Semaphore is used for lock/unlock of a common resource. If resource is in unlock state the resource can be used, if it is in lock state, then it can not be used.

Lets assume that in a Verification env, we have single ethernet port/driver and there are 2 packet generators using this one ethernet port, one generating normal frames and other generating flow control (pause) frames.

So each of this two packet generators can use semaphone to if ethenet driver is free (unlock) state, before it can transmit the frame. So if the driver is free, then it locks the drivers and transmits the frame, once tramitted, it unlocks the drivers.

When driver is in lock state, it will wait till driver is in unlock state.

Semaphore in Vera supports following methods for above operations.

• Semaphore allocation
• Using semaphore keys
• Returing semaphore keys.

Semaphore Allocation

function integer alloc(SEMAPHORE, integer semaphore_id,
integer semaphore_count, integer key_count);

Where

• semaphore_id : Is the ID number of the particular semaphore being created. It must be an integer value. You should generally use 0. When you use 0, a semaphore ID is automatically generated by the simulator. Using any other number explicitly assigns an ID to the semaphore being created.
• semaphore_count : Specifies how many semaphore buckets you want to create. It must be an integer value.
• key_count : Specifies the number of keys initially allocated to each semaphore bucket you are creating.

Using semaphore keys

function integer semaphore_get(NO_WAIT | WAIT,
integer semaphore_id, integer key_count);

Where

• NO_WAIT : Continues code execution even if there are not enough keys available.
• WAIT : Suspends the process until there are enough keys available, at which time execution continues.
• semaphore_id : Specifies which semaphore to get keys from.
• key_count : Specifies the number of keys being taken from the semaphore.

If there are enough keys available, a 1 is returned and execution continues. If there are not enough keys available, a 0 is returned and the process is suspended depending on the wait option.

Returning keys

task semaphore_put(integer semaphore_id, integer key_count);

Where

• semaphore_id : Specifies which semaphore to return the keys to.
• key_count : Specifies the number of keys being returned to the semaphore.

  1 program semaphore {
  2   // Declare a integer, which will be used for semaphore
  3   integer  bus;
  4   // Alloc it as semaphore, with 1 lock
  5   bus =  alloc(SEMAPHORE,0,1,1);
  6   fork 
  7     { agent("AGENT 0",5);}
  8     { agent("AGENT 1",20);}
  9   join all
 10 
 11 }
 12 
 13 task agent(string name, integer wait) {
 14   integer i = 0;
 15   for (i = 0 ; i < 4; i ++ ) {
 16     // Check if bus is avaible
 17     if (semaphore_get(WAIT,bus,1) == 1) {
 18       printf("[%0d] Lock bus for %s\n", get_time(LO),name);
 19       delay(wait);
 20       printf("[%0d] Release bus for %s\n", get_time(LO),name);
 21       semaphore_put(bus,1);
 22       delay(wait);
 23     }
 24   }
 25 }

 [0] Lock bus for AGENT 0
 [5] Release bus for AGENT 0
 [5] Lock bus for AGENT 1
 [25] Release bus for AGENT 1
 [25] Lock bus for AGENT 0
 [30] Release bus for AGENT 0
 [35] Lock bus for AGENT 0
 [40] Release bus for AGENT 0
 [45] Lock bus for AGENT 1
 [65] Release bus for AGENT 1
 [65] Lock bus for AGENT 0
 [70] Release bus for AGENT 0
 [85] Lock bus for AGENT 1
 [105] Release bus for AGENT 1
 [125] Lock bus for AGENT 1
 [145] Release bus for AGENT 1

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Deepak Kumar Tala - All rights reserved

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