Indexing vectors and arrays with +:

system-verilog
DOS picture DOS · Aug 5, 2013 · Viewed 126k times · Source

I am seeing a code in SystemVerilog which has something like this:

if(address[2*pointer+:2])
  do_something;

How should I understand the +: when indexing this vector?

I found that it is called bit slicing, but I can't find an explanation about it.

Answer

Greg picture Greg · Aug 5, 2013

Description and examples can be found in IEEE Std 1800-2017 § 11.5.1 "Vector bit-select and part-select addressing". First IEEE appearance is IEEE 1364-2001 (Verilog) § 4.2.1 "Vector bit-select and part-select addressing". Here is an direct example from the LRM:

logic [31: 0] a_vect;
logic [0 :31] b_vect;
logic [63: 0] dword;
integer sel;
a_vect[ 0 +: 8] // == a_vect[ 7 : 0]
a_vect[15 -: 8] // == a_vect[15 : 8]
b_vect[ 0 +: 8] // == b_vect[0 : 7]
b_vect[15 -: 8] // == b_vect[8 :15]
dword[8*sel +: 8] // variable part-select with fixed width

If sel is 0 then dword[8*(0) +: 8] == dword[7:0]
If sel is 7 then dword[8*(7) +: 8] == dword[63:56]

The value to the left always the starting index. The number to the right is the width and must be a positive constant. the + and - indicates to select the bits of a higher or lower index value then the starting index.

Assuming address is in little endian ([msb:lsb]) format, then if(address[2*pointer+:2]) is the equivalent of if({address[2*pointer+1],address[2*pointer]})

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