packed vs unpacked vectors in system verilog

vector system-verilog
Nathan Fellman picture Nathan Fellman · Jan 25, 2009 · Viewed 74k times · Source

Looking at some code I'm maintaining in System Verilog I see some signals that are defined like this:

node [range_hi:range_lo]x;

and others that are defined like this:

node y[range_hi:range_lo];

I understand that x is defined as packed, while y is defined as unpacked. However, I have no idea what that means.

What is the difference between packed and unpacked vectors in System Verilog?

Edit: Responding to @Empi's answer, why should a hardware designer who's writing in SV care about the internal representation of the array? Are there any times when I shouldn't or can't use packed signals?

Answer

empi picture empi · Jan 25, 2009

This article gives more details about this issue: http://electrosofts.com/systemverilog/arrays.html, especially section 5.2.

A packed array is a mechanism for subdividing a vector into subfields which can be conveniently accessed as array elements. Consequently, a packed array is guaranteed to be represented as a contiguous set of bits. An unpacked array may or may not be so represented. A packed array differs from an unpacked array in that, when a packed array appears as a primary, it is treated as a single vector.

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