File I/O for test vectors ($readmemh, $writememh) in Verilog - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output content of memory after $readmemh?

Given the following Verilog code snippet and the file data.hex containing hexadecimal values, what will be the content of mem[0] and mem[1] after the $readmemh call?

Verilog

reg [7:0] mem [0:3];
initial begin
  $readmemh("data.hex", mem);
end

// Contents of data.hex:
// 1A
// 2B
// 3C
// 4D

A mem[0] = 8'h1A, mem[1] = 8'h2B

B mem[0] = 8'h00, mem[1] = 8'h00

C mem[0] = 8'h3C, mem[1] = 8'h4D

D mem[0] = 8'h2B, mem[1] = 8'h1A

💡 Hint

The $readmemh command loads file lines sequentially into memory starting at index 0.

The $readmemh reads the file line by line and stores each hexadecimal value into consecutive memory locations starting at mem[0]. So the first line 1A goes to mem[0], the second line 2B goes to mem[1].

❓ Predict Output

intermediate

What does $writememh output to the file?

Consider the following Verilog code that writes memory content to a file using $writememh. What will be the content of the output file out.hex?

Verilog

reg [7:0] mem [0:2];
initial begin
  mem[0] = 8'hAA;
  mem[1] = 8'hBB;
  mem[2] = 8'hCC;
  $writememh("out.hex", mem);
end

A

AA
BB
CC

B AA BB CC

C

10101010
10111011
11001100

D

0xAA 0xBB 0xCC

💡 Hint

$writememh writes hexadecimal values line by line without prefixes.

The $writememh command writes each memory element as a hexadecimal string on its own line without any prefix or spacing.

❓ Predict Output

advanced

What error occurs with incorrect $readmemh file format?

What error or behavior occurs when $readmemh tries to read a file containing invalid hexadecimal characters?

Verilog

reg [7:0] mem [0:1];
initial begin
  $readmemh("bad.hex", mem);
end

// Contents of bad.hex:
// 1G
// ZZ

A Memory locations contain garbage values

B Syntax error and simulation stops

C Simulation warning: invalid hex digit, memory location set to 0

D No error, memory loaded as zeros

💡 Hint

Invalid hex digits cause warnings but do not stop simulation.

$readmemh issues a warning for invalid hex digits and sets the corresponding memory location to zero. It does not cause a syntax error or stop simulation.

❓ Predict Output

advanced

What is the value of mem[2] after $readmemh with offset?

Given the following code, what is the value of mem[2] after the $readmemh call?

Verilog

reg [7:0] mem [0:4];
initial begin
  $readmemh("data.hex", mem, 1, 3);
end

// Contents of data.hex:
// 10
// 20
// 30
// 40
// 50

A 8'h40

B 8'h20

C 8'h30

D 8'h10

💡 Hint

The $readmemh with start and end indices loads the first three lines from the file into memory mem[1] to mem[3].

The call $readmemh("data.hex", mem, 1, 3) loads first line (10) to mem[1], second line (20) to mem[2], third line (30) to mem[3]. So mem[2] gets 8'h20.

🧠 Conceptual

expert

Why use $readmemh instead of $readmemb for test vectors?

Which reason best explains why $readmemh is often preferred over $readmemb for loading test vectors in Verilog simulations?

A <code>$readmemh</code> automatically converts decimal to binary during loading

B <code>$readmemh</code> supports loading floating-point numbers directly

C <code>$readmemh</code> is faster because it reads files asynchronously

D <code>$readmemh</code> files are more compact and easier to read because hexadecimal uses fewer digits than binary

💡 Hint

Think about file size and readability of hex vs binary.

Hexadecimal representation is more compact and easier to read and edit than binary, so $readmemh is preferred for test vectors. The other options are incorrect because $readmemh does not support floating-point directly, does not convert decimal, and file reading is synchronous.

Practice

1. What is the primary purpose of the Verilog system task $readmemh?

easy

A. To load hexadecimal data from a file into a memory array

B. To write memory contents to a file in binary format

C. To initialize registers with random values

D. To display memory contents on the console

Solution

1. Step 1: Understand $readmemh usage

   $readmemh is used to read hexadecimal data from a file and load it into a memory array in Verilog.

