Hardware Reference
In-Depth Information
as
protected mode
, which alludes to the fact that software programs running in that mode
are protected from overwriting one another in memory. Such protection makes the system
much more crash-proof because an errant program can't easily damage other programs or
theOS.Inaddition,acrashedprogramcanbeterminatedwhiletherestofthesystemcon-
tinues to run unaffected.
Knowing that new OSs and applications—which take advantage of the 32-bit protected
mode—would take some time to develop, Intel wisely built a backward-compatible real
mode into the 386. That enabled it to run unmodified 16-bit OSs and applications. It ran
them quite well—much more quickly than any previous chip. For most people, that was
enough.Theydidnotnecessarilywantnew32-bitsoftware;theyjustwantedtheirexisting
16-bitsoftwaretorunmorequickly.Unfortunately,thatmeant thechipwasneverrunning
in the 32-bit protected mode, and all the features of that capability were being ignored.
When a 386 or later processor is running DOS (real mode), it acts like a “Turbo 8088,”
which means the processor has the advantage of speed in running any 16-bit programs; it
otherwise can use only the 16-bit instructions and access memory within the same 1MB
memory map of the original 8088. Therefore, if you have a system with a current 32-bit
or 64-bit processor running Windows 3.x or DOS, you are effectively using only the first
megabyte of memory, leaving all the other RAM largely unused!
New OSs and applications that ran in the 32-bit protected mode of the modern processors
wereneeded.Beingstubborn,weasusersresistedalltheinitialattemptsatbeingswitched
over to a 32-bit environment. People are resistant to change and are sometimes more con-
tent with running older software more quickly than with adopting new software with new
features. I'll be the first one to admit that I was (and still am) one of those stubborn users
myself!
Because of this resistance, true 32-bit OSs took quite a while before getting a mainstream
share in the PC marketplace. Windows XP was the first true 32-bit OS that became a true
mainstream product, and that is primarily because Microsoft coerced us in that direction
with Windows 9x/Me (which are mixed 16-bit/32-bit systems). Windows 3.x was the last
16-bit OS, which some did not really consider a complete OS because it ran on top of
DOS.
IA-32 Virtual Real Mode
The key to the backward compatibility of the Windows 32-bit environment is the third
mode in the processor: virtual real mode.
Virtual real
is essentially a virtual real mode
16-bit environment that runs inside 32-bit protected mode. When you run a DOS prompt
window inside Windows, you have created a virtual real mode session. Because protected
modeenablestruemultitasking,youcanactuallyhaveseveralrealmodesessionsrunning,
