File I/O for test vectors ($readmemh, $writememh) in Verilog - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the Verilog system task $readmemh do?

$readmemh reads hexadecimal data from a text file and loads it into a memory array in your Verilog simulation.

beginner

How is $writememh used in Verilog?

$writememh writes the contents of a memory array to a text file in hexadecimal format, useful for saving simulation results.

intermediate

What is the difference between $readmemh and $readmemb?

$readmemh reads hexadecimal data, while $readmemb reads binary data from a file into memory.

beginner

How do you specify the memory array and file name in $readmemh?

You provide the file name as a string and the memory array variable. Example: $readmemh("data.hex", memory_array);

beginner

Why is using $readmemh and $writememh helpful in testbenches?

<p>They let you load input test vectors and save output results easily, making simulation setup and analysis simpler and more flexible.</p>

Which format does $readmemh expect in the input file?

A Hexadecimal

B Binary

C Decimal

D ASCII text

$readmemh reads hexadecimal values from the file.

What is the main purpose of $writememh in Verilog?

A To compile the Verilog code

B To write memory contents to a file in hex format

C To initialize registers

D To read memory contents from a file

$writememh saves memory contents to a file in hexadecimal format.

If you want to load binary data into memory, which system task should you use?

A $readmemb

B $readmemh

C $writememh

D $writememb

$readmemb reads binary data from a file.

Which of the following is a correct way to load a file named input.hex into a memory array mem?

A $writememh("input.hex", mem);

B $readmemh(mem, "input.hex");

C $readmemb("input.hex", mem);

D $readmemh("input.hex", mem);

The correct syntax is $readmemh("filename", memory_array);

What happens if the file specified in $readmemh does not exist?

A Simulation stops with an error

B Memory is zeroed out

C Memory is left unchanged and a warning is shown

D File is created automatically

The memory remains unchanged and the simulator usually shows a warning message.

Explain how to use $readmemh to load test vectors into a memory array in Verilog.

Think about what arguments the task needs and what format the file should have.

Describe the benefits of using $readmemh and $writememh in Verilog testbenches.

Consider how these tasks help with input and output during simulation.

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