PMODE/W allows DOS programs to run in full 32 bit protected mode, with
access to all memory available in the system. PMODE/W basically extends the
DOS environment to protected mode and provides a simple interface to the
real mode DOS system services for your code. PMODE/W takes care of all
aspects of running the system in protected mode and maintaining
compatibility with the underlying real mode software. PMODE/W deals with
low level necessities such as descriptor tables, memory management, IRQ and
interrupt redirection, real/protected mode translation functions, exception
handling, and other miscellaneous aspects of running in protected mode.
Your code does not need to deal with specific aspects of different systems,
such as XMS/EMS/VCPI/DPMI availability. PMODE/W will run on top of almost
any system and provide common protected mode services to your program
through the DPMI interface specification, as well as most standard DOS
functions extended for protected mode use.
PMODE/W is the stub and extender in one. The generated executable contains
the PMODE/W extender within it as the stub. When run, PMODE/W will take
care of setting up the system and executing the protected mode portion of
the program. Several years have gone into the development of PMODE/W. It is
now a fairly mature DOS extender, and has gone through its fair share of
bugs and incompatibilities. It is at this point, a very stable protected
mode system. Great pains have gone into the optimization and testing of
PMODE/W. Our major goals have been speed, size, and stability. We now feel
that we have achieved a good deal of those things. But don't take our word
for it, try it yourself, as you can do that very easily before you actually
decide to buy PMODE/W. Just plug PMODE/W into any popular program out there
which uses DOS/4GW.
To sum it up, if you are looking for a good solid, stable, and fast
extender, PMODE/W may be just what you need.
Here are the advantages of PMODE/W:
No external extender required (everything needed to execute is in the EXE).
Small size (less than 12k for the entire extender program). Compression of
protected mode executables. Low extended memory overhead. Does not require
ANY extended memory to load OR execute. Fast execution.
Our major concerns in developing PMODE/W were speed, size, and stability.
PMODE/W itself was written entirely in assembly. When running under
PMODE/W, your code will be running at a privilege level of zero, the
highest and fastest. PMODE/W does not virtualize what it does not need to,
and does not invoke any protected mode mechanism that is slow. For example,
if the system is running clean or under XMS, PMODE/W does not turn on
paging. Under a memory manager which provides both VCPI and DPMI services,
PMODE/W will opt for VCPI protected mode which is significantly faster than
DPMI. When PMODE/W makes calls to real mode, it switches the system into
actual real mode rather than the slower V86 mode (when it can, under VCPI
this is not possible, control must be passed back to the VCPI server). In
terms of speed, when your code is running under PMODE/W, it is running as
fast as the system will allow. In terms of size on disk, we need say no
more than for you to look at the size of the PMODE/W executable and compare
it to other extenders. In terms of memory size, you may do tests yourself
to confirm that PMODE/W does indeed suck up a lot less memory at run-time
than the competition. In fact, PMODE/W will run even if there is absolutely
no extended memory in the system (assuming of course there is enough low
memory for the program). To be fair, we must say that we squished the
PMODE/W executable with our own compression program written expressly for
the purpose. This demonstrates the extent we took most of our optimizations
to.
When run under a clean system, XMS, or VCPI, PMODE/W has control of
protected mode. In this case, it can set up the system to run as fast as
possible under the various conditions. Under DPMI, the DPMI host of the
system will have full protected mode control and PMODE/W will install its
DOS extensions on top of that. If the system provides both VCPI and DPMI
services, PMODE/W will use the VCPI services for faster execution, unless
instructed not to by the setup program. When PMODE/W does have protected
mode control under clean/XMS/VCPI, it runs all code at a privilege level of
zero. In addition, under a clean or XMS system, paging will not be enabled.
This is only a minor speed increase, but there is no real need to manage
paging.
PMODE/W provides a subset of DPMI 0.9 function calls and general
functionality when a DPMI host is not present. PMODE/W will pass any
software interrupts from protected mode to their default real mode
handlers, provided no protected mode handlers have been installed for them,
just as DPMI will. The general registers will be passed on to the real mode
handler, but the segment registers can not be as they have different
meanings in real mode and protected mode. The flags will be passed back
from the real mode handler. This provides a simple interface to all real
mode interrupt routines which do not take parameters in the segment
registers, for example, INT 16h function 00h.
Any IRQs that occur in protected mode and have not been hooked by a
protected mode handler will be sent on to their real mode handlers. If an
IRQ occurs in real mode, and a protected mode handler has hooked that IRQ,
it will be sent to the protected mode handler first. The protected mode
handler may chain to the real mode handler for that IRQ by calling the
previous protected mode handler for that IRQ. This behavior is in
accordance with the DPMI standard. If you hook a protected mode IRQ (INT
31h function 0205h), then hook the same IRQ in real mode (INT 31h function
0201h), the protected mode handler will be called if the IRQ occurs in
protected mode, and the real mode handler will handle the IRQs if they
occur in real mode. Setting up two handlers like this assures minimal
latency. This means a handler will get control when the IRQ occurs as soon
as physically possible. PMODE/W does have to intervene in the IRQ process,
however, when the low 8 IRQs are mapped to INTs 08h-15h to differentiate
IRQs from CPU exceptions.
