-configured by the customer. A hardware description language (HDL) is used to
-configure a FPGA. FGPAs are composed of programmable logic components, called
-logic blocks. These blocks can be configured to perform simple (AND, XOR, ...)
-or complex combinational functions. Logic blocks are interconnected by
-reconfigurable links. Modern FPGAs contain memory elements and multipliers which
-enable to simplify the design and to increase the speed. As the most complex
-operation on FGPAs is the multiplier, design of FGPAs should use simple
-operations. For example, a divider is not an operation available and it should
-be programmed using simplest operations.
-
-FGPAs programming is very different from classic processors programming. When
-logic blocks are programmed and linked to perform an operation, they cannot be
-reused anymore. FPGAs are cadenced more slowly than classic processors but they
-can perform pipeline as well as parallel operations. A pipeline provides a way
-to manipulate data quickly since at each clock top it handles a new
-data. However, using a pipeline consumes more logics and components since they
-are not reusable. Nevertheless it is probably the most efficient technique on
-FPGA. Parallel operations can be used in order to manipulate several data
-simultaneously. When it is possible, using a pipeline is a good solution to
-manipulate new data at each clock top and using parallelism to handle
-simultaneously several pipelines in order to handle multiple data streams.
-
-%% parler du VHDL, synthèse et bitstream
+configured by the customer. FGPAs are composed of programmable logic components,
+called configurable logic blocks (CLB). These blocks mainly contain look-up
+tables (LUT), flip/flops (F/F) and latches, organized in one or more slices
+connected together. Each CLB can be configured to perform simple (AND, XOR, ...)
+or complex combinational functions. They are interconnected by reconfigurable
+links. Modern FPGAs contain memory elements and multipliers which enable to
+simplify the design and to increase the performance. Nevertheless, all other
+complex operations, like division, trigonometric functions, $\ldots$ are not
+available and must be done by configuring a set of CLBs. Since this
+configuration is not obvious at all, it can be done via a framework, like
+ISE~\cite{ISE}. Such a software can synthetize a design written in a hardware
+description language (HDL), map it onto CLBs, place/route them for a specific
+FPGA, and finally produce a bitstream that is used to configure the FPGA. Thus,
+from the developer's point of view, the main difficulty is to translate an
+algorithm in HDL code, taking into account FPGA resources and constraints like
+clock signals and I/O values that drive the FPGA.
+
+Indeed, HDL programming is very different from classic languages like
+C. A program can be seen as a state-machine, manipulating signals that
+evolve from state to state. Moreover, HDL instructions can executed
+concurrently. Basic logic operations are used to aggregate signals to
+produce new states and assign it to another signal. States are mainly
+expressed as arrays of bits. Fortunately, libraries propose some
+higher levels representations like signed integers, and arithmetic
+operations.
+
+Furthermore, even if FPGAs are cadenced more slowly than classic
+processors, they can perform pipeline as well as parallel
+operations. A pipeline consists in cutting a process in a sequence of
+small tasks, taking the same execution time. It accepts a new data at
+each clock top, thus, after a known latency, it also provides a result
+at each clock top. However, using a pipeline consumes more logics
+since the components of a task are not reusable by another
+one. Nevertheless it is probably the most efficient technique on
+FPGA. Because of its architecture, it is also very easy to process
+several data concurrently. Whenever possible, the best performance
+is reached using parallelism to handle simultaneously several
+pipelines in order to handle multiple data streams.
+
