Memory initialization with $readmemh in Verilog - Step-by-Step Execution

Concept Flow - Memory initialization with $readmemh

Start Simulation

↓

Invoke $readmemh

↓

Open Hex File

↓

Read Each Line

↓

Convert Hex to Binary

↓

Store Data in Memory Array

↓

Memory Initialized

↓

Continue Simulation

The simulation starts and calls $readmemh to open a hex file, read each line, convert it to binary, and store it in the memory array before continuing.

Execution Sample

Verilog

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

This code initializes a 4-byte memory array with values from a hex file named data.hex.

Execution Table

Step	Action	File Line Read	Hex Value	Binary Stored	Memory Index
1	Open file data.hex	-	-	-	-
2	Read line 1	0A	0A	00001010	mem[0]
3	Read line 2	1F	1F	00011111	mem[1]
4	Read line 3	FF	FF	11111111	mem[2]
5	Read line 4	00	00	00000000	mem[3]
6	End of file reached	-	-	-	-
7	Memory initialization complete	-	-	-	-

💡 All lines read from data.hex and stored in memory array mem[0..3]

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 5	Final
mem[0]	x	00001010	00001010	00001010	00001010	00001010
mem[1]	x	x	00011111	00011111	00011111	00011111
mem[2]	x	x	x	11111111	11111111	11111111
mem[3]	x	x	x	x	00000000	00000000

Key Moments - 3 Insights

Why does mem[0] get its value at step 2 and not before?

Because $readmemh reads the first line of the file at step 2 and stores it into mem[0], before that mem[0] is uninitialized (x).

What happens if the hex file has fewer lines than the memory size?

Only the memory locations corresponding to the lines read are initialized; others remain unchanged or undefined, as shown by the variable tracker.

Why is the binary stored shown as 8 bits for each hex value?

Because mem is declared as 8 bits wide, each hex value is converted to an 8-bit binary number before storing.

Visual Quiz - 3 Questions

Look at the execution table, what is the binary value stored in mem[2] after step 4?

A 00011111

B 11111111

C 00000000

D 00001010

💡 Hint

Check the 'Binary Stored' column at step 4 for mem[2]

At which step does the memory initialization complete?

A Step 5

B Step 6

C Step 7

D Step 4

💡 Hint

Look for the row with 'Memory initialization complete' in the Action column

If the hex file had only 2 lines, what would be the value of mem[3] after initialization?

A x (undefined)

B 00000000

C 11111111

D 00001010

💡 Hint

Refer to variable_tracker and key moment about fewer lines than memory size

Concept Snapshot

Memory initialization with $readmemh:
- Use $readmemh("file.hex", memory_array) inside initial block
- Reads hex values line by line from file
- Converts hex to binary and stores in memory
- Memory size and file lines should match or handle undefined
- Used for simulation memory preload

Full Transcript

This visual execution shows how Verilog's $readmemh system task initializes a memory array from a hex file. The simulation starts and calls $readmemh, which opens the file and reads each line. Each hex value is converted to an 8-bit binary number and stored in the corresponding memory index. The execution table traces each step, showing the file line read, the hex value, the binary stored, and the memory index updated. The variable tracker shows how each memory location changes from undefined to the stored value after each step. Key moments clarify common confusions like why memory is undefined before reading and what happens if the file has fewer lines than the memory size. The visual quiz tests understanding of the stored values and the process completion step. The concept snapshot summarizes the syntax and behavior of $readmemh for quick reference.