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There are mainly two types of simulators AVAILABLE. 

• Event Driven 
• Cycle Based 

Event-based Simulator: 

This Digital Logic Simulation method sacrifices performance for rich functionality: every active signal is calculated for every device it propagates through during a clock cycle. Full Event-based simulators support 4-28 states; simulation of Behavioral HDL, RTL HDL, gate, and transistor representations; full TIMING calculations for all devices; and the full HDL standard. Event-based simulators are like a Swiss Army knife with many different features but none are particularly fast. 

Cycle Based Simulator: 

This is a Digital Logic Simulation method that eliminates unnecessary calculations to achieve huge performance gains in verifying Boolean logic: 

1. Results are only examined at the end of every clock cycle; and 
2. The digital logic is the only part of the DESIGN SIMULATED (no timing calculations). By limiting the calculations, Cycle based Simulators can provide huge increases in performance over conventional Event-based simulators. 

Cycle based simulators are more like a high speed electric carving knife in comparison because they focus on a subset of the biggest problem: logic verification. 

Cycle based simulators are almost invariably used along with Static Timing verifier to compensate for the lost timing information coverage. 

There are mainly two types of simulators available. 

Event-based Simulator: 

This Digital Logic Simulation method sacrifices performance for rich functionality: every active signal is calculated for every device it propagates through during a clock cycle. Full Event-based simulators support 4-28 states; simulation of Behavioral HDL, RTL HDL, gate, and transistor representations; full timing calculations for all devices; and the full HDL standard. Event-based simulators are like a Swiss Army knife with many different features but none are particularly fast. 

Cycle Based Simulator: 

This is a Digital Logic Simulation method that eliminates unnecessary calculations to achieve huge performance gains in verifying Boolean logic: 

Cycle based simulators are more like a high speed electric carving knife in comparison because they focus on a subset of the biggest problem: logic verification. 

Cycle based simulators are almost invariably used along with Static Timing verifier to compensate for the lost timing information coverage. 

This is a POPULAR CODING error. You used the bit WISE AND operator (&) where you meant to USE the logical AND operator (&&). 

This is a popular coding error. You used the bit wise AND operator (&) where you meant to use the logical AND operator (&&). 

wire [3:0] x;
always @(...) begin
CASE (1'b1)
x[0]: SOMETHING1;
x[1]: SOMETHING2;
x[2]: SOMETHING3;
x[3]: SOMETHING4;
endcase
end

The case STATEMENT walks down the list of cases and executes the first one that matches. So here, if the lowest 1-bit of x is bit 2, then something3 is the statement that will get executed (or selected by the LOGIC). 

wire [3:0] x;
always @(...) begin
case (1'b1)
x[0]: SOMETHING1;
x[1]: SOMETHING2;
x[2]: SOMETHING3;
x[3]: SOMETHING4;
endcase
end

The case statement walks down the list of cases and executes the first one that matches. So here, if the lowest 1-bit of x is bit 2, then something3 is the statement that will get executed (or selected by the logic). 

module bidirec (OE, clk, INP, outp, bidir);

// Port DECLARATION

input oe;
input clk;
input [7:0] inp;
output [7:0] outp;
INOUT [7:0] bidir; 
reg [7:0] a;
reg [7:0] B;
assign bidir = oe ? a : 8'bZ ;
assign outp = b;

// Always Construct

always @ (posedge clk)

begin
b <= bidir;
a <= inp;
end
endmodule

module bidirec (oe, clk, inp, outp, bidir);

// Port Declaration

input oe;
input clk;
input [7:0] inp;
output [7:0] outp;
inout [7:0] bidir; 
reg [7:0] a;
reg [7:0] b;
assign bidir = oe ? a : 8'bZ ;
assign outp = b;

// Always Construct

always @ (posedge clk)

begin
b <= bidir;
a <= inp;
end
endmodule

(wire,TRI)Physical connection between structural ELEMENTS. Value assigned by a CONTINUOUS assignment or a gate OUTPUT. REGISTER type: (reg, integer, time, real, real time) represents abstract data storage element. Assigned values only within an always statement or an initial statement. The main difference between wire and reg is wire cannot hold (store) the value when there no connection between a and b like a->b, if there is no connection in a and b, wire loose value. But reg can hold the value even if there in no connection. Default values:wire is Z,reg is x. 

