Program Metropolis - Software Boutique 95/SOFTWARECD.iso/qemm75/qemm.qip/XMS.DOC

Text File | 1994-09-17 | 15.5 KB | 359 lines

                                   XMS.COM

XMS.COM allows you to exercise all the functions of the XMS specification
at the DOS prompt.

When you run XMS.COM with no parameters it reports the state of XMS.

When you run XMS.COM with the "?" parameter it displays all its
functions with their abbreviations.

When you run XMS.COM with the HELP parameter it displays a usage
message for all parameters.

Here is an example of a status report XMS.COM returns with no
parameters used:

XMS Version Number:                      3.00
XMS Internal Version Number:             7.50
The HMA exists.
The HMA is not available.
The A20 line is enabled.
Total Free XMS:                          6928K
Largest free EMB:                        6896K
Highest ending address of any EMB:       0083:FFFF
Total free XMS handles:                  61
Largest free UMB:                        182.234K
Address of largest free UMB:             B5B9
Amount of HMA DOS is not using:          18800 bytes
The first available address in the HMA:  FFFF:B690

Here is an explanation of the above categories:

XMS version number -  The XMS specification has gone through three
revisions.  The number indicates the level of the XMS specification the
XMM (XMS Memory Manager) supports.

XMS internal version number -  Indicates the version of the XMM,
determined by its author.

HMA - The first 64K (minus 16 bytes) of extended memory comprises the
HMA (High Memory Area). In DOS segment-offset addressing the HMA begins
at FFFF:0010 and ends at FFFF:FFFF.  In flat 32-bit addresses, the HMA
begins at 10:0000 and ends at 10:FFEF.  For background on the HMA read
the history section.

A20 - IBM and compatible PCs originally used the 8088 processor
which had only 20 address lines (A0 - A19). Beginning with the 80286
PCs had additional address lines (necessary to address more than
one megabyte of memory).  To use extended memory (including the HMA)
you must enable the 21st address line (A20); thus the XMS
specification keeps track of the state of the A20 line. Read the
history section for more information on the A20 line.

Total free XMS - tells you how much extended memory (not including
the HMA) you have available.

Largest free EMB - tells you the largest Extended Memory Block (EMB)
you can allocate.  When you allocate extended memory through the XMS
specification, the XMM gives you an EMB.  The XMM can allocate only
contiguous EMBs; you cannot allocate an EMB larger than Largest Free
EMB no matter what Total Free XMS reports.

Highest ending address of any EMB - reported if your XMM supports XMS 3
or higher.  This number, a 32-bit flat address, tells you the highest
address of any extended memory.

Total free XMS handles -  returns the number of free XMS handles.
When you allocate an EMB the XMM gives you a handle by which you refer
to it. To quote the XMS specification:

     "Extended memory handles are scarce resources.  Programs should
     try to allocate as few as possible at any one time.  When all
     of a driver's handles are in use, any free extended memory is
     unavailable."

Largest free UMB - returns the size of the largest available Upper
Memory Block if your XMS manager provides UMBs (memory in the
address space above the top of conventional memory and below the
beginning of extended memory - typically between 640K and 1024K).

Address of largest free UMB -  returns the segment address of that
largest UMB.

Amount of HMA DOS is not using - tells you how much of the HMA is
not in use if you are using DOS=HIGH

The first available address in the HMA -  tells you the
first available address in the HMA.



HMA FUNCTIONS

XMS has two HMA functions:  AllocateHMA (AH) and ReleaseHMA
(RH).

AllocateHMA xxxx allocates the HMA.  If the HMA is already allocated,
the call fails and tells you so.  Because the XMS specification has no
provision to share the HMA, it allows XMMs to require that the HMA
requestor use a certain minimum amount of the HMA. (The XMS
specification recommends that the XMM author let the user set this
minimum size with the HMAMIN parameter.)  Thus when you request the HMA
you must specify a size.  XMS.COM defaults to interpreting the size you
specify as a number of bytes; add a K to the end of the number if you
want it interpreted as a number of kilobytes. If you specify a size
smaller than HMAMIN, the XMM will refuse the request.  HMAMIN defaults
to 1 unless it is set by a parameter to the XMM.

ReleaseHMA releases the HMA.  It takes no parameters.



A20 FUNCTIONS

XMS has four A20 functions: GlobalEnableA20 (GE), GlobalDisableA20
(GD), LocalEnableA20 (LE), and LocalDisableA20 (LD). These four
switches turn on/off the 21st address line, A20.  When you turn A20
off, you cannot access any odd megabytes of extended memory,
including the HMA. The XMS specification states that the Global
functions "should only be used by programs which have control of the
HMA." Additionally, the Local functions "should only be used by a
programs which need direct access to extended memory." Read the
history section if you want background on these functions.



EMB FUNCTIONS

XMS has seven EMB functions (not counting its SuperEMB functions,
described later):  CreateEMB (CE), FreeEMB (FE), MoveEMB (ME), LockEMB
(LKE), UnlockEMB (UE), QueryEMB (QE), and ResizeEMB (RE).

