Verification of Parameterised FPGA Circuit Descriptions with Layout ...

CHAPTER 3. GENERATING PARAMETERISED LIBRARIES WITH LAYOUT 37
so that, for example, block R can be positioned explicitly at an offset of the (unknown)
size of block S. We do this by providing height() and width() functions that can be used in
expressions to explicitly refer to the size of a particular block.
Quartz combinators can therefore be used to provide relative placement capabilities though
they are themselves described in terms of explicit coordinates. This approach has the ad-
vantage that layout and function can be described by the same code and in many cases a
geometrically sensible layout is a natural by-product of the functional description.
The Quartz layout system provides an architecture independent abstract homogeneous grid
of resources. This allows the layout infrastructure to be developed in an architecture indepen-
dent manner and layout information can be translated into the specific format required for
the target architecture during compilation. The assumption that the available resources are
arranged in a homogeneous grid is an approximation to the complexity of modern FPGAs
where the device fabric will typically also include specialised resources such as embedded
RAM or fast multipliers however it is sufficiently close to reality for practical use.
The Quartz layout grid is composed of elements of uniform size 1 × 1, so all positions on the
grid can be addressed using purely integer co-ordinates. This is, once again, an approximation
to FPGA structure where a single computational block has multiple resources, for example
the Xilinx Virtex-II slice illustrated in Chapter 2 includes a look-up table and also other
logic gates and multiplexers. How this simplified grid is used to describe real circuits will be
demonstrated in Chapter 6.
3.3 Requirements
Figure 3.1 shows the different Quartz statements. Each type of statement produces particular
kinds of structures with specific layout requirements. Our layout infrastructure must be able
to support all of these structures. It is important to note that what we are essentially
discussing how the size of each statement can be described since if the explicit coordinate
system is to support fully relative placement then it is necessary to be able to place statements
relative to the size of other statements. The same approach can be extended to whole blocks
and each block should be given a size function, parameterised in the same way as the block