(wire,tri)Physical connection between structural elements. Value assigned by a continuous assignment or a gate output. Register type: (reg, integer, time, real, real time) represents abstract data storage element. Assigned values only within an always statement or an initial statement. The main difference between wire and reg is wire cannot hold (store) the value when there no connection between a and b like a->b, if there is no connection in a and b, wire loose value. But reg can hold the value even if there in no connection. Default values:wire is Z,reg is x. 

The easiest and efficient WAY to generate SINE WAVE is using CORDIC ALGORITHM. 

The easiest and efficient way to generate sine wave is using CORDIC Algorithm. 

output of "==" can be 1, 0 or X.
output of "===" can only be 0 or 1.

When you are comparing 2 nos using "==" and if one/both the numbers have one or more bits as "x" then the output would be "X" . But if use "===" outpout would be 0 or 1.

e.g A = 3'b1x0
B = 3'b10x
A == B will give X as output.
A === B will give 0 as output.

"==" is used for comparison of only 1's and 0's .It can't compare XS. If any bit of the input is X output will be X

"===" is used for comparison of X also.

output of "==" can be 1, 0 or X.
output of "===" can only be 0 or 1.

When you are comparing 2 nos using "==" and if one/both the numbers have one or more bits as "x" then the output would be "X" . But if use "===" outpout would be 0 or 1.

e.g A = 3'b1x0
B = 3'b10x
A == B will give X as output.
A === B will give 0 as output.

"==" is used for comparison of only 1's and 0's .It can't compare Xs. If any bit of the input is X output will be X

"===" is used for comparison of X also.

'timescale directive is a compiler directive.It is used to MEASURE SIMULATION TIME or DELAY time. Usage :`timescale / reference_time_unit : Specifies the unit of measurement for times and delays. time_precision: specifies the PRECISION to which the delays are rounded off. 

'timescale directive is a compiler directive.It is used to measure simulation time or delay time. Usage :`timescale / reference_time_unit : Specifies the unit of measurement for times and delays. time_precision: specifies the precision to which the delays are rounded off. 

#5 a = B;
a = #5 b;
#5 a = b;

Wait FIVE time units before doing the ACTION for "a = b;". 

a = #5 b; The value of b is calculated and STORED in an internal temp REGISTER,After five time units, assign this stored value to a. 

#5 a = b;
a = #5 b;
#5 a = b;

Wait five time units before doing the action for "a = b;". 

a = #5 b; The value of b is calculated and stored in an internal temp register,After five time units, assign this stored value to a. 

always @(clk) begin
a = 0;
a <= 1;
$display(a);
end

This is a tricky ONE! VERILOG scheduling semantics basically imply a four-level deep queue for the current simulation time:

1. Active Events (blocking statements)
2. Inactive Events (#0 delays, ETC)
3. Non-Blocking Assign Updates (non-blocking statements)
4. Monitor Events ($display, $monitor, etc).

Since the "a = 0" is an active event, it is scheduled into the 1st "queue".

The "a <= 1" is a non-blocking event, so it's PLACED into the 3rd queue.

FINALLY, the display statement is placed into the 4th queue. Only events in the active queue are completed this sim cycle, so the "a = 0" happens, and then the display shows a = 0. If we were to look at the value of a in the next sim cycle, it would show 1. 

always @(clk) begin
a = 0;
a <= 1;
$display(a);
end

This is a tricky one! Verilog scheduling semantics basically imply a four-level deep queue for the current simulation time:

Since the "a = 0" is an active event, it is scheduled into the 1st "queue".

The "a <= 1" is a non-blocking event, so it's placed into the 3rd queue.

