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1. Resident time variables
A number of variables are permanently defined and are available for use in a model. Their value can be used directly in any variable expression of a model, and cannot be modified in assignment statements in the model. Their names are visible in any model, and can be used directly without being declared in a VAR declaration.
The following resident variables provide access to the time characteristics of the simulation of a model.
Variable 
Description 
A separate set of values of these variables exists locally for each use of each model. The timestep can be declared locally in the header of each model TIMESTEP min:recip(SampleFreq) 

t 
the present value of the simulation time in the model; 
prevtime 
the previous value of the simulation time in the model; 
timestep 
the present value of the simulation interval in the model (=tprevtime); 
endtime 
the external simulation time to which the model is to be updated; 
fullstep 
the total time interval over which the model is to be updated (= endtime  previous endtime); 
maxstep 
the calculated present maximum allowed value of the time step in the model; 
minstep 
the calculated present minimum allowed value of the time step in the model; 
Other resident variables are defined globally for a simulation: 

starttime 
the value of time at the start of the simulation; 
stoptime 
the value of time at which the simulation will end; 
startstep 
the value of the outermost time step at the start of the simulation. 
In addition to representing the present value of the simulation time in a model, the variable t can also be used as a regular reference to the value of time in expressions describing history functions in a model (see HISTORY).
2. Resident constants
A number of numerical and logical constants are permanently defined and are available for use in model descriptions. Their value can be used directly in any expression of a model. Their names are visible in any model, and can be used directly without being declared in a CONST declaration.
The following resident constants are defined:
Constant 
Value 
Resident numerical constants: 

pi 
= 3.14159... 
inf 
= very large number fitting the computer in use 
undefined 
= 88888.88888 (value held by any element before it is assigned a first value) 
Resident logical constants: 

false 
= 0 
true 
= 1 
no 
= 0 
yes 
= 1 
open 
= 0 
closed 
= 1 
off 
= 0 
on 
= 1 