Verification of Parameterised FPGA Circuit Descriptions with Layout ...

CHAPTER 7. CONCLUSION AND FUTURE WORK 170
7.3.3 Ruby and Lava
Both Ruby [26] and Lava [7] have been used to generate placed circuit descriptions using
their higher-order combinators. Ruby and Lava combinators can describe both function and
placement, as with Quartz. Neither the Ruby nor Lava system supports the generation of
parameterised output, instead they generate flattened netlists.
Ruby uses a variable-free notation of relations to describe circuits and it is thus relatively
easy to give key combinators, such as beside and below a layout interpretation which can then
be used to generate placed output. Ruby’s design style does not support explicit instantiation
and signal connection and can not support explicit co-ordinates. This is a limiting factor and
the system can not support the irregular grid example. No explicit verification infrastructure
is available for Ruby layouts, however since the output is a flattened netlist and placement
is limited to beside/below relationships it should be impossible to describe invalid layouts.
Lava is a more flexible system based on Haskell. Lava provides a combinators which place
components below each other, beside each other or at the same location. Unlike Ruby, it
does not enforce a variable-free notation and is thus more flexible. A version of Lava has also
been demonstrated that supports layout with explicit co-ordinates [77].
While Lava provides constructs to aid the construction of correct layouts (using beside and
below), it also permits possibly invalid layouts (by placing components in the same slice 2 ,
either explicitly or using the combinator provided for this purpose). Unlike Ruby, Lava does
therefore permit the description of invalid layouts and there is no infrastructure available for
verifying Lava layouts however our layout verification infrastructure could be adapted to this
purpose.
Ruby and Lava use recursion as their only means of repetition. The languages do not support
iterative descriptions, which can be a clearer way of describing some circuit arrangements
than recursion (although they are not more powerful per se), although this is less relevant
than for Quartz since they do not produce output in a format that supports iteration.
Ruby and Lava do not support giving combinators different layout interpretations in the
2 The version of Lava which supports placement was developed at Xilinx and is designed specifically to
target Xilinx FPGAs.