Digital gate definition language

   1.  Simple digital gates in LOG can be described entirely in the
       modest interpreted language described here.  For more complicated
       functionality, for displays and user interaction, or for access
       to the gate's attributes, you must write Pascal code using a
       CALL statement.  The interpreted language is usually referred to
       as "LOGED language," though it is really part of the digital
       simulator, not of LOGED proper.  The language is also called
       Gate Description Language, or GDL.

   2.  A GDL program consists of a sequence of statements, displayed one
       per line.  These statements are typically either assignments to
       pins or variables, or IF/ELSE/END constructions.  GDL contains no
       loops, arrays, or arithmetic.  The program is executed once per
       LOG time-step, from top to bottom, performing boolean operations
       on values in the gate.

   3.  Resources available to a GDL program are pins, notated as in "#3";
       internal nodes, notated as in "##6"; and state variables, such
       as "A", "P", or "V26".  A state variable is much like a Pascal
       variable; an assignment to it replaces the previously assigned
       value, and lasts until the next assignment.  It is a 1-bit quantity
       whose value is interpreted as a boolean logic 1 or 0.  A pin or
       internal node is a true LOG node; it uses a five-valued logic
       (One, Zero, undriven or "None", and weak One and Zero) and must be
       assigned to once per time-step.  The assigned value does not show up
       on the node until the next time-step.  If the gate uses only nodes
       (no state variables), the order of execution of statements within
       the gate is essentially arbitrary.

   4.  Pin numbers range from 1 to the number of pins on the gate.
       The 16 state variables A through P are always available.  For
       internal nodes and additional state variables, you can use the
       INST statement described below.  All state variables are initially
       Zero when the gate is created.

   4.  Each GDL statement must be one of the following:


       < variable >  =  < expression >
                  Assign the value of the expression to the variable.
                  The previous value of the variable is replaced.  This
                  is exactly like a variable assignment in a conventional
                  language.  If the value of the expression's value is
                  None, the variable is set to One.


       < node >  = < expression > 
                  Output the value of the expression to the pin or
                  internal node.  If two statements (in the same or
                  different gates) output conflicting values to the
                  node in the same time-step, a conflict is registered
                  on the node.  Outputting None to a node has no effect.


       < node >  = PULLUP   or   PULLDN
                  Output a weak One or Zero (respectively) to the
                  node.  This essentially registers a default value
                  for the node; if no other GDL statement assigns a
                  strong value to the node in this time-step, the
                  weak default will be used.  A strong value overrides
                  a weak value without conflict.  A conflict will be
                  registered if the node is weakly pulled to One and
                  Zero simultaneously.

       < node >  '< ' < expression > 
                  Output an open-collector value to the node.  If
                  the expression's value is Zero, it is driven to the
                  node.  If the expression's value is One or None,
                  the node is left alone.


       IF < condition >  < expression > 
          < statements > 
       ELSE
          < statements > 
       END
                  If the condition is satisfied, the first set of
                  statements are executed.  Otherwise, the second set
                  (if any) are executed.  The conditions available
                  cover all possibilities of One, Zero, and None
                  values for the expression:

                        IF x        True if x=One or None.
                        IFONE x     True if x=One.
                        IFZN x      True if x=Zero or None.
                        IFZERO x    True if x=Zero.
                        IFCONN x    True if x=One or Zero.
                        IFNONE x    True if x=None.


       CALL < procedure-name > 
                  Call the Pascal procedure specified.  The name has the
                  usual "modulename_procedurename" form.  If there are
                  CALL statements in a gate's procedure, those procedures
                  are called in order when the gate is simulated, drawn,
                  tapped, created, destroyed, copied, configured, etc.


       INST < num-nodes >  , < num-vars > 
                  This statement, if used, must be the first statement of
                  the program (before even comments and blank lines).
                  The statement "INST 17,6" reserves 17 internal nodes,
                  ##0 through ##16, and 6 additional state variables,
                  V0 through V5, for the gate.  If the number of variables
                  is zero, it and the comma can be omitted.


   5.  GDL statements may also be blank lines or comments.  A comment
       line begins with a "#" not followed by a digit.  Blank lines and
       comments do have a small impact on the simulator's performance.


   6.  A GDL expression is a series of "factors" joined by binary
       operators.  All operators have the same precedence and are
       evaluated left-to-right.  In the following list of operators,
       X is any expression and Y is any factor.


       x AND y    Boolean AND, OR, or exclusive OR of X and Y.  If
       x OR y     either X or Y is None, the result is the other input.
       x XOR y    If both are None, the result is None.


       x NAND y   Equivalent to "NOT (x AND y)".


       x NOR y    Equivalent to "NOT (x OR y)".


       x SAME y   Both X and Y must be pins.  Equal to One if both
                  pins are connected to the same electrical node, or
                  Zero if the pins are not on the same node.


   7.  A GDL factor is either an expression in parentheses, or one of
       the following (X is any factor; P is any pin or internal node):


       < var >       A state variable.  The result is One or Zero depending
                  on the current value of the variable.


       < node >      A pin number or internal node.  The result is either
                  One, Zero, or None, depending on the value driven on
                  the node in the previous time-step.


       ONE        The constant value, One.  This can also be written
                  "TRUE" or "1".


       ZERO       The constant value, Zero.  Also "FALSE" or "0".


       NOT x      The result is One if X is Zero, Zero if X is One, or
                  None if X is None.


       RISE p     The result is One if the specified node is receiving
                  a rising transition, that is, the previous time-step
                  drove the pin to a One, but the time-step before that
                  drove it to a Zero.  Otherwise, the result is Zero.


       FALL p     The result is One if the specified node is receiving
                  a falling (One to Zero) transition.


       FIX x      The value of X, with None converted to Zero.


       STRONG p   The result is One or Zero if the specified node was
                  driven by a strong (i.e., normal) value on the previous
                  time-step, or None if the node was undriven or was only
                  weakly driven.


   8.  For examples of GDL programs, consult the many gates in the files
       "log.gate" and "actel.gate".

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