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Showing posts from February, 2009

Runtime Error (Exception) Handling

ADA handles programmer defined exceptions and system defined exceptionsSome of the system defined exceptions are like  NUMERIC_ERROR - raised whenever the abnormal precision in the program like divide by zero STORAGE-ERROR - raised whenever the program runs out of memory spaceCONSTRAINT_ERROR - asserts when a variable goes out of its boundTASKING_ERROR - raised during the incorrect use of tasksPROGRAM_ERROR- raised whenever an exception is not captured by any other conditionsExample on Exceptions
//Program to create two programmer defined exceptions
declare  P,PRESSURE:float;
begin   loop     P:=READ_PRESSURE(PRESSURE);     if P<100> raise LOW_PRESSURE;
    elseif P>150 then  raise HIGH_PRESSURE;
end if;
end loop;
exception  when LOW_PRESSURE => put("Warning: Very Low Pressure");
when HIGH_PRESSURE => put ("Warning: High Pressure");

When exceptions are raised in multiple procedures or block and Procedures, handling the exceptions…

Packages in ADA

Benefits of PackagesPackages can be compiled, debugged independentlysoftware maintenance is very easierOnce package is developed, they can be made accessible to everyone who has access to it.packages provide more security                                                                                                            Packages contains two parts 1) a specification and 2) a body Specification interfaces to the outside world. body contains the executable statements associated with the functions
Examples of packages includ, COMPLEX NUMBERS Calculations, Creation of TIMER and CLOCK using packages and made as a library
Example //Package specification describes the functions and variables relevant to complex number and their addition and subtraction operations
package COMPLEX_NUMBER is type COMPLEX is  record  real, imag:float; end record; function ADD(a,b:COMPLEX) return COMPLEX; function SUB(a,b:COMPLEX) return COMPLEX; end COMPLEX_NUMBER;
//Paackage body will implement the function specified …

Blocks and Procedures in ADA

BlocksA Block in ADA contains two parts specification and a body. The specification specifies the variable used inside the block and the body executes the statement for the block
syntax block begin --- --- end
The main purpose of blocks are information hiding.
A Variable declared inside a block is accessible only within the block and nowhere outside the block.Example var x,y:integer begin    y:=0;  for x in 1..10 loop   block    var x:integer;     begin      for x in 1..5 loop     y:=y+x;    end loop; end; end loop;
(NB: the above example shows two variables x within the block and outside the block, The x inside the block is accessible only within that block)
Procedures and Functions Blocks are repeated whenever they are needed, but functions can be written once, called anytimesExamples //to define a function to show the smaller among two integers function SMALLER (A,B: integer) return integer is  begin if A < B then
return A; else
return B;
end if;
//function calling SMALLER D:=C+SMALLER(x,y);

ADA - Data types

Simple Data types
//creates three variables for TEMPERATURE which is of type float
type TEMPERATURE is new float;ta, tb, tc: TEMPERATURE;

//creates three variables for PRESSURE which is of type float
type PRESSURE is new float;pa, pb, pc: PRESSURE;
ta=pa is illegal as it is implicit type conversion
ta= TEMPERATURE(pa);is legal

//creates range between o to 10000
type HEIGHT is new float range 0..100000;

Sub types
type TIME is new float range 0..23.59;subtype MORNING is TIME range 0..11.59;subtype NOON is TIME range 12..16.00;subtype EVENING is TIME range 16.01..18.59

MONTH is (jan,feb,mar,apr,may,jun,jul,aug,sep,oct,nov,dec);

floating point

type var1 is digits 8 range -1e20..1e20;
type var2 is digits 12 range -1e20..1e20;

fixed point

type var3 is delta 0.001 range 0.000..0.999;

//SMALLINT is a array variable can have the range between 0 and 20, 10 such variables are declared

type SMALLINT is integer range 0..20;
type VAR4 is array(1..10) of SMALLINT;

ADA - programming language Features

Object oriented programming Strong typingGeneric programming/templates Exception handling Facilities for modular organization of code Standard libraries for I/O, string handling, numeric computing Systems programming Concurrent programming Real-time programming
Helps for periodic and event driven tasksAvoids unbounded priority inversionEfficient implementation of Priority ceiling in ADA
Distributed systems programming Interfaces to other languages (C, COBOL, Fortran)

Strongly Typed Language

What is a strongly typed language?
Each variable must be declared with its own data typeeach data type should be associated with a value and operation (which operates on those values)implicit type conversions are not allowedexplicit type conversions are legal.
for example, if i is integer and a is float,
then i=a; says it is implicit which is illegal, if i=(integer)a; then it is perfectly legal.

