Holy cow, I wrote a book!
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Sean wants to know what the role of MS-DOS was in Windows 95.
I may regret answering this question since it's
clear Slashdot bait.
(Even if Sean didn't intend it that way,
that's what it's going to turn into.)
Remember, what I write here may not be 100% true, but it is "true enough."
(In other words, it gets the point across without getting bogged down
in nitpicky details.)
MS-DOS served two purposes in Windows 95.
When Windows 95 started up, a customized version of
MS-DOS was loaded, and it's that customized version that
processed your CONFIG.SYS file,
which ran your AUTOEXEC.BAT and which
eventually ran WIN.COM, which began the
process of booting up the VMM, or the 32-bit virtual machine manager.
The customized version of MS-DOS was fully functional as far
as the phrase "fully functional" can be applied to MS-DOS in the
It had to be, since it was all that was running
when you ran Windows 95 in "single MS-DOS application mode."
The WIN.COM program started booting what most people
think of as "Windows" proper.
It used the copy of MS-DOS to load the virtual machine manager,
read the SYSTEM.INI file,
load the virtual device drivers,
and then it turned off any running copy of EMM386
and switched into protected mode.
It's protected mode that is what most people think of as "the real Windows."
Once in protected mode, the virtual device drivers did their magic.
Among other things those drivers did was
"suck the brains out of MS-DOS,"
transfer all that state to the 32-bit file system manager,
and then shut off MS-DOS.
All future file system operations would get routed to the
32-bit file system manager.
If a program issued an int 21h,
the 32-bit file system manager would be responsible for handling it.
And that's where the second role of MS-DOS comes into play.
For you see, MS-DOS programs and device drivers
loved to mess with the operating system itself.
They would replace the int 21h service vector,
they would patch the operating system,
they would patch the low-level disk I/O services
int 25h and int 26h.
They would also do crazy things to the BIOS interrupts such
as int 13h, the low-level disk I/O interrupt.
When a program issued an int 21h call to access MS-DOS,
the call would go first to the 32-bit file system manager,
who would do some preliminary munging and then, if it detected
that somebody had hooked the int 21h vector,
it would jump back into the 16-bit code to let the hook run.
Replacing the int 21h service vector is logically
analogous to subclassing a window.
You get the old vector and set your new vector.
When your replacement handler is called, you do some stuff,
and then call the original vector to do "whatever would normally happen."
After the original vector returned, you might do some more work
before returning to the original caller.
One of the 16-bit drivers loaded by CONFIG.SYS was called
The job of this 16-bit driver was to hook MS-DOS first
before the other drivers and programs got a chance!
This driver was in cahoots with the 32-bit file system manager,
for its job was to jump from 16-bit code back into 32-bit code
to let the 32-bit file system manager continue its work.
In other words, MS-DOS was just an extremely elaborate
Any 16-bit drivers and programs would patch or hook what they thought
was the real MS-DOS, but which was in reality just a decoy.
If the 32-bit file system manager detected that somebody bought the decoy,
it told the decoy to quack.
Let's start with a system that didn't contain any "evil" drivers
or programs that patched or hooked MS-DOS.
This was paradise.
The 32-bit file system manager was able to do all the work
without having to deal with pesky drivers that did bizarro things.
Note the extra step of updating the state variables inside MS-DOS.
Even though we extracted the state variables from MS-DOS during
the boot process, we keep those state variables in sync because
drivers and programs frequently "knew" how those state variables
worked and bypassed the operating system and accessed them directly.
Therefore, the file system manager had to maintain the charade that
MS-DOS was running the show (even though it wasn't) so that those drivers and
programs saw what they wanted.
Note also that those state variables were per-VM.
(I.e., each MS-DOS "box" you opened got its own copy of those state
each MS-DOS box had its idea of what the current directory was,
what was in the file tables, that sort of thing.
This was all an act, however, because the real list of open files was kept in
by the 32-bit file system manager.
It had to be, because disk caches had to be kept coherent,
and file sharing need to be enforced globally.
If one MS-DOS box opened a file for exclusive access,
then an attempt by a program running in another MS-DOS box to open the file
should fail with a sharing violation.
Okay, that was the easy case.
The hard case is if you had a driver that hooked
I don't know what the driver does, let's say that
it's a network driver that intercepts I/O to network drives
and handles them in some special way.
Let's suppose also that there's some TSR running in the MS-DOS box
which has hooked int 21h so it can
print a 1 to the screen when the int 21h is active
and a 2 when the int 21h completes.
Let's follow a call to a local device (not a network device,
so the network driver doesn't do anything):
Notice that all the work is still being done by the 32-bit
file system manager.
It's just that the call gets routed through all the 16-bit
stuff to maintain the charade that 16-bit MS-DOS is still
running the show.
The only 16-bit code that actually ran (in red)
is the stuff that the TSR and network driver installed,
plus a tiny bit of glue in the 16-bit IFSMGR hook.
Notice that no 16-bit MS-DOS code ran.
The 32-bit file manager took over for MS-DOS.
A similar sort of "take over but let the crazy stuff happen
if somebody is doing crazy stuff" dance took place when the
I/O subsystem took over control of your hard drive from
16-bit device drivers.
If it recognized the drivers, it would "suck their brains out"
and take over all the operations, in the same way that
the 32-bit file system manager took over operations from
On the other hand, if the driver wasn't one that the I/O subsystem
it let the driver be the one in charge of the drive.
If this happened, it was said that you were going through the
"real-mode mapper" since "real mode" was name for the CPU mode
when protected mode was not running;
in other words, the mapper was letting the 16-bit drivers do the work.
Now, if you were unlucky enough to be using the real-mode mapper,
you probably noticed that system performance to that drive was
That's because you were using the old clunky single-threaded 16-bit drivers
instead of the faster, multithread-enabled 32-bit drivers.
(When a 16-bit driver was running, no other I/O could happen because
16-bit drivers were not designed for multi-threading.)
This awfulness of the real-mode mapper actually came in handy
in a backwards way,
because it was an early indication that your computer got infected
with an MS-DOS virus.
After all, MS-DOS viruses did what TSRs and drivers did:
They hooked interrupt vectors and took over control of your hard drive.
From the I/O subsystem's point of view,
they looked just like a 16-bit hard disk device driver!
When people complained,
"Windows suddenly started running really slow,"
we asked them to look at the system performance page in the
control panel and see if it says that
"Some drives are using MS-DOS compatiblity."
If so, then it meant that the real-mode mapper was in charge,
and if you didn't change hardware, it probably means a virus.
Now, there are parts of MS-DOS that are unrelated to file I/O.
For example, there are functions for allocating memory,
parsing a string containing potential wildcards into FCB format,
that sort of thing.
Those functions were still handled by MS-DOS since they were
just "helper library" type functions and there was no
benefit to reimplementing them in 32-bit code aside from just
being able to say that you did it.
The old 16-bit code worked just fine, and if you let it do the
work, you preserved compatibility with programs that patched MS-DOS
in order to alter the behavior of those functions.