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CUSTOMER_CODE
SMUDE
DIVISION_CODE
SMUDE
EVENT_CODE
APR2016
ASSESSMENT_CODE MIT4023_APR2016
QUESTION_TYPE
DESCRIPTIVE_QUESTION
QUESTION_ID
14321
QUESTION_TEXT
Explain the 2 types of Real Time Operating Systems.
SCHEME OF EVALUATION
1.Centralized system (5 marks)
2.Hierarchical system (5 marks)
QUESTION_TYPE
DESCRIPTIVE_QUESTION
QUESTION_ID
14324
QUESTION_TEXT
Explain on the limitations of Multitasking.
SCHEME OF
EVALUATION
1.A co–routine is executed after an explicit call from another co routine.
Therefore a multitasking solution based on using co routines relies on
cooperation between tasks. Given that in many cases tasks within an
operating system are logically independent then this is not a satisfactory
solution.
2.The problem of organizing the calls between co–routines so that all
co–routines have a fair share of the processor would be very complex
and prone to error. Indeed in many cases tasks may not get a share of the
processor at all.
3.Co routines do not contain any scheduler. This makes it difficult to
determine which tasks out of those available are actually ready to run.
4.Basic memory management. (10 marks)
QUESTION_TYPE
DESCRIPTIVE_QUESTION
QUESTION_ID
14325
QUESTION_TEXT
Discuss on various Unit Level Testing Methods.
SCHEME OF
EVALUATION
1. Unit Level Testing: Several methods can be used to test individual
modules or units. These techniques can be used by the unit author and by
the independent test team to exercise each unit in the system.
[1] Black Box Testing: In Black Box Testing, only inputs and outputs of
the unit are considered; how the outputs are generated based on a
particular set of inputs are ignored. Such a technique, being independent
of the implementation of the module, can be applied to any number of
modules with the same functionality. Some wifely used Black Box
Testing techniques include the following: (3 marks)
2. [ a ] Exhaustive Testing: Brute-force or exhaustive testing involves
each code unit with every possible input combination. Brute-force
testing can work well in the case of a small number of inputs, each with
a limited input range. A major problem with Brute-force testing is the
combinational explosion in the number of test cases. (1 mark)
3. [ b ] Boundary-Value Testing: Boundary-value or corner-case testing
solves the problem of combinational explosion by testing some very tiny
subset of the input combinations identified as meaningful “boundaries”
of input. For example, consider a code unit with different inputs, each of
which is a 16-bit signed number. Approaching the testing of this code
unit using exhaustive testing would require 216 • 216 • 216 • 216 • 216 =
280 test cases. (1 mark)
4. [ c ] Random Test-Case Generation: Random Test-Case Generation,
or statistically based testing, can be used for both unit- and system-level
testing. This kind of testing involves subjecting the code unit to many
randomly generated test cases over some period of time. The purpose of
this approach is to simulate execution of the software under realistic
conditions. Randomly generated test cases are based on determining the
underlying statistics of the expected inputs. The major drawback of such
a technique is that the underlying probability distribution functions for
the input variables may be unavailable or incorrect. (1 mark)
5. [ d ] Worst-Case Testing: Worst-Case or Pathological-Case testing
deals with those test scenarios that might be considered highly unusual
and unlikely. It is often the case that these exceptional cases are exactly
those for which the code is likely to be poorly designed, and therefore, to
fail. For example, the inertial measurement system, while it might be
highly unlikely that the system will achieve the maximum accelerations
that can be represented in a 16-bit scaled number, this worst case still
needs to be tested. (1 mark)
6. [2] White Box Testing: One disadvantage of Black-Box Testing is
that it can often bypass unreachable or dead code. In addition, it may not
test all of the control paths in the module. Black Box Testing only tests
what is expected to happen, not what was intended. White-Box Tests are
data driven, white-box tests are logic driven, that is they are designed to
exercise all paths in the code unit. White-box testing also has the
advantage that it can discover those code paths that cannot be executed.
This unreachable code is undesirable because it is likely a sign that the
logic is incorrect, because it wastes code space memory, and because it
might inadvertently be executed in the case of the corruption of the
computer’s program counter. (3 marks)
QUESTION_TYPE
DESCRIPTIVE_QUESTION
QUESTION_ID
126048
QUESTION_TEXT
What are the basic requirements of RTOS? Explain.
