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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