21

I am going through a Verilog test case, and I found this statement:

assign XYZ = PQR_AR[44*8 +: 64];

What is the "+:" operator known as? I tried to find this on google, but I didn't get any relevant answer.

toolic
  • 5,637
  • 5
  • 20
  • 33
shailendra
  • 591
  • 5
  • 16
  • 29

2 Answers2

31

That syntax is called an indexed part-select. The first term is the bit offset and the second term is the width. It allows you to specify a variable for the offset, but the width must be constant.

Example from the SystemVerilog 2012 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
dwikle
  • 966
  • 7
  • 7
3

Lastly i got the source page for this, this is called as Indexed Vector part Select ("+:").

To explain it a bit more

PQR_AR[44*8 +: 64];

With Indexed vector part select, which is added in Verilog 2000, you can select a part of bus rather then selecting whole bus.

44*8 part is starting point of part select variable and 64 is the width of part select andis constant.It means that if initially we have initialized 

input [415:0] PQR;

we are selecting a particular part of PQR using 

PQR_AR[44*8 +: 64];

that is PQR_AR[352+:64] or it means that we are taking a part from 352 to 415 out of 0 to 415.

shailendra
  • 591
  • 5
  • 16
  • 29
  • From dwikle answer derived from LRM, when we initialize a bus like: input [415:0] PQR; Then, PQR_AR[44*8 +: 64] should be PQR_AR[352:288] and not PQR_AR[415:352]. Please correct me if I am going in wrong direction. – ABX Dec 01 '15 at 05:09
  • @ABX I agree about the direction, except it should be PQR_AR[352:289] to be a 64 bit wide bus – peterbc May 22 '19 at 08:50