Jemm

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Contents

1. About Jemm

2. Jemm's Features

3. Commandline Options

4. EMS Implementation Notes

5. Emulation of privileged Opcodes

6. Additional Tools 6.1 UMBM 6.2 JEMFBHLP 6.3 CPUSTAT 6.4 EMSSTAT 6.5 XMSSTAT 6.6 MEMSTAT 6.7 VCPI 6.8 MOVEXBDA 6.9 CPUID

7. Troubleshooting, Hints

8. License

9. About Jemm

Jemm is an "Expanded Memory Manager" (EMM), 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 eXtended Memory Manager (XMM; examples: Himem[S]X, MS Himem, XMGR ) 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.

Currently there's a third variant supplied, JemmExL. It's a stripped down version of JemmEx, missing the enhanced XMS capabilities added to JemmEx in v5.80. Since the same effect can be achieved by setting MAXSEXT=0 (see below), JemmExL will be removed in future releases.

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). ]
• JemmEx supports XMS v3.5, allowing access to extended memory beyond the 4 GB barrier. See XMS35.txt for technical details.

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. The current implementation sends a "system reset" command to the keyboard controller. 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 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). 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 in read-only memory 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 is below 32 MB). The MAX= prefix is optional (a number found as option will be handled as if it has a MAX= prefix). Default value is 120 MB. MIN=nnn preallocates 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 is 0. MOVEXBDA move XBDA into UMB, thus increasing low DOS memory. This option may cause a system "lock" because the XBDA might contain buffers used for DMA operations. In such cases either use MOVEXBDA.EXE or UMBPCI+UMBM instead. NOCHECK disallows access via Int 15h, AH=87h to address regions which aren't backuped by RAM. As for Jemm386: be aware that some old XMMs return wrong values for highest physical RAM - then this option may cause strange results. Also, this option may prevent real-mode programs from using the VESA LFB. NODYN disables XMS dynamic memory allocation. Jemm will allocate XMS memory for EMS/VCPI on initialization. Default is size of largest XMS block/2, but max. 32MB. With option MIN=xxx one may override this. 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: since v5.80, this option is virtually obsolete, because either Jemm should find RAM activated by UMBPCI automatically, or, if UMBM.EXE has been loaded in CONFIG.SYS, the RAM is already used by DOS. SB Soundblaster driver compatibility mode on. 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. This option should only be set if a resident program is to be installed that can handle GPFs in V86-mode. 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. NOVME is the default. X=mmmm-nnnn exclude a memory range to be used as UMBs or page frame. 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 : 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 kB. MAXSEXT=l limit extended memory beyond 4GB barrier to kB. setting MAXSEXT=0 will make JemmEx behave like a v3.0 XMM. NOE801 don't use int 15h, ax=E801h to get amount of extended memory. NOE820 for JemmEx, removed in v5.80, since required by XMS v3.5. JemmExL still understands this option, which tells it not to 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 =32767. XMSHANDLES=n set number of XMS handles (default=48, min=10, 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= 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>, ;special_reg = CRn, DRn, TRn
• mov , <special_reg> ;reg = eax, ebx, ecx, edx, esi, edi, ebp
• WBINVD
• INVD
• WRMSR
• RDMSR
• RDTSC

6. Additional Tools

6.1. UMBM

UMBM is a small tool only useful in conjunction with Uwe Sieber's UMBPCI. The main purpose of UMBM is to allow DOS to load the XMM into upper memory. This driver must be loaded before Jemm386 and therefore DOS can't use UMBs provided by Jemm386. For JemmEx, UMBM is usually not needed, since the XMM is included and needs no low memory.

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 (note: for this to work line DOS=UMB is required in CONFIG.SYS). After the UMBs have been allocated, UMBM will be removed from memory automatically.

Additionally, UMBM understands the /XBDA option. This will cause UMBM to move the XBDA into its first UMB, thus freeing even more low DOS memory.

UMBs based on "shadow" RAM, as it is supplied by UMBPCI+UMBM, may have limitations depending on the motherboard's chipset. Sometimes the memory is inaccessible for DMA (read the documentation coming with UMBPCI for more details). OTOH, logical and physical addresses for these UMBs are identical, which may be an advantage, especially for the XBDA.

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 displays some system registers. Most of them aren't accessible in v86-mode. So this program should be seen as a test if the emulation of the privileged opcodes works as expected.

