GPOS- GP Opertaing System
RTOS -Real Time Operating System

An RTOS is a special case of GPOS. 
1. In RTOS every task must have a priority.
2. Pre-emption is a must feature.
3. Priority Inheritance is must otherwise priority 
inversion may cause unbounded wait.

The standard UNIX kernel is a nonpreemptive kernel; it does
not allow a user process to preempt a process executing in
kernel mode. Once a running process issues a system call and
enters kernel mode, preemptive context switches are disabled
until the system call is completed. Although there are
context switches, a system call may take an arbitrarily long
time to execute without voluntarily giving up the processor.
During that time, the process that made the system call may
delay the execution of a higher-priority, runnable, realtime
process.

The maximum process preemption latency for a nonpreemptive
kernel is the maximum amount of time it can take for the
running, kernel-mode process to switch out of kernel mode
back into user mode and then be preempted by a
higher-priority process. Under these conditions it is not
unusual for worst-case preemption to take seconds, which is
clearly unacceptable for many realtime applications.

This might be because, you might be allocating memory
dynamically from the heap. But care is not taken to
deallocate the same.
as a result, there are memory leaks and finally the os says
that there is no free memory

The problem is because of memory fragmentation..

Though 10KB of memory is available but it will be in 
smaller chunks (less than 5KB).. when we request for 5KB 
malloc returns NULL as it coudnt find a chunk of size 5KB...

1.The operation must be atomic
2.The atomicity is ensured by disabling the interrupts and 
immediately after crictical section enabling the 
interrupts..here slight precausion has to be taken i.e do 
not forget the enabling of interrupts ....

Adv:
1) Supports various types of processor platforms
2) Possible to Customize the Kernel and can reduce its size
(upto ~300KB kernel size)
3) Especially for the embedded device platforms

Disadv:
1) Process limits, max. 32 process at a time in WinCE 5.0
2) Limited space allocation for each processes, 32MB/each
process

I am not sure in every cases, whether this is true..??
May be the reason is,
While compared to PocketPc, smartphone has a limited h/w
resources, like memories etc. In order to reduce the
kernel+application size, they are removing the libraries for
the MFC .

 If anyone know a good reason for this, please post it

Well there is nothing to related with limited H/w I believe,
Because MFC is just a collection classes designed using the
Windows APIs , to provide ease of use. 
So the proper answer may Be...
To design any application on mobile.. We have very limited
APIs which user can directly used. Generally in Embedded
system (even in Soft RTOS also ) , if application design
using  less ( comfort )layer like MFC ( and other if it is
there :) ),then your aplication would be more efficient.
That's why even today's era... The application written in
Assembly is the most efficient application then others.

I still have more points to discuss..

there are four levels in Win CE architecture.
They are 
1)Hardware layer
2)OEM layer
3)OS layer
4)Application layer

for more info refer to
http://msdn2.microsoft.com/en-us/library/ms905093.aspx

MSFT standard SDK's are,

1. Windows Mobile for Smartphone = Windows Mobile Standard.
2. Windows Mobile for Pocket PC = Windows Mobile Classic.
3. Windows Mobile for Pocket PC Phone Edition = Windows 
Mobile Professional.

normal os                       Wince
unlimited process               32 process
non preemptive                  preemptive
no XIP                           XIP

Normal OS(Desktop OS like XP)     Windows CE
-----------------------------     -----------
Not a Realtime OS                 Real TimeOS

non preemptive                    Preemptive

suitable for Desktop              suitable for small  
systems                           gadgets with like Portable
                       .                                
media players,Mobile phones 

Large memory footprint            Small memory foot print.

Normal OS                   wince 
drivers runs under          User space(part of device manager)
kernel space

cannot be customized       customized for embedded application
for embedded application   like mobile phones,thin client...
(exception linux)

kernel - monolithic        kernel- micro kernel

In the name "Windows CE," the letters "CE" are not an 
abbreviation for anything, but rather they imply a number 
of the precepts around which Windows CE is designed, 
including "Compact," Connectable," 
Compatible," "Companion," and "Efficient."

Its stands for "Compact Edition"

consumer electronics

CE -> Compact Edition

It is surely meant for "Consumer Electronics"

It's Compat Edition. I have read in many books... [ :) ]

Windows CE (also known officially as Windows Embedded 
Compact post version 6.0 [2][3], and sometimes abbreviated 
WinCE) is Microsoft's operating system for minimalistic 
computers and embedded systems. Windows CE is a distinctly 
different operating system and kernel, rather than a 
trimmed-down version of desktop Windows. It is not to be 
confused with Windows XP Embedded which is NT-based

(Refer http://en.wikipedia.org/wiki/Windows_CE)

- Supports various processor x86, MIPS, SH, ARM
- Real TimeOS
- Preemptive
- Max 32 process supported on or previous version of wince 
6.x
- XIP
- Small memory foot print.
- Suitable for small gadgets with like Portable media 
players, Mobile phones

It is software which talks to the device

A device driver is a code that performs device control
operations specific to the device being addressed.
It is software layer that lies between applications and the
actual device.

or a more formal definition would be
Device drivers are distinct black boxes that make a
particular piece of hardware respond to a well defined
internal programming interface.

How a vehicle needs a driver to make the vehicle run 
properly, similarly for an embedded systems a device driver 
will make it run or work according to the user events or 
timer events or through some h/w interrupts. 

Its a software abstraction layer which will sit just above 
the h/w. So an application can be written using the device 
driver API calls. So depending on the API call being called 
from application, the device which has been programmed to 
do a specific task will be performed.

Device Driver lies between the Hardware & operating system.
Through this layer only all the request/response between the
OS & H/w are happening,

Device Driver is a software abstraction to the under lying
hardware, which allows an application programmer to access
the  device with out knowing the complexities of the hardware. 

In windows CE a device driver is a part of BSP. 

There are two types of drivers in Windows CE. 

    1. Native Device Drivers (Loaded and managed by GWES.exe)
    2. Stream Device Drivers (Loaded and managed by the
device   
       manager), which exposes a set of functions called
       stream device interface. The interfaces gives a file 
       like view for the device.

a device driver is typical for each kind of device. you may
find many diver softwares for an audio device. in linux a DD
is part of kernel. Driver is the software that makes a real
hardware device(chip/ ic) work. Its job is to configure the
device , create special files for that , read/write to the
device.