Manipulating columns in a 2-D array in verilog

Question

I am working with a 1023*1023 MATRIX in verilog. I am trying to do the following : 1) Pick each row one at a time , do some modifications , write the output back to the corresponding row. For example , I first pick row0 , make changes to it and write the output back to the row0. I do this for all the 1023 rows. 2) Then, I pick each column one at a time, do some modifications and then write the data back.

I can create a 2-d array like reg [1022:0] my_array[1022:0]. Now, accessing the rows is easy. I can write my_array[0] to access the first row and so on. But how do I access the columns ?

Matthew · Accepted Answer · 2019-03-21 16:47:13Z

It's best not to think of

reg [1022:0] my_array[1022:0];

as a 1023 by 1023 array; it's best to think of it as an array of 1023 1023-bit numbers. This is a true 1023 by 1023 array:

reg my_array[1022:0][1022:0];

SystemVerilog generalises arrays and introduces some new terminology that is useful when talking about Verilog. (Verilog is only an unofficial subset of SystemVerilog, anyway.) In SystemVerilog, you declare arrays like this

<type> <packed dimensions> <name> <unpacked dimensions>

So, the equivalent of your original declaration would be

logic [1022:0] my_array [1022:0];

So, the first [1022:0] is a packed dimension and the second [1022:0] is an unpacked dimension. In SystemVerilog, you can have as many packed dimensions as you like and as many unpacked dimensions as you like. In Verilog, you can have as many unpacked dimensions as you like, but you can only have one unpacked dimension.

The rules about accessing packed dimensions are different to the rules about accessing unpacked dimensions. Basically, the rules for accessing unpacked dimensions are much more strict. (This is why you perceive your difference in behaviour between rows and columns.) Basically, for unpacked dimensions, you can either

access a single element or
access the whole array at once.

That's it. So, when you're dealing with the unpacked dimension, you're going to have to use some kind of loop to do what you want to do. And, if you are really working with an array of 1023 by 1023 single-bit numbers, then, given it's always going to be more work to access the unpacked dimensions, you might want to consider making both dimensions unpacked and using loops for both dimensions.

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