Concatenation operator (&) in VHDL - Step-by-Step Execution

Concept Flow - Concatenation operator (&)

Start with signals A and B

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Apply & operator: A & B

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Create new signal with bits of A followed by bits of B

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Use concatenated signal in design

The & operator joins two bit vectors or signals by placing bits of the first signal before bits of the second, creating a longer combined signal.

Execution Sample

VHDL

signal A : std_logic_vector(3 downto 0) := "1010";
signal B : std_logic_vector(3 downto 0) := "1100";
signal C : std_logic_vector(7 downto 0);

C <= A & B;

This code concatenates two 4-bit signals A and B into an 8-bit signal C.

Execution Table

Step	Action	Signal A	Signal B	Concatenation (A & B)	Signal C Assigned
1	Initialize A	1010	-	-	-
2	Initialize B	-	1100	-	-
3	Concatenate A & B	1010	1100	10101100	-
4	Assign concatenation to C	1010	1100	10101100	10101100
5	End	1010	1100	10101100	10101100

💡 Concatenation completed and assigned to signal C

Variable Tracker

Variable	Start	After Concatenation	Final
A	1010	1010	1010
B	1100	1100	1100
C	undefined	10101100	10101100

Key Moments - 2 Insights

Why does the concatenated signal C have 8 bits instead of 4?

Because the & operator joins all bits of A (4 bits) followed by all bits of B (4 bits), making a total of 8 bits as shown in execution_table row 3.

Can we concatenate signals of different lengths?

Yes, the & operator can join signals of different lengths, resulting in a combined signal whose length is the sum of both, similar to how A and B are joined here.

Visual Quiz - 3 Questions

Look at the execution_table at step 3, what is the value of the concatenation A & B?

A 1010

B 11001010

C 10101100

D 1100

💡 Hint

Check the 'Concatenation (A & B)' column at step 3 in the execution_table.

At which step is the concatenated value assigned to signal C?

A Step 4

B Step 3

C Step 2

D Step 5

💡 Hint

Look at the 'Signal C Assigned' column in the execution_table.

If signal B was "0011" instead of "1100", what would be the concatenated result at step 3?

A 11001010

B 10100011

C 00111010

D 10101100

💡 Hint

Concatenation joins A bits then B bits, so replace B's bits in the concatenation column.

Concept Snapshot

Concatenation operator (&) in VHDL joins two signals by placing bits of the first signal before the second.
Syntax: result <= signal1 & signal2;
Result length = length(signal1) + length(signal2).
Used to combine bit vectors into longer vectors.
Example: "1010" & "1100" = "10101100".

Full Transcript

In VHDL, the concatenation operator & combines two signals by placing the bits of the first signal before the bits of the second. For example, if signal A is "1010" and signal B is "1100", then A & B results in "10101100". This combined signal can be assigned to another signal, like C. The length of the new signal is the sum of the lengths of the two original signals. This operation is useful to build wider signals from smaller parts.

Practice

1. What does the VHDL concatenation operator & do?

easy

A. Joins two or more bit vectors or bits into one longer vector

B. Performs bitwise AND operation between two vectors

C. Splits a vector into smaller parts

D. Assigns a value to a signal

Solution

1. Step 1: Understand the operator's purpose

   The & operator in VHDL is used to join bits or bit vectors together.

2. Step 2: Differentiate from other operators

   Unlike bitwise AND, it concatenates bits side by side to form a longer vector.

3. Final Answer:

   Joins two or more bit vectors or bits into one longer vector -> Option A

4. Quick Check:

   Concatenation = Joining bits [OK]

Hint: Remember & joins bits side by side, not bitwise AND [OK]

Common Mistakes:

• Confusing & with bitwise AND
• Thinking & splits vectors
• Assuming & assigns values

2. Which of the following is the correct syntax to concatenate two std_logic_vectors A and B in VHDL?

easy

A. C <= A & B;

B. C <= A + B;

C. C <= A and B;

D. C <= A | B;

Solution

1. Step 1: Identify concatenation syntax

   In VHDL, concatenation uses the & operator between vectors.

2. Step 2: Check other operators

   + is arithmetic addition, and is bitwise AND, | is not valid in VHDL for concatenation.

3. Final Answer:

   C <= A & B; -> Option A

4. Quick Check:

   Concatenation uses & operator [OK]

Hint: Use & to join vectors, not + or and [OK]

Common Mistakes:

• Using + instead of & for concatenation
• Using bitwise operators like and
• Using | which is invalid in VHDL

3. Given the following VHDL code:

signal A : std_logic_vector(3 downto 0) := "1010";
signal B : std_logic_vector(1 downto 0) := "11";
signal C : std_logic_vector(5 downto 0);

C <= A & B;

What is the value of C after the assignment?

medium

A. "1010"

B. "101011"

C. "111010"

D. "1111"

Solution

1. Step 1: Understand concatenation order

   The operator & joins A first, then B, so bits of A come first.

2. Step 2: Combine the bit vectors

   A is "1010" and B is "11", so concatenation is "1010" & "11" = "101011".

3. Final Answer:

   "101011" -> Option B

4. Quick Check:

   Concatenate A then B = 101011 [OK]

Hint: Left side bits come first in concatenation [OK]

Common Mistakes:

• Reversing order of concatenation
• Confusing concatenation with addition
• Ignoring vector sizes

4. Consider this VHDL snippet:

signal X : std_logic_vector(3 downto 0) := "1100";
signal Y : std_logic_vector(3 downto 0);

Y <= X & '1';

What is the error in this code?

medium

A. Single quotes used incorrectly for bit literal

B. Cannot concatenate a vector and a single bit directly

C. Signal Y has wrong size for concatenation result

D. No error, code is correct

Solution

1. Step 1: Check sizes of signals

   X is 4 bits, concatenated with one bit '1' makes 5 bits total.

2. Step 2: Compare with target signal size

   Y is declared as 4 bits, which cannot hold 5 bits from concatenation.

3. Final Answer:

   Signal Y has wrong size for concatenation result -> Option C

4. Quick Check:

   Concatenation size must match target signal size [OK]

Hint: Check target signal size matches concatenation length [OK]

Common Mistakes:

• Assuming single bit can't concatenate with vector
• Ignoring size mismatch errors
• Thinking single quotes are wrong for bits

5. You want to create an 8-bit vector DATA by concatenating a 3-bit vector A, a 2-bit vector B, and a 3-bit vector C. Which of the following assignments is correct in VHDL?

hard

A. DATA <= (A & B) or C;

B. DATA <= B & A & C;

C. DATA <= A and (B & C);

D. DATA <= A & B & C;

Solution

1. Step 1: Determine the correct concatenation

   To join A (3 bits), B (2 bits), C (3 bits) into 8-bit DATA, use DATA <= A & B & C; which appends them in order.

2. Step 2: Eliminate incorrect options

   B reverses order to B A C, C mixes 'and' operator, D uses 'or' after partial concat.

3. Final Answer:

   DATA <= A & B & C; -> Option D

4. Quick Check:

   3+2+3=8 bits, A&B&C [OK]

Hint: Always concatenate in the specified order using & [OK]

Common Mistakes:

• Reversing the concatenation order
• Mixing bitwise operators (and, or) with concatenation (&)
• Assuming different operator precedence changes order