This features allows two new instructions in the tech file "drc"
section: "exception <name>|none" and "exemption <name>|none". The
purpose of these rules is to allow DRC rule exceptions for certain
areas which are marked, usually by a GDS identifier layer. Because
GDS identifier layers can be cast as "mask-hint" properties, the
DRC rule exceptions make use of that (recent) feature. So rules
can be specified as "exceptions" (rule applies for layout inside
the marked area) or "exemptions" (rule is ignored for layout inside
the marked area). Rules following the statements will have the
exception or exemption applied until another exception or exemption
statement is given. Either "exception none" or "exemption none" will
stop applying any rule exception or exemption. This is especially
useful for applications like SRAM where there may be a number of
rule exceptions on different layers. The previous way to handle
that was to create new layers in magic for any layer that needed a
rule exception. That mainly resulted in very messy tech files,
and a large number of defined layers. This implementation is both
simpler and cleaner.
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