Verification of Parameterised FPGA Circuit Descriptions with Layout ...

CHAPTER 3. GENERATING PARAMETERISED LIBRARIES WITH LAYOUT 63
function max(a : integer ; b : integer ) return integer is
begin
if a > b then
return a ;
else
return b ;
end if ;
end function max;
Figure 3.18: VHDL max function
template can be parameterised in its upper and lower bounds and thus only one function
needs to be generated for each usage of sum over the same function. The semantics of this
VHDL sum function are the same as those of the Quartz/LE-Pebble function. If the range
of the sum is zero then the default value of a (0) is returned, otherwise the sum is returned.
An equivalent template can be instantiated for maxf. The n-input Quartz/LE-Pebble max
function can be transformed to use nested calls to a 2-input VHDL max function defined as
in Figure 3.18. Conditional expressions can be implemented using VHDL conditionals.
Placement co-ordinates themselves are compiled into the appropriate constructs for a partic-
ular target architecture, as specified by a directive in the source file. We have implemented
placement support for the Xilinx Virtex and Virtex-II architectures, generating RLOC place-
ment macros. Virtex and Virtex-II use different co-ordinate schemes and the Pebble compiler
maps from the basic abstract grid onto these co-ordinate schemes.
For the Virtex-II architecture, for example (see Chapter 2), each slice contains two look-up
tables and other logic that can be explicitly instantiated. However placement co-ordinates
are described in terms of individual slices. When mapping from Pebble to VHDL the grid is
squashed so each set of two vertically adjacent 1 × 1 blocks are placed in the same slice. The
basic layout element is thus a half-slice.
3.9 Summary and Comparison with Related Work
In this chapter we have described a layout infrastructure for the Quartz language which allows
designers to apply explicit absolute or relative placement information to hardware designs.
Our infrastructure allows Quartz higher-order combinators to be given layout interpretations