Difference between Verilog and vhdl?

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.

structure of Verilog code you follow?

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

how blocking and non blocking statements get executed?

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.

What is meant by inferring latches,how to avoid it?

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.

What is difference between Verilog full case and parallel case?

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."

Difference between $monitor,$display & $strobe?

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, ... );

Difference between inter statement and intra statement delay?

//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