What is the function of $readmemh and $writememh in Verilog?

Question

I have been looking at some Verilog testbench code that heavily uses $readmemh and $writememh.

I have a vague understanding that these functions basically read to and write from memory. What is their specific function and how do they work?

Answer 1

I agree its not too easy to find something about readmem/writemem. You can find a little bit here: https://www.fullchipdesign.com/readmemh.htm

Anyway there is not too much to say about these functions, the syntax is:

$readmem[hb]("File", ArrayName, StartAddr, EndAddr)
$writemem[hb]("File", ArrayName, StartAddr, EndAddr)

Verilog is very picky about the file format, the number of bit in the text file have to match the number of bits in the array.

I recommend you play around a little bit by defining an array, filling it up with data write it out with writememh/writememb and print it out afterwards.

Something like this should get you started (not tried out!).

integer i;
reg [7:0] memory [0:15]; // 8 bit memory with 16 entries

initial begin
    for (i=0; i<16; i++) begin
        memory[i] = i;
    end
    $writememb("memory_binary.txt", memory);
    $writememh("memory_hex.txt", memory);
end