Finally, the display statement is placed into the 4th queue. Only events in the active queue are completed this sim cycle, so the "a = 0" happens, and then the display shows a = 0. If we were to look at the value of a in the next sim cycle, it would show 1. 

yes case can INFER priority register DEPENDING on CODING style
reg r; 
// Priority encoded mux, 
ALWAYS @ (a or b or C or select2) 
begin 
r = c; 
case (select2) 
2'b00: r = a; 
2'b01: r = b; 
endcase 
end 

yes case can infer priority register depending on coding style
reg r; 
// Priority encoded mux, 
always @ (a or b or c or select2) 
begin 
r = c; 
case (select2) 
2'b00: r = a; 
2'b01: r = b; 
endcase 
end 

$deposit(VARIABLE, value);

This system task SETS a Verilog register or NET to the specified value. variable is the register or net to be changed; value is the new value for the register or net. The value remains until there is a subsequent driver transaction or another $deposit task for the same register or net. This system task operates IDENTICALLY to the ModelSim force -deposit command.

The force command has -freeze, -drive, and -deposit options. When none of these is specified, then -freeze is assumed for unresolved signals and -drive is assumed for resolved signals. This is designed to provide compatibility with force files. But if you PREFER -freeze as the default for both resolved and unresolved signals. 

$deposit(variable, value);

This system task sets a Verilog register or net to the specified value. variable is the register or net to be changed; value is the new value for the register or net. The value remains until there is a subsequent driver transaction or another $deposit task for the same register or net. This system task operates identically to the ModelSim force -deposit command.

The force command has -freeze, -drive, and -deposit options. When none of these is specified, then -freeze is assumed for unresolved signals and -drive is assumed for resolved signals. This is designed to provide compatibility with force files. But if you prefer -freeze as the default for both resolved and unresolved signals. 

there r MAINLY 4 ways 2 WRITE fsm CODE

1. using 1 process where all input decoder, present state, and output decoder r combine in one process.
2. using 2 process where all comb ckt and sequential ckt separated in different process
3. using 2 process where input decoder and persent state r combine and output decoder seperated in other process
4. using 3 process where all three, input decoder, present state and output decoder r separated in 3 process. 

there r mainly 4 ways 2 write fsm code

Ants can move only along edges of triangle in either of direction, let’s SAY one is represented by 1 and another by 0, since there are 3 sides EIGHT COMBINATIONS are possible, when all ants are going in same direction they won’t collide that is 111 or 000 so PROBABILITY of not collision is 2/8=1/4 or collision probability is 6/8=3/4

Ants can move only along edges of triangle in either of direction, let’s say one is represented by 1 and another by 0, since there are 3 sides eight combinations are possible, when all ants are going in same direction they won’t collide that is 111 or 000 so probability of not collision is 2/8=1/4 or collision probability is 6/8=3/4

Programming Language Interface (PLI) of Verilog HDL is a mechanism to interface Verilog programs with programs written in C language. It also PROVIDES mechanism to access internal databases of the SIMULATOR from the C program. 

PLI is used for implementing SYSTEM calls which would have been hard to do OTHERWISE (or impossible) using Verilog syntax. Or, in other words, you can take advantage of both the paradigms - parallel and hardware related features of Verilog and sequential flow of C - using PLI. 

Programming Language Interface (PLI) of Verilog HDL is a mechanism to interface Verilog programs with programs written in C language. It also provides mechanism to access internal databases of the simulator from the C program. 

PLI is used for implementing system calls which would have been hard to do otherwise (or impossible) using Verilog syntax. Or, in other words, you can take advantage of both the paradigms - parallel and hardware related features of Verilog and sequential flow of C - using PLI. 

Synchronous reset, synchronous means clock dependent so reset MUST not be present in sensitivity DISK

eg: ALWAYS @ (POSEDGE clk )

begin if (reset)
. . . end

Asynchronous means clock INDEPENDENT so reset must be present in sensitivity list.