In accordance with DPMI specifications, PMODE/W will pass up software
interrupts 1ch (BIOS timer tick), 23h (DOS CTRL+C), and 24h (DOS critical
error) from real mode to protected mode. This means that those interrupts
can be hooked directly in protected mode without having to set up a
callback mechanism yourself. PMODE/W will also pass interrupt 1bh (BIOS
CTRL+BREAK) from real mode up to protected mode. This is not a DPMI
requirement, but it is necessary for the sake of compatibility with
DOS/4GW. Another departure by PMODE/W from official DPMI specifications is
in extended memory allocation. DPMI documentation states that the block of
extended memory allocated through function 0501h is guaranteed at least
paragraph alignment. The PMODE/W DPMI implementation will enforce only
DWORD alignment.
When a PMODE/W executable is run, PMODE/W will attempt to switch the system
into protected mode and load the protected mode portion of the same
executable. If there is some error, not enough memory, or a system
incompatibility, PMODE/W will exit with an error message. If loading was
successful, PMODE/W will pass execution control on to the program. PMODE/W
will load any 16 bit code and data into low memory, but 32 bit code and
data may be loaded into low or extended memory depending on availability.
There are a number of modifiable parameters in the PMODE/W extender
executable that affect protected mode execution. For the most part, these
parameters deal with memory. PMODE/W allocates one large block of extended
memory for its pool from which it provides memory to its client program.
There is a maximum value for the extended memory to be allocated. By
default, the maximum is all of the extended memory in the system. The
maximum value reflects the size of the block you want PMODE/W to take from
the system, not necessarily the size of the largest block available to the
default GetMem procedures. You may set the maximum to zero to indicate you
do not want PMODE/W to allocate ANY extended memory. The amount of memory
that you allow PMODE/W to allocate from the system determines how much
extended memory will be left to other if you shell out of your PMODE/W
program.
Another variable specifies the amount of low memory for PMODE/W to TRY to
keep free. If PMODE/W can, it will accommodate this value by loading 32 bit
code and data into extended memory. If there is not enough extended memory
available for this, 32 bit code and data will be loaded into low memory
anyway. If PMODE/W can not keep this much low memory free, it will not exit
with an error message. Setting this parameter to a high value will, in
effect, duplicate the DOS/4GW behavior of loading all 32 bit code and data
into extended memory. If you do not necessarily need any extra low memory
free during the execution of your program, you may set this value to zero.
There is a group of parameters that specify the number and size of nested
mode switch stacks. Whenever you make a call to real mode, or a callback or
IRQ is passed from real mode to its routine in protected mode, a nested
stack is used. These parameters have meaning only if the program is not run
under a DPMI system. If a DPMI host is in place when the program is run, it
provides its own nested stacks for mode switches. The number of nested
stacks directly affects the number of nested mode switches your program can
make using the various mode switch methods. The size of both the real mode
and protected mode nested stacks can also be specified. By default, these
values are high enough for normal operation. However, if you intend to use
a lot of stack variables in a protected mode IRQ handler, or a lot of
recursive calls, you may need to increase the size of the protected mode
nested stacks. The more nested stacks you specify, and the larger they are,
the more low memory is needed by PMODE/W during execution.
Another group of variables that has meaning only under clean/XMS/VCPI
execution specify the number of selectors and DPMI callbacks you want
PMODE/W to make available. The more selectors and callbacks you ask for,
the more low memory is used by PMODE/W, though the amount of low memory
used for each is quite low so that large numbers of each can be specified.
There will usually be a little less than the number of selectors and
callbacks you request available to your program due to the protected mode
system and Pascal code using some of them. For this reason you should
request 20h-40h more selectors and 2-4 more callbacks than you will need in
your program.
There are four other miscellaneous parameters that can be set. There is a
maximum number of page tables to use under a VCPI system. Each page table
allocated requires 4k of low memory to be used by PMODE/W and maps 4M of
memory. This directly affects the maximum amount of extended memory
available under a VCPI system. This parameter is only the maximum number of
page tables to allow. At run-time, only as many page tables will be
allocated as are needed. Under a clean/XMS system, no page tables are
required, so this parameter has no meaning. But under VCPI, you may want to
restrict the number of page tables to save low memory if you do not need
more than a certain amount of extended memory. This puts a maximum ceiling
on extended memory under VCPI which may be lower than the maximum actually
specified in the other variable. The second parameter specifies the order
of DPMI and VCPI detection. By default, VCPI will be checked before DPMI,
but you may set DPMI to be checked before VCPI so that under a system which
supports both VCPI and DPMI, DPMI will be used. The third variable
specifies how many pages to reserve for physical address mapping calls (INT
31h function 0800h) under VCPI. Under XMS or a clean system, paging is not
enabled, and PMODE/W does not need pages for physical address mapping. Each
page will allow you to map up to 4M of address space and takes up 4k of
extended memory. So for example, if you intend to map a 2M frame buffer of
a video card, you will need only one page. You may set this parameter to
zero if you do not intend to map any physical addresses. The fourth
parameter specifies whether PMODE/W displays its banner at startup. This
may be desirable to indicate that the program is indeed running, and has
not crashed, as allocating memory from certain VCPI servers can be a slow
process.
PMODE/W MSDOS Extender