2. Step 2: Compare with other options

   Other options describe writing data, random initialization, or display, which are not the purpose of $readmemh.

3. Final Answer:

   To load hexadecimal data from a file into a memory array -> Option A

4. Quick Check:

   $readmemh loads hex data [OK]

Hint: Remember: $readmemh reads hex data into memory [OK]

Common Mistakes:

• Confusing $readmemh with $writememh
• Thinking it writes data instead of reading
• Assuming it reads binary instead of hex

2. Which of the following is the correct syntax to write memory contents to a hex file in Verilog?

easy

A. $readmemh("data.hex", mem_array);

B. $writememh("output.hex", mem_array);

C. $writehex("output.hex", mem_array);

D. $dumpmemh("output.hex", mem_array);

Solution

1. Step 1: Identify the correct system task for writing

   $writememh is the Verilog system task used to write memory contents to a hex file.

2. Step 2: Check syntax correctness

   The syntax $writememh("output.hex", mem_array); correctly specifies the file name and memory array.

3. Final Answer:

   $writememh("output.hex", mem_array); -> Option B

4. Quick Check:

   Write memory to hex file = $writememh [OK]

Hint: Use $writememh to save memory to hex file [OK]

Common Mistakes:

• Using $readmemh instead of $writememh
• Misspelling the system task name
• Using non-existent tasks like $writehex

3. Given the following Verilog code snippet, what will be the content of mem[0] after $readmemh executes if the file data.hex contains:
0A
1F
2B

reg [7:0] mem [0:2];
initial begin
  $readmemh("data.hex", mem);
end

medium

A. 8'h00

B. 8'h1F

C. 8'h2B

D. 8'h0A

Solution

1. Step 1: Understand $readmemh file loading order

   $readmemh loads the first line of the file into mem[0], second line into mem[1], and so on.

2. Step 2: Match file content to memory

   The first line is 0A, so mem[0] will be 8'h0A.

3. Final Answer:

   8'h0A -> Option D

4. Quick Check:

   First file line -> mem[0] = 0A [OK]

Hint: First file line loads into mem[0] with $readmemh [OK]

Common Mistakes:

• Assuming mem[0] gets last file line
• Confusing hex values with decimal
• Forgetting zero-based indexing

4. Identify the error in the following Verilog code snippet that attempts to write memory contents to a hex file:

reg [7:0] mem [0:3];
initial begin
  $writememh(mem, "output.hex");
end

medium

A. Missing file extension in filename

B. Memory array size is invalid

C. Arguments to $writememh are in wrong order

D. Cannot use $writememh inside initial block

Solution

1. Step 1: Check $writememh argument order

   The correct syntax is $writememh("filename", memory_array);. The code reverses these arguments.

2. Step 2: Verify other aspects

   Memory size and file extension are valid, and $writememh can be used inside initial blocks.

3. Final Answer:

   Arguments to $writememh are in wrong order -> Option C

4. Quick Check:

   Filename first, memory second in $writememh [OK]

Hint: Filename is first argument in $writememh [OK]

Common Mistakes:

• Swapping filename and memory array arguments
• Thinking file extension is mandatory
• Believing $writememh can't be in initial block

5. You want to test a memory module by loading test vectors from a hex file and then save the modified memory contents back to another hex file. Which sequence of Verilog system tasks correctly achieves this?

hard

A. $readmemh("input.hex", mem); // test$writememh("output.hex", mem); // save

B. $writememh("input.hex", mem); // test$readmemh("output.hex", mem); // save

C. $readmemh(mem, "input.hex"); // test$writememh(mem, "output.hex"); // save

D. $readmemh("output.hex", mem); // test$writememh("input.hex", mem); // save

Solution

1. Step 1: Load test vectors correctly

   Use $readmemh("input.hex", mem); to load test vectors from the input file into memory.

2. Step 2: Save modified memory correctly

   Use $writememh("output.hex", mem); to write the memory contents to the output file.

3. Final Answer:

   $readmemh("input.hex", mem); // test
   $writememh("output.hex", mem); // save -> Option A

4. Quick Check:

   Read first, then write with correct syntax [OK]

Hint: Read test vectors first, write results second [OK]

Common Mistakes:

• Reversing read and write order
• Swapping filename and memory arguments
• Using wrong filenames for read/write