Use of << in given Verilog code?

Question

In the following Verilog code snippet for implementing an input buffer for a router, in second line, what is the role of 1<<`BUF_WIDTH? I understand that << is the left shift operator, but what happens by left shifting 1 by `BUF_WIDTH? Or is there some other function of << operator?

`define BUF_WIDTH 3    // BUF_SIZE = 16 -> BUF_WIDTH = 4, no. of bits to be used in pointer
`define BUF_SIZE ( 1<<`BUF_WIDTH )

module fifo13( clk, rst, buf_in, buf_out, wr_en, rd_en, buf_empty, buf_full, fifo_counter );

input                 rst, clk, wr_en, rd_en;   
input [7:0]           buf_in; // data input to be pushed to buffer

output[7:0]           buf_out;// port to output the data using pop.

output                buf_empty, buf_full; // buffer empty and full indication 

output[`BUF_WIDTH :0] fifo_counter; // number of data pushed in to buffer   

reg[7:0]              buf_out;
reg                   buf_empty, buf_full;
reg[`BUF_WIDTH :0]    fifo_counter;
reg[`BUF_WIDTH -1:0]  rd_ptr, wr_ptr;           // pointer to read and write addresses  
reg[7:0]              buf_mem[`BUF_SIZE -1 : 0]; 

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The entire code is available on http://electrosofts.com/verilog/fifo.html

mkrieger1 · Accepted Answer · 2015-06-13 19:04:10Z

1

You assume correctly that << is the left-shift operator, it has no other special meaning.

Shifting the binary representation of a number to the left is equivalent to multiplying the number by 2. So, by shifting 1 to the left N times, you get 2 to the power of N as a result.

The way this is used in the code sample ensures that the buffer has exactly as many entries (BUF_SIZE) as can be uniquely addressed by a pointer of size BUF_WIDTH.

edited Jun 13, 2015 at 19:04

answered Jun 13, 2015 at 18:56

mkrieger1

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Sami Kuhmonen · Accepted Answer · 2015-06-13 18:49:11Z

0

It is the bit shift operator. Think what it does: it shifts bits left. You have a definition of BUF_WIDTH being 3. Then you take 1, shift it by that many places and you get 8 for BUF_SIZE. With three bits you can have 8 different values.

So this is a way to define these two constants so that you only have to change one value. If they would be two constants, someone might accidentally only change one and not the other and this would cause problems.

answered Jun 13, 2015 at 18:49

Sami Kuhmonen

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