A one-dimensional array with elements of type reg is also called a memory. These memories can be used to model read-only memories (ROMs), random access memories (RAMs), and reg files. Each reg in the array is known as an element or word and is addressed by a single array index.

Array declarations examples:

Array declarations examples:
reg [7:0] mema[0:255]; // declares a memory mema of 256 8-bit // registers. The indices are 0 to 255 
integer inta[1:64]; // an array of 64 integer values

Assignment to above array elements-
mema = 0; // Illegal syntax- Attempt to write to entire array 
arrayb[1] = 0; // Illegal Syntax - Attempt to write to elements // [1][0]..[1][255] 
arrayb[1][12:31] = 0; // Illegal Syntax - Attempt to write to // elements [1][12]..[1][31] 
mema[1] = 0; // Assigns 0 to the second element of mema 
arrayb[1][0] = 0; // Assigns 0 to the bit referenced by indices // [1][0] 
inta[4] = 33559; // Assign decimal number to integer in array 
chng_hist[t_index] = $time; // Assign current simulation time to element addressed by integer index

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Verilog Introduction

Verilog is a language using which a designer can specify the behavior, or the functionality, or the structure of some given hardware; some specified hardware circuit. this language is also called Hardware Description Languages (HDL).
HDL’s allows the design to be simulated earlier in the design cycle in order to correct errors or experiment with different architectures. Designs described in HDL are technology-independent, easy to design and debug, and are usually more readable than schematics, particularly for large circuits.

Verilog can be used at several levels-

1) High level Behavioral

  • Behavioral models in Verilog contain procedural statements, which control the simulation and manipulate variables of the data types, these all statements are contained within the procedures. Each of the procedure has an activity flow associated with it.
  • Below is a Half Adder example-

//below is code for half adder            
module half_adder(a,b,sum,cout);
input a,b;
output sum,cout; // sum and carry

assign sum = a^b;
assign cout = a&b ;

2) Register transfer level

  • Designs using the Register-Transfer Level specify a circuit’s characteristics using operations and the transfer of data between the registers

3) Gate level
Designing circuits using basic logic gates is known as gate-level modeling.
It is a low-level abstraction that describes design in terms of gates.

Below is simple gate level design of a Half Adder

// code your half adder design             
module half_add(a,b,s,c); 
  input a,b;
  output s,c;
// gate level design of half adder  
  xor x1(s,a,b);
  and a1(c,a,b);
endmodule :half_add