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Explain exact semantics of switch and case rules in the manual.

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whitequark 2019-06-19 05:22:40 +00:00
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manual/CHAPTER_Overview.tex
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@ -350,6 +350,18 @@ and this bit is a one (the second ``1'').} for {\tt \textbackslash{}reset == 1}
sets {\tt \$0\textbackslash{}q[0:0]} to the value of {\tt \textbackslash{}d} if {\tt
\textbackslash{}enable} is active (lines $6 \dots 11$).
A case can specify zero or more compare values that will determine whether it matches. Each of the compare values
must be the exact same width as the control signal. When more than one compare value is specified, the case matches
if any of them matches the control signal; when zero compare values are specified, the case always matches (i.e.
it is the default case).
A switch prioritizes cases from first to last: multiple cases can match, but only the first matched case becomes
active. This normally synthesizes to a priority encoder. The {\tt parallel\_case} attribute allows passes to assume
that no more than one case will match, and {\tt full\_case} attribute allows passes to assume that exactly one
case will match; if these invariants are ever dynamically violated, the behavior is undefined. These attributes
are useful when an invariant invisible to the synthesizer causes the control signal to never take certain
bit patterns.
The lines $13 \dots 16$ then cause {\tt \textbackslash{}q} to be updated whenever there is
a positive clock edge on {\tt \textbackslash{}clock} or {\tt \textbackslash{}reset}.