Eg: Always @(posedge clock or posedge reset)

begin
if (reset)
. . . end

Synchronous reset, synchronous means clock dependent so reset must not be present in sensitivity disk

eg: always @ (posedge clk )

begin if (reset)
. . . end

Asynchronous means clock independent so reset must be present in sensitivity list.

Eg: Always @(posedge clock or posedge reset)

begin
if (reset)
. . . end

In earlier version of Verilog ,we USE 'or' to specify more than one element in sensitivity list . In Verilog 2001, we can use comma as SHOWN in the example below.

// Verilog 2k example for usage of comma
always @ (i1,i2,I3,i4)

Verilog 2001 allows us to use star in sensitive list instead of listing all the variables in RHS of combo logics . This removes typo mistakes and thus avoids simulation and synthesis mismatches, Verilog 2001 allows port direction and data type in the port list of modules as shown in the example below

module memory (
input r,
input WR,
input [7:0] data_in,
input [3:0] addr,
output [7:0] data_out
);

In earlier version of Verilog ,we use 'or' to specify more than one element in sensitivity list . In Verilog 2001, we can use comma as shown in the example below.

// Verilog 2k example for usage of comma
always @ (i1,i2,i3,i4)

Verilog 2001 allows us to use star in sensitive list instead of listing all the variables in RHS of combo logics . This removes typo mistakes and thus avoids simulation and synthesis mismatches, Verilog 2001 allows port direction and data type in the port list of modules as shown in the example below

module memory (
input r,
input wr,
input [7:0] data_in,
input [3:0] addr,
output [7:0] data_out
);

$display, $displayb, $displayh, $displayo, $write, $writeb, $writeh, $writeo. 

The most useful of these is $display.This can be USED for displaying strings, expression or values of variables. 

Here are some examples of usage. 

$display("Hello oni");
--- output: Hello oni
$display($TIME) // current simulation time.
--- output: 460
counter = 4'b10;
$display(" The COUNT is %b", counter);
--- output: The count is 0010
$reset resets the simulation back to time 0; $stop halts the SIMULATOR and puts it in interactive mode where the user can enter commands; $finish exits the simulator back to the operating system

$display, $displayb, $displayh, $displayo, $write, $writeb, $writeh, $writeo. 

The most useful of these is $display.This can be used for displaying strings, expression or values of variables. 

Here are some examples of usage. 

$display("Hello oni");
--- output: Hello oni
$display($time) // current simulation time.
--- output: 460
counter = 4'b10;
$display(" The count is %b", counter);
--- output: The count is 0010
$reset resets the simulation back to time 0; $stop halts the simulator and puts it in interactive mode where the user can enter commands; $finish exits the simulator back to the operating system

Compilation

VHDL. Multiple design-units (entity/architecture pairs), that reside in the same system file, MAY be separately compiled if so desired. However, it is good design practice to keep each design unit in it's own system file in which case separate compilation should not be an issue. 

Verilog. The Verilog language is still ROOTED in it's native interpretative mode. Compilation is a means of speeding up simulation, but has not changed the original nature of the language. As a result care must be taken with both the compilation order of code written in a single file and the compilation order of multiple files. Simulation results can change by simply changing the order of compilation. 

Data types 

VHDL. A multitude of language or USER defined data types can be used. This may mean dedicated conversion FUNCTIONS are needed to CONVERT objects from one type to another. The choice of which data types to use should be considered wisely, especially enumerated (abstract) data types. This will make models easier to write, clearer to read and avoid unnecessary conversion functions that can clutter the code. VHDL may be preferred because it allows a multitude of language or user defined data types to be used. 

Verilog. Compared to VHDL, Verilog data types a re very simple, easy to use and very much geared towards modeling hardware structure as opposed to abstract hardware modeling. Unlike VHDL, all data types used in a Verilog model are defined by the Verilog language and not by the user. There are net data types, for example wire, and a register data type called reg. A model with a signal whose type is one of the net data types has a corresponding electrical wire in the implied modeled circuit. Objects, that is signals, of type reg hold their value over simulation delta cycles and should not be confused with the modeling of a hardware register. Verilog may be preferred because of it's simplicity. 

