Gate level Modelling

The module is implemented in terms of logic gates and interconnections between these gates. Designer should know the gate-level diagram of the design

Gate primitives are predefined in Verilog, which are ready to use. They are instantiated like modules. There are two classes of gate primitives: Multiple input gate primitives and Single input gate primitives.
Multiple input gate primitives include and, nand, or, nor, xor, and xnor. These can have multiple inputs and a single output. They are instantiated as follows:

// Two input AND gate.
and and_1 (out, in0, in1);

// Three input AND gate.
and and_2 (out, in0, in1, in2);

// Two input OR gate.
or or_1 (out, in0, in1);

// Four input NOR gate.
or or_2 (out, in0, in1, in2, in3);

// Five input XOR gate.
xor xor_1 (out, in0, in1, in2, in3, in4);

// Two input XNOR gate.
xnor and_1 (out, in0, in1);

Single input gate primitives include not, buf, notif1, bufif1, notif0, and bufif0. These have a single input and one or more outputs. Gate primitives notif1, bufif1, notif0, and bufif0 have a control signal. The gates propagate if only control signal is asserted, else the output will be high impedance state (z). They are instantiated as follows:

// Inverting gate.
not not_1 (out, in);

// Two output buffer gate.
buf buf_1 (out0, out1, in);

// Single output Inverting gate with active-high control signal.
notif1 notif1_1 (out, in, ctrl);

// Double output buffer gate with active-high control signal.
bufif1 bufif1_1 (out0, out1, in, ctrl);

// Single output Inverting gate with active-low control signal.
notif0 notif0_1 (out, in, ctrl);

// Single output buffer gate with active-low control signal.
bufif0 bufif1_0 (out, in, ctrl);

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