A.	Test block

	1. Customers arrive at Starbucks ever 2 +/- 1 minute and queue up
	for service.  There are two servers.  60% of all customers
	want a special cafe, which takes 5 +/- 1 minute to prepare.
	The remaining customers just want a regular coffee, which takes
	2 minutes to prepare.  Simulate for an 8 hour day.

	SIMULATE

 SERVER	STORAGE		2

	GENERATE	2,1
	QUEUE		LINEUP
	ENTER		SERVER
	DEPART		LINEUP
	TRANSFER	.60,COFFEE,CAFE
 COFFEE	ADVANCE		2
	LEAVE		SERVER
	TERMINATE

 CAFE	ADVANCE		5,1
	LEAVE		SERVER
	TERMINATE

	GENERATE	480
	TERMINATE	1

	START		1
	END


	2. Same scenario, only if the queue has five or more people
	in it, then new arrivals go away and 50% come back in two minutes.

	How can we model this?  Instead of a queue, use a storage.

	SIMULATE

 SERVER	STORAGE		2
 LINEUP	STORAGE		5

	GENERATE	2,1
 ENTRY	TRANSFER	BOTH,,GOAWAY
	ENTER		LINEUP
	ENTER		SERVER
	LEAVE		LINEUP
	TRANSFER	.60,COFFEE,CAFE
 COFFEE	ADVANCE		2
	LEAVE		SERVER
	TERMINATE

 CAFE	ADVANCE		5,1
	LEAVE		SERVER
 EXIT	TERMINATE

 GOAWAY	TRANSFER	.50,,EXIT
	ADVANCE		2
	TRANSFER	,ENTRY

	GENERATE	480
	TERMINATE	1

	START		1
	END

	3.  Same scenario, only now 20% of new arrivals will go away if
	the queue has five or more people, the rest join the queue.

	Now we must allow the queue to have an arbitrary length, but
	still transfer away from it if it is too long.

	Solution: use a TEST block.

	syntax:	TEST <auxiliary operation>	A,B,C

	auxiliary operation: L LE G GE E NE
	A,B:	SNA
	C:	Destination block if test if "false"


	SIMULATE

 SERVER	STORAGE		2

	GENERATE	2,1
 ENTRY	TEST L		Q(LINEUP),5,GOAWAY
 STAY	QUEUE		LINEUP
	ENTER		SERVER
	DEPART		LINEUP
	TRANSFER	.60,COFFEE,CAFE
 COFFEE	ADVANCE		2
	LEAVE		SERVER
	TERMINATE

 CAFE	ADVANCE		5,1
	LEAVE		SERVER
 EXIT	TERMINATE

 GOAWAY	TRANSFER	.80,,STAY
	TRANSFER	.50,,EXIT
	ADVANCE		2
	TRANSFER	,ENTRY

	GENERATE	480
	TERMINATE	1

	START		1
	END


B.	SNAs

	The basic elements of a GPSS simulation model are
		transactions, resources, blocks, control statements,
		compiler directives, standard numerical attributes

	transactions are the dynamic entities (generated).  resources
	are static entities (facilities, storages).  blocks are actions
	taken by transactions. standard numerical attributes are functions
	that provide information about entities, built-in or user-defined.

	transaction attributes:
		built-in:
			M1 - transit time (or mark time)
			PR - priority
			Pnumber  - parameter
		some built-ins are inaccessible, like current block where
		a transaction resides.
	block attributes:
		N(name or number) - total entry count
		W(name or number) - current entry count
	facility attributes:
		F(name or number) - status (0=idle,1=busy)
		FT(name or number) - average time per transaction
		FR() - utilisation (parts/1000)
	queue attributes:
		Q(name or number) - current length
		QA() - average contents
		QM() - maximum contents
		QT() - average time per transaction (all transactions)
		QX() - average time for those without zero delay
		QZ() - how many zero delays
	Standard numerical attributes in GPSS are 

C.	CLEAR control statement

	Rerun the simulation for seven days.  Not seven "identical"
	days, but seven days with the same characteristics.

	How?  Can't just rerun the model seven times as it will generate
	the same results seven times.

	Use the CLEAR statement, which clears all statistics
	and removes all transactions.  Statistics for each separate
	run are generated in the listing file.


	SIMULATE

 SERVER	STORAGE		2

	GENERATE	2,1
 ENTRY	TEST L		Q(LINEUP),5,GOAWAY
 STAY	QUEUE		LINEUP
	ENTER		SERVER
	DEPART		LINEUP
	TRANSFER	.60,COFFEE,CAFE
 COFFEE	ADVANCE		2
	LEAVE		SERVER
	TERMINATE

 CAFE	ADVANCE		5,1
	LEAVE		SERVER
 EXIT	TERMINATE

 GOAWAY	TRANSFER	.80,,STAY
	TRANSFER	.50,,EXIT
	ADVANCE		2
	TRANSFER	,ENTRY

	GENERATE	480
	TERMINATE	1

	START		1
	CLEAR
	START		1
	CLEAR
	START		1
	CLEAR
	START		1
	CLEAR
	START		1
	CLEAR
	START		1
	CLEAR
	START		1
	END

- - - - -

D.	RESET control statement

	The coffee shop simulation starts from an empty shop.
	Simulate for just the last hour of business.

	RESET clears statistics but leaves transactions in place.