CreateEMB xxxx allocates a EMB of size xxxxK and returns the number of
the handle.

FreeEMB xxxx frees the EMB handle xxxx.

MoveEMB tttt uuuu vvvv:wwww xxxx yyyy:zzzz moves a block of data of
length tttt (in bytes) from EMB handle uuuu beginning at address
vvvv:wwww to EMB handle xxxx beginning at address yyyy:zzzz. (vvvv:wwww
and yyyy:zzzz are 32-bit flat addresses that are offsets from the base
address of their respective handles uuuu and xxxx).

LockEMB xxxx locks EMB handle xxxx. You cannot free or resize a
locked EMB. You can lock a handle up to 255 times.

UnlockEMB xxxx unlocks EMB handle xxxx.  Unlocking only removes one
lock.

QueryEMB xxxx returns the size and lock count of EMB handle xxxx.

ResizeEMB xxxx yyyy resizes EMB handle xxxx to size yyyyK.


UMB FUNCTIONS

XMS has three UMB functions:  CreateUMB (CU), FreeUMB (FU), and
ResizeUMB (RU).

CreateUMB xxxx creates a UMB of size xxxxP (P stands for paragraphs,
which are 16-byte units) and returns the segment address of the UMB.

FreeUMB xxxx frees the UMB at the segment xxxx.

ResizeUMB xxxx yyyy resizes the UMB at segment xxxx to size yyyyP (p
stands for paragraphs).




SUPER FUNCTIONS

Beginning with version 3, the XMS specification can handle EMBs of
size up to 4,096 megabytes. This required adding new functions. If
you have a version 3 manager and more than 64 megabytes of extended
memory, you can use these functions. XMS has three SuperEMB
functions:  SuperCreateEMB (SCE), SuperQueryEMB (SQE),
SuperResizeEMB (SRE).

SuperCreateEMB xxxx creates an EMB of size xxxxM (megabytes).

SuperQueryEMB xxxx returns the size (up to 4,096M) and lock count of
EMB handle xxxx.

SuperResizeEMB xxxx yyyy resizes EMB handle xxxx to size yyyyM
(megabytes).


ADDITIONAL XMS.COM FUNCTIONS

In addition to the functions that allow you to exercise the XMS
specification, XMS.COM has some additional functions:

DIR lists all your EMBs by handle.  It reports the length, starting
address and lock count for each handle.

OFF turns off all XMS functions.

ON turns XMS functions back on again if turned off with XMS OFF.

SAVE xxxx yyyyyyyy.zzz saves EMB handle xxxx as file yyyyyyyy.zzz

LOAD yyyyyyyy.zzz allocates an EMB, loads file yyyyyyyy.zzz into
this EMB, then reports the name of the handle into which it loaded
it. Because an XMM can allocate EMBs only in increments of
kilobytes, LOAD will round up to the next kilobyte to accommodate
the file.

HELP displays a usage message for all parameters.

? displays a list of all parameters and their abbreviations.

Please note that there is absolutely no security in the XMS
specification. If some program (even DOS) uses the HMA, the XMS
specification does not prevent another program from deallocating the HMA
and using it for its own purposes. This holds true for deallocating an
EMB or a UMB. If you deallocate the HMA, a UMB or an EMB that another
program is using and use that memory for another purpose, the first user
of the HMA (or that UMB or EMB) will malfunction when it tries to use
that memory, quite possibly causing a crash.  XMS.COM has NO active
security to prevent you from doing anything the XMS specification
allows.



HISTORY

EXTENDED MEMORY

The original IBM PC could address only one megabyte of address
space. With the advent of the AT we had to figure out what to do
with sixteen megabytes of address space. Had we shifted to an
operating system written for the 80286, we would have had no problem.
But we stuck with DOS. DOS could do nothing with the memory above
one megabyte (or so we originally thought). Indeed, because of the way
the 8088 worked, PC manufacturers had to disable the 21st address line
(A20) on 80286s to prevent access to the memory above one megabyte. They
did this by putting a electronic switch on A20 that could be flipped
by sending an instruction to the keyboard controller chip

Because we continued to use DOS with the 80286, we had to separate
"normal" memory (beneath one megabyte) from the memory beginning at
one megabyte. We called the latter extended memory. In the
beginning, no one had a use for extended memory.  Then IBM wrote a
ram disk, VDISK, that worked by suspending the operation of DOS for a
brief period of time, switching into protected mode (the mode
necessary to use extended memory, and a mode in which DOS cannot run),
moving the desired information to/from its target in "normal"
memory, then switching back to real mode (the mode in which DOS
runs).  This may sound slow but it worked much faster than accessing
even a fast hard disk.  Later, some disk caches and print spoolers used
extended memory and then some very large programs figured out how to run
in protected mode but still use the services of DOS (DOS extenders).