Design reusability 

VHDL. Procedures and functions may be placed in a package so that they are avail able to any design-unit that wishes to use them. 

Verilog. There is no concept of packages in Verilog. Functions and procedures used within a model must be defined in the module. To make functions and procedures generally accessible from different module statements the functions and procedures must be placed in a separate system file and included using the `include compiler directive. 

Compilation

VHDL. Multiple design-units (entity/architecture pairs), that reside in the same system file, may be separately compiled if so desired. However, it is good design practice to keep each design unit in it's own system file in which case separate compilation should not be an issue. 

Verilog. The Verilog language is still rooted in it's native interpretative mode. Compilation is a means of speeding up simulation, but has not changed the original nature of the language. As a result care must be taken with both the compilation order of code written in a single file and the compilation order of multiple files. Simulation results can change by simply changing the order of compilation. 

Data types 

VHDL. A multitude of language or user defined data types can be used. This may mean dedicated conversion functions are needed to convert objects from one type to another. The choice of which data types to use should be considered wisely, especially enumerated (abstract) data types. This will make models easier to write, clearer to read and avoid unnecessary conversion functions that can clutter the code. VHDL may be preferred because it allows a multitude of language or user defined data types to be used. 

Verilog. Compared to VHDL, Verilog data types a re very simple, easy to use and very much geared towards modeling hardware structure as opposed to abstract hardware modeling. Unlike VHDL, all data types used in a Verilog model are defined by the Verilog language and not by the user. There are net data types, for example wire, and a register data type called reg. A model with a signal whose type is one of the net data types has a corresponding electrical wire in the implied modeled circuit. Objects, that is signals, of type reg hold their value over simulation delta cycles and should not be confused with the modeling of a hardware register. Verilog may be preferred because of it's simplicity. 

Design reusability 

VHDL. Procedures and functions may be placed in a package so that they are avail able to any design-unit that wishes to use them. 

Verilog. There is no concept of packages in Verilog. Functions and procedures used within a model must be defined in the module. To make functions and procedures generally accessible from different module statements the functions and procedures must be placed in a separate system file and included using the `include compiler directive. 

A good template for your Verilog file is shown below. 

// timescale directive tells the simulator the base units and precision of the simulation 
`timescale 1 ns / 10 ps 
module name (input and OUTPUTS); 
// PARAMETER declarations 
parameter parameter_name = parameter VALUE; 
// Input output declarations 
input in1; 
input in2; // single bit inputs 
output [msb:lsb] out; // a bus output 
// internal signal register TYPE declaration - register types (only assigned within always statements). reg register
variable 1; 
reg [msb:lsb] register variable 2; 
// internal signal. net type declaration - (only assigned outside always statements) wire net variable 1; 
// hierarchy - instantiating another module 
reference name instance name ( 
.pin1 (net1), 
.pin2 (net2), 
. 
.pinn (netn) 
); 
// synchronous procedures 
always @ (posedge clock) 
begin 
. 
end 
// combinatinal procedures 
always @ (signal1 or signal2 or signal3) 
begin 
. 
end 
assign net variable = combinational LOGIC; 
endmodule 

A good template for your Verilog file is shown below. 

// timescale directive tells the simulator the base units and precision of the simulation 
`timescale 1 ns / 10 ps 
module name (input and outputs); 
// parameter declarations 
parameter parameter_name = parameter value; 
// Input output declarations 
input in1; 
input in2; // single bit inputs 
output [msb:lsb] out; // a bus output 
// internal signal register type declaration - register types (only assigned within always statements). reg register
variable 1; 
reg [msb:lsb] register variable 2; 
// internal signal. net type declaration - (only assigned outside always statements) wire net variable 1; 
// hierarchy - instantiating another module 
reference name instance name ( 
.pin1 (net1), 
.pin2 (net2), 
. 
.pinn (netn) 
); 
// synchronous procedures 
always @ (posedge clock) 
begin 
. 
end 
// combinatinal procedures 
always @ (signal1 or signal2 or signal3) 
begin 
. 
end 
assign net variable = combinational logic; 
endmodule 

Yes in a pure combinational circuit is it necessary to mention all the inputs in sensitivity DISK other WISE it will result in pre and POST SYNTHESIS mismatch. 

