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Diffstat (limited to 'manual/CHAPTER_Overview.tex')
| -rw-r--r-- | manual/CHAPTER_Overview.tex | 21 |
1 files changed, 17 insertions, 4 deletions
diff --git a/manual/CHAPTER_Overview.tex b/manual/CHAPTER_Overview.tex index ae5cf094..4136efed 100644 --- a/manual/CHAPTER_Overview.tex +++ b/manual/CHAPTER_Overview.tex @@ -331,8 +331,9 @@ to update {\tt \textbackslash{}q}. An RTLIL::Process is a container for zero or more RTLIL::SyncRule objects and exactly one RTLIL::CaseRule object, which is called the {\it root case}. -An RTLIL::SyncRule object contains an (optional) synchronization condition -(signal and edge-type) and zero or more assignments (RTLIL::SigSig). +An RTLIL::SyncRule object contains an (optional) synchronization condition (signal and edge-type) and zero or +more assignments (RTLIL::SigSig). The {\tt always} synchronization condition is used to break combinatorial +loops when a latch should be inferred instead. An RTLIL::CaseRule is a container for zero or more assignments (RTLIL::SigSig) and zero or more RTLIL::SwitchRule objects. An RTLIL::SwitchRule objects is a @@ -350,6 +351,18 @@ 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}. @@ -428,8 +441,8 @@ memory object has the following properties: All read accesses to the memory are transformed to {\tt \$memrd} cells and all write accesses to {\tt \$memwr} cells by the language frontend. These cells consist of independent read- and write-ports -to the memory. The \B{MEMID} parameter on these cells is used to link them together and to the -RTLIL::Memory object they belong to. +to the memory. Memory initialization is transformed to {\tt \$meminit} cells by the language frontend. +The \B{MEMID} parameter on these cells is used to link them together and to the RTLIL::Memory object they belong to. The rationale behind using separate cells for the individual ports versus creating a large multiport memory cell right in the language frontend is that |