1.
2.
SCHEME OF
EVALUATION
3.
4.
a.
b.
c.
Multi-tasking and pre-emptable: To support multiple tasks in real–time
applications, an RTOS must be multi-tasking and pre-emptible. The scheduler
should be able to preempt any task in the system and give the resource to the task
that needs it most. An RTOS should also handle multiple levels of interrupts to
handle multiple priority levels.
Dynamic deadline identification: In order to achieve preemption, an RTOS should
be able to dynamically identify the task with earliest deadline. To handle
deadlines, deadline information may be converted to priority levels that are used
for resource allocation. Although such an approach is error prone, nonetheless it is
employed for lack of a better solution.
Predictable synchronization: For multiple threads to communicate among
themselves in a timely fashion, predictable inter-task communication and
synchronization mechanisms are required. Semantic integrity as well as timeliness
constitutes predictability.
Predefined latencies: The timing of system calls must be defined using the
following specifications:
Task switching latency or the time to save the context of a currently executing
task and switch to another.
Interrupt latency or the tome elapsed between the execution of the last instruction
of the Interrupted task and the first instruction of the interrupt handler.
Interrupt dispatch latency or the time to switch from the last instruction in the
interrupt handler to the next task scheduled to run.
(2.5 marks each=10 marks)
QUESTION_TYPE DESCRIPTIVE_QUESTION
QUESTION_ID
126050
QUESTION_TEXT What is Practical real-time operating system? Discuss Windows CE.
SCHEME OF
EVALUATION
Practical real time operating systems are software that run on common
processors and have sizable user bases. The common capabilities of
these Operating systems are: Efficiency, Non-pre-emptible sytem
calls, Prioritized scheduling, Priority inversion control, Memory
management support.
(2 marks)
Windows CE is a modular, portable real time embedded OS for small
memory, mobile 32-bit devices. Windows CE slices CPU time
among threads and provides to run in kernel mode.
(2 marks)
Windows CE kernel has the following features:*
While executing non-preemptive code in the kernel, translation
look-aside buffer misss are avoided by moving all kernel data into
physical memory.
*
Kcalls, all non-pre-emptible portions of the kernel, are broken into
small sections reducing the duration of non-pre-emptible code.
*
All kernel objects are dynamically allocated in virtual memory.
*
For portability, an equipment adaptation layer isolates device
dependent routines. The equipment manufacturer can specify trusted
modules and processes to prevent unauthorized applications from
accessing system application programming
interfaces.
(4x1.5=6 marks)
QUESTION_TYPE DESCRIPTIVE_QUESTION
QUESTION_ID
126052
QUESTION_TEXT Discuss on Interrupts.
1.
2.
SCHEME OF
EVALUATION
3.
4.
An interrupt is an asynchronous hardware-supported request for a
specific task activation caused by an event external to the currently
active computation. This definition of an interrupt does not include
an exception, i.e., a synchronous break in the control flow caused by
a condition within the task. In an Event Triggered system, the
significant external event triggers are often relayed to the computer
system by means of the interrupt mechanism. (2 marks)
Whenever the interrupt mechanism is enabled, and an interrupt
occurs, the execution of the current task is preempted, and a context
switch to the interrupt handler is enforced by the hardware. To
reduce the size of the hardware context, i.e. the voltage level of an
external signal line is only checked at well-defined points during a
hardware instruction, or at the end of the hardware instruction. After
the termination of the interrupt handler, another context switch is
initiated to either continue the preempted task, or to hand control
over to an interrupt service task. These context switches require a
worst-case administrative overhead, WACO. (3 marks)
Every interrupt reduces the CPU capacity that is available to the
application by an amount up to the size of the WCAO, as shown in
figure below.
(2 marks)
The administrative overhead required even if the interrupt handler
decides that the interrupt was erroneous, and no application task
needs to be activated. If the interrupt frequency reaches the value
1/WCAO, then, no CPU capacity may be left over for the application
tasks. It is therefore of paramount importance to limit the frequency
of interrupts, particularly if the interrupts could be erroneous. This
can be difficult, because the source of the interrupt is outside the
sphere of control, SOC, of the considered node. Hence, it cannot be
known a priori whether a pending interrupt is in error, or whether it
carries relevant event information that must not be lost. (3 marks)