Yes in a pure combinational circuit is it necessary to mention all the inputs in sensitivity disk other wise it will result in pre and post synthesis mismatch. 

The sensitivity list indicates that when a change occurs to any ONE of elements in the list change, begin…end statement INSIDE that always BLOCK will GET executed.

The sensitivity list indicates that when a change occurs to any one of elements in the list change, begin…end statement inside that always block will get executed.

Signals

Signals

Execution of blocking assignments can be viewed as a one-step process:

1. Evaluate the RHS (right-hand side equation) and update the LHS (left-hand side expression) of the blocking assignment without interruption from any other Verilog statement. A blocking assignment "blocks" TRAILING assignments in the same always block from occurring until after the current assignment has been completed 

Execution of nonblocking assignments can be viewed as a two-step process: 

1. Evaluate the RHS of nonblocking statements at the beginning of the TIME step.
2. Update the LHS of nonblocking statements at the end of the time step. 

Execution of blocking assignments can be viewed as a one-step process:

1. Evaluate the RHS (right-hand side equation) and update the LHS (left-hand side expression) of the blocking assignment without interruption from any other Verilog statement. A blocking assignment "blocks" trailing assignments in the same always block from occurring until after the current assignment has been completed 

Execution of nonblocking assignments can be viewed as a two-step process: 

Consider the following : 

always @(s1 or s0 or i0 or i1 or i2 or i3)
CASE ({s1, s0}) 
2'd0 : out = i0;
2'd1 : out = i1;
2'd2 : out = i2;
endcase

in a case statement if all the possible combinations are not compared and default is also not SPECIFIED like in example above a latch will be inferred ,a latch is inferred because to reproduce the previous value when unknown branch is specified. 

For example in above case if {s1,s0}=3 , the previous stored value is reproduced for this storing a latch is inferred. 

The same MAY be observed in IF statement in case an ELSE IF is not specified. 

To avoid inferring latches make sure that all the cases are mentioned if not default condition is provided. 

Consider the following : 

always @(s1 or s0 or i0 or i1 or i2 or i3)
case ({s1, s0}) 
2'd0 : out = i0;
2'd1 : out = i1;
2'd2 : out = i2;
endcase

in a case statement if all the possible combinations are not compared and default is also not specified like in example above a latch will be inferred ,a latch is inferred because to reproduce the previous value when unknown branch is specified. 

For example in above case if {s1,s0}=3 , the previous stored value is reproduced for this storing a latch is inferred. 

The same may be observed in IF statement in case an ELSE IF is not specified. 

To avoid inferring latches make sure that all the cases are mentioned if not default condition is provided. 

A "full" CASE statement is a case statement in which all possible case-EXPRESSION BINARY patterns can be matched to a case item or to a case default. If a case statement does not include a case default and if it is possible to find a binary case expression that does not match any of the DEFINED case items, the case statement is not "full." 

A "parallel" case statement is a case statement in which it is only possible to match a case expression to one and only one case item. If it is possible to find a case expression that would match more than one case item, the MATCHING case items are called "overlapping" case items and the case statement is not "parallel." 

A "full" case statement is a case statement in which all possible case-expression binary patterns can be matched to a case item or to a case default. If a case statement does not include a case default and if it is possible to find a binary case expression that does not match any of the defined case items, the case statement is not "full." 

A "parallel" case statement is a case statement in which it is only possible to match a case expression to one and only one case item. If it is possible to find a case expression that would match more than one case item, the matching case items are called "overlapping" case items and the case statement is not "parallel." 

These commands have the same syntax, and display text on the screen during simulation. They are much less CONVENIENT than waveform display tools like cwaves?. $display and $strobe display once every time they are executed, whereas $monitor displays every time one of its parameters CHANGES. 