HMA

To maintain compatibility with the 8080 (the 8-bit processor the
8088 replaced) the 8088 addresses memory by adding the contents of two
numbers, a segment register, shifted left four times (i.e., multiplied
by 16), plus an offset (supplied from another register, a memory
location, or a processor instruction).  This allows you to run CP/M on
your 8088 (or even your 80486, for that matter).  On an 8088, when these
two numbers add up to a sum greater than one megabyte, the 8088 "wraps
around" and accesses the bottom 64K of the address space instead of the
non-existent address above one megabyte. On a processor like the 80286,
the address greater than one megabyte can actually exist, so the CPU
accesses it instead: unless you disable A20.

Quarterdeck programmers saw that if you enabled A20, you could
actually use the first 64K (minus 16 bytes) of extended memory without
having to leave DOS's real mode. This strategy required a few tricks to
make it work safely, but by 1987 DESQview put nearly 64K bytes of itself
into this piece of the address space, decreasing its overhead in lower
memory by about the same amount.

Beginning with DOS version 5 Microsoft figured out how to put a
piece of DOS into this portion of the address space. Microsoft
calls this piece of memory the HMA.

How do you understand the addressing quirk that makes the HMA possible?
In hexadecimal, one megabyte = 100000. DOS only knows about 16-bit
registers.  You can only fit four hex digits into a 16-bit register
limiting it to FFFF (64K minus 1).  To access more than 64K, the 8088
generates 20-bit addresses (five hexadecimal digits) by shifting the
segment register left 4 times (or one place in hex) and adding another
4-digit number, which we call the offset.   For example:

     1000:1000 = 10000 + 1000 =  11000  = 68K
     1100:0000 = 11000 + 0000 =  11000  = 68K
     1010:0F00 = 10100 + 0F00 =  11000  = 68K
     5678:1234 = 56780 + 1234 =  57A14  = 350K + 532 bytes
     FFFF:FFFF = FFFF0 + FFFF = 10FFEF  = 1087K + 1007 bytes

An 8088 would have no address lines to accommodate the most significant
digit of this last sum, so it would access 0FFEF, 63K + 1007 bytes,
instead. With the A20 line disabled, so would an 80286 (or 80386 or
80486 or Pentium...), thus maintaining compatibility with the 8088.
With A20 enabled, segment-offset addressing thus extends almost 64K
beyond the first megabyte.


UMBs

When IBM partitioned the one megabyte of address space that the 8088
could address, it reserved the top 384K for ROMs and hardware.
Almost everyone has a system BIOS ROM in the top 64K (F000-FFFF,
960K - 1024K). The VGA card puts graphics in A000-AFFF (640K -
704K) and color text in B800 - BFFF (736K - 768K) or monochrome text in
B000 - B7FF (704K - 736K).  Most VGA cards have a 32K ROM at
C000-C7FF (768K - 800K).  You may have a network card or disk
controller or some other adapter card that uses a portion of the
address space in this area for a ROM or RAM. Almost everyone has
vacant address space in this region though--address space DOS
programs can use if only usable RAM occupied it.

The EMS (Expanded Memory Services) specification has provision for
mapping memory into vacant addresses in this portion of the address
space. Before the 386, computers with suitable EMS cards could map
memory into this portion of the address space and use it with DOS
programs. Quarterdeck's DESQview pioneered use of this address
space. When the 386 came out, many programs could use EMS but few
could use extended memory.  386 owners bought EMS emulators, such as
QEMM, to turn their then not-so-useful extended memory into EMS.
These EMS emulators could map memory into this portion of the
address space. Programs that so chose could allocate this memory and
have more conventional memory available.

Then Quarterdeck wrote the LOADHI programs to load programs into this
memory.  The programs loaded by LOADHI knew nothing about the High RAM
an EMM could create.  Version 2 of the XMS specification added support
for Upper Memory Blocks, which is the XMS term equivalent to the High
RAM that QEMM puts between 640K and 1024K.  Upper Memory Blocks have
nothing to do with extended memory.  Even today an XMS manager does not
have to support any UMB functions (e.g., Microsoft's HIMEM.SYS).
And computers without extended memory can still have UMBs (e.g., users
of Quarterdeck's QRAM).

XMS SPECIFICATION

We had no rules for the use of extended memory when it first came
into existence. IBM created a de facto specification with the way
VDISK reserved extended memory. Some programs that used extended
memory reserved all extended memory for themselves to ensure that no
subsequent program tried to use their extended memory mistakenly.

In 1988 Microsoft drew up a specification for the use of extended
memory They called the 64K (minus 16 bytes) just above one megabyte
the HMA.  The specification allowed for the enabling and disabling
of the A20 line and the allocation and deallocation of the HMA.  It
included separate enabling and disabling of the A20 line for HMA
users and extended memory users. It had functions to allocate
extended memory above the HMA in increments of one kilobyte up to
64MB (the amount you can specify with a 16-bit register). Version 2
added support for UMBs (see UMBS section above). Version 3 of the
XMS specification added support for resizing UMBs and for handling
EMBs up to 4,096M in size.

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