Long description

The figure consists of four rectangular blocks, the first block is inactive and labeled PG underscore active equals 0, PG underscores referenced equals 0. The second block is placed below the first block and labeled PG underscore active equals 0, PG underscores referenced equals 1. An arrow is pointing from the first block to the second block which is labeled Used and a dotted lined arrow is pointing from the second block to the first block which is labeled Timeout. The third block is active and labeled PG underscore active equals 1, PG underscore referenced equals 0, and the fourth block is labeled PG underscore active equals 1, PG underscore referenced equals 1. An arrow is pointing from the third block to the fourth block which is labeled Used and a dotted lined arrow is pointing from the fourth block to the third block which is labeled Timeout. An arrow is pointing from the fourth block to the third block and the second block to the first block which is labeled Refill. An arrow is pointing from the third block to the first block which is labeled Used and a dotted arrow is pointing from the fourth block to the second block is labeled Refill.