Myopic Offline Scheduling Algorithm

Myopic Offline Scheduling is a multiprocessor Scheduling Algorithm
This algorithm is meant for non preemptive tasksSince it is an offline scheduling algorithm, release time, deadline and execution time are known apriori.It proceeds like building up a schedule treeEvery level should be checked for feasibility. If not strongly feasible, then backtrack. If strongly feasible, then proceeds the next level until all the tasks are scheduled. Heuristics are based on
Deadline (Di)
Earliest Start Time Laxity (Di-ei)weighted sum of any of the aboveFor example if the heuristics H(i) = Di, then the schedule is carried out based on the tasks with earlier deadline

Example - Multiprocessor Scheduling with precedence

The feasible interval (release Time and Deadline) of each task in the precedence graph is given next to its name. The execution time for T1,T2....T7 is respectively 2, 2, 3, 1, 1, 1, 3. The priority is in the following order, T1>T2>T3...>T7 ( T1 being the highest priority and T7 being the Lowest Priority) a) What is the minimum number of processors required for feasibility?
Single Processor

At time 0, T2 and T6 are ready, Since T2 is an independant task it can scheduled.
At time 1, T2 is running and T3,T5 and T6 are ready (but all are dependant tasks) At time 2, T2 completes, T1, T3,T5 and T6 are ready (T1 is starting because of high priority) At time 3, T1 is running, At time 4, T1 completes, T3, T4, T5 and T6 are ready.. Here T3 is ready to execute because of high priority and as well it satisfies the precedence constraint. At time 6, T7 is also ready along with T4, T5 and T6.
Except T5 all the tasks meets their deadlines. So, having one processor is not a feasible schedule..

Examples - Processor Utilisation

A system with three periodic tasks with period and its execution time T1 (3, 1) T2 (5, 2) and T3 (7, x). What is the value of x if the system uses the processor for 95% of the time?Processor Utilisation Factor is given by
so, 0.95 = 1/3+2/5+x/7 x=1.54

Example - IRIS1- Identcal Linear Reward Function

For More information about this post, Refer the Book, Real Time Systems by Krishna and Shin, McGraw Hill.

The mandatory portion of the task T4 is not meeting its deadline and it is just adjusted from the Task T2 (the mandatory portion of the task T2 is meeting its deadline, but the reward is less)

This kind of algorithm says, atleast the mandatory portion of the tasks should meet their deadlines.

IRIS Tasks (Increased Reward with Increased Service)

IRIS means Increased Reward with Increased Service,
If a task runs to its completion, it get full reward. if not run to its completion then it gets zero reward. 
So, Each IRIS tasks has two portions a Mandatory part and an Optional Part. 
If a tasks completes atleast its mandatory part, then it gets a little reward, but if a tasks does not complete even the mandatory part, then it will get zero reward from it.
so the reward function is given below, mathematically,
Where Ri(x) is the reward function, oi being the optional part and mi being the mandatory part.... 
Each task has Mi={M0.....Mn} Mandatory portions and Oi={O0...On} Optional portions so, the execution time of the task is the sum of mandatory portion and optional portion. ei=mi+oi
The schedule is like this, Step1; First try to schedule the task set using EDF as a whole with execution time, deadline and release time, if the task set is schedulable, the that is the feasible schedule.STOP. If not feasible, go to Step2; Step2: Schdeule the …

Shift Register

Requirements:WARP Active HDL Program for Shift Register:
library IEEE;
use IEEE.std_logic_1164.all;
entity shiftreg is
port (
load: in STD_LOGIC;
sp: in STD_LOGIC;
clk: in STD_LOGIC;
si: in STD_LOGIC;
pi: in STD_LOGIC_VECTOR (3 downto 0);
s0: out STD_LOGIC;
p0: out STD_LOGIC_VECTOR (3 downto 0)
end shiftreg;
architecture shiftreg of shiftreg is
signal storage:std_logic_vector(3 downto 0) :="XXXX";
variable i:integer := 1;
if(load = '1')then
case sp is
when '1' =>
storage <= pi;
when '0' =>
if i<5 then
storage(i-1) <= si;
i :=i+1;
end if;
when others => null;
end case;
case sp is
when '1' =>
when '0' =>
if i<5 then
s0 &l…