How do I access an array element using a variable as index?

Question

I'm trying to access an element from an array using an input as index and I keep getting this error:

cache.v:27: error: array 'tagc' index must be a constant in this context.

Here's how I'm trying to do it:

assign tagc[index] = tag;

tagc is an array of 1024 regs; index is a 10 bits input; tag is a 20 bits input.

Is there a way to do that?

We would need to see some context and what you are trying to do. Assigning an incoming signal to a dynamically-changing index in an array doesn't seem like it would be valid at all. — nanofarad, Jun 30 '16 at 23:35

score 1 · Answer 1 · answered Jul 01 '16 at 00:07

Two possibilities:

You're trying to assign an indexed location of tagc to mirror the value of tag, in which case you need index to be a constant (parameter, localparam, or `define).
You're using tagc as a memory that stores the value of tag in a location indexed by a dynamic variable 'index'. In this case, you need to do the assignment in an always block, after deciding what event should trigger an update of tagc.

score 0 · Answer 2 · answered Jul 01 '16 at 00:11

Usually an array assignment using a dynamic index is done in a clocked procedural block.

always @(posedge clk) begin
  tagc[index] <= tag;
end

It can also be done with latching. Use an enable signal and ensure index does not change while enabled.

always @* begin
  if (enable) begin
    tagc[index] <= tag;
  end
end

Or another latch option:

integer i;
always @* begin
  for(i=0; i<PARAM_SIZE_OF_TAG; i=i+1)
    if (index==i) tagc[i] <= tag;
  end
end

FYI: tagc must be defined as a reg not a wire type

2 Answers2