Hardware Reference
In-Depth Information
eachwithitsownsoftwarerunningonavirtualPC.Thesecanallrunsimultaneously,even
while other 32-bit applications are running.
Note that any program running in a virtual real mode window can access up to only 1MB
of memory, which that program will believe is the first and only megabyte of memory in
the system. In other words, if you run a DOS application in a virtual real window, it will
have a 640KB limitation on memory usage. That is because there is only 1MB of total
RAM in a 16-bit environment, and the upper 384KB is reserved for system use. The vir-
tual real window fully emulates an 8088 environment, so that aside from speed, the soft-
ware runs as if it were on an original real mode-only PC. Each virtual machine gets its
own 1MB address space, an image of the real hardware basic input/output system (BIOS)
routines, and emulation of all other registers and features found in real mode.
Virtual real mode is used when you use a DOS window to run a DOS or Windows 3.x
16-bit program. When you start a DOS application, Windows creates a virtual DOS ma-
chine under which it can run.
One interesting thing to note is that all Intel and Intel-compatible (such as AMD and
VIA/Cyrix) processors power up in real mode. If you load a 32-bit OS, it automatically
switches the processor into 32-bit mode and takes control from there.
It's also important to note that some 16-bit (DOS and Windows 3.x) applications misbe-
haveina32-bitenvironment,whichmeanstheydothingsthatevenvirtualrealmodedoes
not support.
Diagnostics software is a perfect example of this. Such software does not run properly
in a real mode (virtual real) window under Windows. In that case, you can still run your
modernsystemintheoriginalno-frillsrealmodebybootingtoaDOSorWindows9x/Me
startup floppy.
Although 16-bit DOS and “standard” DOS applications use real mode, special programs
areavailablethat“extend”DOSandallowaccesstoextendedmemory(over1MB).These
are sometimes called
DOS extenders
and usually are included as part of any DOS or Win-
dows 3.x software that uses them. The protocol that describes how to make DOS work in
protected mode is called
DOS protected mode interface
(DPMI).
Windows 3.x used DPMI to access extended memory for use with Windows 3.x applica-
tions. It allowed these programs to use more memory even though they were still 16-bit
programs. DOSextenders are especially popular inDOSgames because they enable them
to access much more of the system memory than the standard 1MB that most real mode
programs can address. These DOS extenders work by switching the processor in and out
of real mode. In the case of those that run under Windows, they use the DPMI interface
built into Windows, enabling them to share a portion of the system's extended memory.