Optionally, in V86-mode, CPUSTAT may display GDT, IDT and page directory of the v86-monitor. Run "CPUSTAT -?" for more details.

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 check 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. VCPI

VCPI may be used to display the VCPI status of the installed EMM. With option -p it will display the page table entries for the conventional memory.

6.8. 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 1-12 kB.

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. Also, both JemmEx and UMBM have the ability to move the XBDA as well; those tools move the XBDA to upper memory, thus increasing low DOS memory. However, sometimes MOVEXBDA may be the best option available - because moving the XBDA into an UMB supplied by Jemm (or UMBPCI) may cause the system to "lock".

MOVEXBDA should work with any EMM.

6.9. CPUID

CPUID displays cpu features returned by the CPUID instruction.

7. Troubleshooting, Hints

þ 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 (HIMEM[S]X.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 NOINVLPG

  This is the safest combination. If this doesn't work, Jemm most likely isn't compatible with the current DOS/BIOS; for example, the BIOS might try to unconditionally use "unreal" mode to access USB mass storage devices - with Jemm, this will cause an exception.

• X=TEST NOHI 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.

þ Jemm can be loaded from the command line with option LOAD. This may be helpful if there are so many displays during the boot process that one cannot read them carefully. Also, option /V should be added to make Jemm talkative. To load Jemm386 from the command line, ensure that a XMM has been loaded previously. For JemmEx, no XMM must be loaded, since it is included.

þ Jemm has been verified to run on the following virtual environments:

Qemu, VMware, VirtualPC, Bochs, VirtualBox

However, it might be necessary to set option NOINVLPG.

þ Some DOS programs will not work if EMS is enabled without a page frame. MS CodeView v4.1, for example, may refuse to start then.

þ Some DOS programs will crash if too much VCPI memory is offered. Jemm's default is 128MB, it can be changed with option MAX=xxx. Popular programs that may cause troubles are programs that use Borland's DPMI hosts DPMI16BI.OVL/DPMI32VM.OVL (coupled with RTM.EXE/32RTM.EXE). Setting Jemm option MAX=32752K ( that's 32MB minus 16kB) should help; alternately, try setting environment variable "DPMIMEM=MAXMEM 16383".

þ The JEMM ;-) DOS Extender (used for "Strike Commander" and "Privateer") isn't compatible with the VME option. 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.

þ 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.

• older FreeDOS versions may need JEMFBHLP.EXE to be installed prior to the XMM (Himem). Possibly this might also be needed for MS-DOS versions < 5.

þ 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.

þ With some BIOSes disk access speed slows down significantly in V86-mode. Most likely this is because the BIOS routines want to avoid using DMA in this mode. Loading XDMA32 ( and XCDROM32 ) might fix that.

þ 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.

þ 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.

þ 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.

þ 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.

þ 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'.

þ JemmEx will display warning "E820 - too many ext memory blocks, block xxxxxxxx ignored!" if int 15h, ax=e820h returns more than 10 available memory blocks. It's very well possible that super-extended memory isn't available if this warning has occured. The only cure is to adjust constant ?XMS_STATICHDLS in file jemm16.inc and recompile JemmEx.

þ 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. Option MOVEXBDA or tool MOVEXBDA.EXE may fix this issue.
• Once address space A000h is remapped, any attempts to run programs that use VGA graphics most likely will 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 ).
• DOS conventional memory cannot be increased anymore once UMB have been added to the DOS memory pool. So you cannot use tool UMBM here!
• if MOVEXBDA.EXE causes a system lock during boot, add the /A option to the line in CONFIG.SYS that loads the driver. This aligns the XBDA to a kB boundary, which may cure the lock.
• the GRUB boot loader may, under certain conditions, allocate a chunk of conventional memory BELOW the XBDA. In this case conventional memory cannot be increased anymore, even if the XBDA is moved.

8. License

• JEMM386/JEMMEX: partly Artistic License (see ARTISTIC.TXT for details)
• UMBM: Public Domain
• JEMFBHLP: Public Domain
• CPUID: Public Domain
• CPUSTAT: Public Domain
• EMSSTAT: Public Domain
• XMSSTAT: Public Domain
• MEMSTAT: Public Domain
• MOVEXBDA: Public Domain
• VCPI: 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