The difference between $display and $strobe is that $strobe displays the parameters at the very end of the current simulation time unit rather than exactly where it is executed. The format string is like that in C/C++, and may CONTAIN format characters. Format characters include %d (decimal), %h (hexadecimal), %b (binary), %c (CHARACTER), %s (string) and %t (time), %m (hierarchy level). %5d, %5b etc. would give exactly 5 spaces for the number instead of the space needed. Append b, h, o to the task name to change default format to binary, OCTAL or hexadecimal. 

Syntax:
$display (“format_string”, par_1, par_2, ... );
$strobe (“format_string”, par_1, par_2, ... );
$monitor (“format_string”, par_1, par_2, ... );

These commands have the same syntax, and display text on the screen during simulation. They are much less convenient than waveform display tools like cwaves?. $display and $strobe display once every time they are executed, whereas $monitor displays every time one of its parameters changes. 

The difference between $display and $strobe is that $strobe displays the parameters at the very end of the current simulation time unit rather than exactly where it is executed. The format string is like that in C/C++, and may contain format characters. Format characters include %d (decimal), %h (hexadecimal), %b (binary), %c (character), %s (string) and %t (time), %m (hierarchy level). %5d, %5b etc. would give exactly 5 spaces for the number instead of the space needed. Append b, h, o to the task name to change default format to binary, octal or hexadecimal. 

Syntax:
$display (“format_string”, par_1, par_2, ... );
$strobe (“format_string”, par_1, par_2, ... );
$monitor (“format_string”, par_1, par_2, ... );

//define register VARIABLES
reg a, B, c;
//intra assignment delays
initial
begin
a = 0; c = 0;
b = #5 a + c; //Take value of a and c at the time=0, evaluate
//a + c and then wait 5 time units to ASSIGN value
//to b.
end 

//Equivalent method with temporary variables and regular DELAY control
initial
begin
a = 0; c = 0;
temp_ac = a + c;
#5 b = temp_ac; //Take value of a + c at the current time and
//store it in a temporary variable. Even though a and c
//might change between 0 and 5,
//the value assigned to b at time 5 is unaffected.
end

//define register variables
reg a, b, c;
//intra assignment delays
initial
begin
a = 0; c = 0;
b = #5 a + c; //Take value of a and c at the time=0, evaluate
//a + c and then wait 5 time units to assign value
//to b.
end 

//Equivalent method with temporary variables and regular delay control
initial
begin
a = 0; c = 0;
temp_ac = a + c;
#5 b = temp_ac; //Take value of a + c at the current time and
//store it in a temporary variable. Even though a and c
//might change between 0 and 5,
//the value assigned to b at time 5 is unaffected.
end

Function: 

• A function is unable to enable a task however functions can enable other functions. 
• A function will carry out its required duty in zero simulation TIME. ( The program time will not be incremented during the function routine)
• Within a function, no event, delay or timing control statements are permitted 
• In the invocation of a function their must be at least one argument to be passed.
• Functions will only RETURN a single value and can not use either output or inout statements. 

Tasks: 

• Tasks are CAPABLE of enabling a function as well as enabling other versions of a Task 
• Tasks also run with a zero simulation however they can if required be EXECUTED in a non zero simulation time. 
• Tasks are allowed to contain any of these statements. 
• A task is allowed to use zero or more ARGUMENTS which are of type output, input or inout. 
• A Task is unable to return a value but has the facility to pass multiple values via the output and inout statements . 

Function: 

Tasks: 

With temp reg ;
ALWAYS @ (posedge clock)
begin 
temp=b;
b=a;
a=temp;
END

Without temp reg;
always @ (posedge clock)
begin 
a &LT;= b;
b <= a;
end 

With temp reg ;
always @ (posedge clock)
begin 
temp=b;
b=a;
a=temp;
end

Without temp reg;
always @ (posedge clock)
begin 
a <= b;
b <= a;
end