APS/Basic Verilog structures/Concatenation.md

Concatenation (Signal Merging)

Concatenation allows assigning a multi-bit signal the "join" of several lower-width signals, or conversely, assigning a higher-width signal to a group of lower-width signals.

The concatenation operator has the following form: {sig1, sig2, ..., sign}.

Suppose we have the following set of signals:

logic a;
logic b;
logic [7:0] c;
logic [1:0] d;

logic [5:0] e;

And we want the wire e to receive the following signals:

• the MSB of signal e receives signal a
• the next bit receives signal b
• the next 2 bits receive bits [4:3] of signal c
• the 2 LSBs receive signal d

This can be done using 4 continuous assignments:

logic a;
logic b;
logic [7:0] c;
logic [1:0] d;

logic [5:0] e;

assign e[5]   = a;
assign e[4]   = b;
assign e[3:2] = c[4:3];
assign e[1:0] = d;

or with a single assignment using concatenation:

logic a;
logic b;
logic [7:0] c;
logic [1:0] d;

logic [5:0] e;

assign e = {a, b, c[4:3], d};

The reverse is also possible. Suppose we want to drive individual wires from separate bits of signal e:

logic a;
logic b;
logic [7:0] c;
logic [1:0] d;

logic [5:0] e;

assign a      = e[5];
assign b      = e[4];
assign c[4:3] = e[3:2];
assign d      = e[1:0];

This operation can also be expressed as a single concatenation:

logic a;
logic b;
logic [7:0] c;
logic [1:0] d;

logic [5:0] e;

assign {a, b, c[4:3], d} = e;

Additionally, concatenation can be used for signal replication. Replication is expressed as:

{a, {replication_count{signal_to_replicate}} ,b}

For example, to assign three copies of bits [4:3] of signal c, followed by signals a and b:

logic a;
logic b;
logic [7:0] c;

logic [7:0] e;

assign e = { {3{c[4:3]}}, a, b};

Chapter Summary

The concatenation operator can be used to group and replicate signals on either side of an assignment:

• it can be used to assign a group of lower-width signals to a higher-width signal
• it can be used to assign the corresponding bits of a higher-width signal to a group of lower-width signals.