From addf01d45dd795b4fe711012facbe43dd7c2eae7 Mon Sep 17 00:00:00 2001 From: whitequark Date: Wed, 19 Jun 2019 05:22:40 +0000 Subject: [PATCH] Explain exact semantics of switch and case rules in the manual. --- manual/CHAPTER_Overview.tex | 12 ++++++++++++ 1 file changed, 12 insertions(+) diff --git a/manual/CHAPTER_Overview.tex b/manual/CHAPTER_Overview.tex index 2feb0f1cb..1a25c477f 100644 --- a/manual/CHAPTER_Overview.tex +++ b/manual/CHAPTER_Overview.tex @@ -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}.