Five GPSS block diagram symbols

GPSS (General Purpose Simulation System) models are drawn as block diagrams. Transactions (entities) flow from block to block; each block has a standard symbol and a keyword statement. Five basic ones:

1. GENERATE

Creates transactions (arrivals). Operands: A = mean inter-arrival time, B = half-range of the uniform spread. GENERATE 10,3 creates a transaction every 10 ± 3 time units (7 to 13).

2. QUEUE and DEPART

QUEUE records that a transaction joins a queue for statistics; DEPART records leaving it. The difference gives waiting time and queue length. QUEUE LINE / DEPART LINE.

3. SEIZE and RELEASE

SEIZE captures a facility (a single server). If it is busy, the transaction waits. RELEASE frees it. SEIZE TELLER / RELEASE TELLER.

4. ADVANCE

Holds a transaction for a time, representing service or delay. ADVANCE 8,2 keeps it for 8 ± 2 units.

5. TERMINATE

Removes the transaction from the system and decrements the termination counter by A. TERMINATE 1 reduces the counter by 1; the run ends when it reaches zero (set by START 100).

Others: TRANSFER (branching), ENTER/LEAVE (storage, multi-server), TEST (conditional), ASSIGN, SAVEVALUE.

Symbol shapes (text sketch)

 GENERATE  -> QUEUE -> SEIZE -> DEPART
 ADVANCE   -> RELEASE -> TERMINATE

Each block has its own shape in the GPSS convention, with operands written inside or beside it.

Example: barber shop with one barber

GENERATE 18,6      ; customer arrives every 18 +/- 6 min
QUEUE    WAIT
SEIZE    BARBER
DEPART   WAIT
ADVANCE  16,4      ; haircut 16 +/- 4 min
RELEASE  BARBER
TERMINATE 1
START 100

Flow: GENERATE -> QUEUE -> SEIZE -> DEPART -> ADVANCE -> RELEASE -> TERMINATE. The output report gives barber utilisation, average waiting time and maximum queue length.