Editing GPSS (section)

== Basic Transaction commands ==
Transactions can :

* Automatically enter the simulation (GENERATE)
* Use a facility or storage (SEIZE/ENTER respectively)
* Stop using a facility or storage (RELEASE/LEAVE respectively)
* Wait (ADVANCE)
* Transfer to another section of code (TRANSFER)
* Exit the simulation (TERMINATE)

Along with these main tasks, they can also hold parameters with the ASSIGN command. Transactions are implicitly sectioned off in the code.

There can be multiple transactions in the code that happen at once. A transaction starts with GENERATE and ends with TERMINATE. The code inbetween could potentially be shared between multiple transactions by means of the TRANSFER command, but besides that your transaction lifecycles will be separated. It helps to add whitespace between lines of action of one transaction versus another.

=== GENERATE ===
 GENERATE 0.1
Transaction generated every 0.1 time unit.
 GENERATE 15,4
Transaction is generation every 15 time units, plus or minus 4 time units.

[https://athena.ecs.csus.edu/~mitchell/csc148/gpssW/Reference%20Manual/r7.htm#GENERATE GENERATE]

=== ADVANCE ===
To have a transaction wait, use the ADVANCE command. It has similar arguments to GENERATE.
 ADVANCE 10,6
Transaction waits 10±6 seconds.

[https://athena.ecs.csus.edu/~mitchell/csc148/gpssW/Reference%20Manual/r7.htm#ADVANCE ADVANCE]

=== SEIZE, RELEASE (Facilities) ===
To use a '''Facility''', which only allows one use at a time, use the SEIZE command. To stop using it, use the RELEASE command.
          GENERATE 30,5   ; Generate a customer every 30±5 time units
          SEIZE Barber    ; Use a facility
          ADVANCE 15,4    ; Wait 15±4 time units
          RELEASE Barber  ; Stop using the facility
          TERMINATE 1     ; Leave the barber shop
If you want more than one use at a time, use '''Storage'''.

[https://athena.ecs.csus.edu/~mitchell/csc148/gpssW/Reference%20Manual/r7.htm#SEIZE SEIZE]

[https://athena.ecs.csus.edu/~mitchell/csc148/gpssW/Reference%20Manual/r7.htm#RELEASE RELEASE]

=== ENTER, LEAVE (Storages) ===
 Seating  STORAGE 100     ; 100 people allowed in lounge seating
 
          GENERATE 10,5   ; Generate a person every 10±5 time units
          ENTER Seating,1 ; Person sits down
          ADVANCE 15,4    ; Wait 15±4 time units
          LEAVE Barber,1  ; Person stops sitting
          TERMINATE 1     ; Exit the seating area
This way, multiple people can sit in the Seating at once. If this was a ''Facility'' (using SEIZE/RELEASE), it would block others who tried to use the resource.

The ENTER command takes the storage reference as Argument A and the amount to reserve with Argument B. The LEAVE command's arguments are the same.

[https://athena.ecs.csus.edu/~mitchell/csc148/gpssW/Reference%20Manual/r7.htm#ENTER ENTER]

[https://athena.ecs.csus.edu/~mitchell/csc148/gpssW/Reference%20Manual/r7.htm#LEAVE LEAVE]

=== TERMINATE ===
To remove the transaction, use TERMINATE.

The optional argument decrements the '''completion counter''', which is a variable that is chosen by the user when running the simulation. Say you wanted to test 100 customers: you would start the execution of your simulation with <code>START 100</code>. <code>TERMINATE 1</code> at the end of each transaction would decrement initial value 100 by 1 (99, 98, 97 ...) until it reached zero. At this point, the simulation stops and results are returned. If you omit the argument in TERMINATE, it will be assumed to be 0. This means your simulation will run forever (unless, of course, you have another TERMINATE that does decrement this counter.

[https://athena.ecs.csus.edu/~mitchell/csc148/gpssW/Reference%20Manual/r7.htm#TERMINATE TERMINATE]

==== Timer ====
To have your program run for a predetermined time, make sure none of your TERMINATES decrement the counter and include a section like this:
 GENERATE      ; Generate one transaction
 ADVANCE 100   ; Run for 100 time units
 TERMINATE 1   ; End
Then run your program with START 1. It will run for 100 time units.

=== ASSIGN (Parameter "Metadata") ===
Use the ASSIGN control block to assign a value to a transaction parameter. Called with Pj (j=parameter number)
             ASSIGN 2,V$Orderqty &nbsp; &nbsp; ;Parameter 2=Order quantity
 Custwait &nbsp;&nbsp; ADVANCE 5 &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; ;Lead time is 5 days
             ENTER Stock,P2 &nbsp; &nbsp; &nbsp; &nbsp;&nbsp; ;Stock increases by P2
[https://athena.ecs.csus.edu/~mitchell/csc148/gpssW/Reference%20Manual/r7.htm#ASSIGN ASSIGN] : ASSIGN Blocks are used to place or modify a value in a Transaction Parameter.