public Domain
- XMSSTAT: Public Domain
- MEMSTAT: Public Domain
- MOVEXBDA: Public Domain
Binaries and source are distributed in separate packages. The binaries'
package has a 'B' suffix in its name, the package containing the source
has a 'S'.
Japheth
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