\section{Montium Tile Processor}
-This section describes the Montium Tile Processor (Montium) in moderate detail.
-It is not meant to be a full spec, but it provides the context necessary for
-understanding the next sections and getting a feel for the challenges involved.
+The Montium Tile Processor (Montium) is the main product of Recore Systems. It
+is a reconfigurable processor that is aimed for inclusion in a tiled,
+heterogenous multi- or manycore system on chip (SoC), connected to other tiles
+and the outside world through a network on chip (NoC).
+
+The Montium has a number of fundamental differences with "regular" processors
+and DSP engines, that make it both interesting and challenging to program for
+both application programmers and compilers.
+
+\begin{figure}
+ \epsfig{file=Img/MontiumOverview.eps, width=.5\textwidth}
+ \caption{Overview of the Montium design}
+\end{figure}
+
+\subsection{Overall design}
+The Montium is built from a few parts. The central part is the interconnect,
+which ties memories, Arithmetic and Logic Units (ALU) and the Communication
+and Configuration Unit (CCU) together. The memories store data locally, the
+ALU's process data and the CCU moves data and configuration on and off the
+Montium. Furthermore, there is a sequencer, which is the closest thing to a
+normal processor in the Montium: It accepts and executes instructions one by
+one, is capable of performing (conditional) jumps and some other limited control
+flow.
+
+\subsubsection{Sequencer}
+The Sequencer executes its instructions one by one and controls all other
+elements through the configuration registers (CR). To keep the size of sequencer
+instructions limited, while not limiting the flexibility of the other elements,
+two levels of configuration registers is introduced. These registers are wide
+and contain multiple sets of input signals to the various multiplexers, function
+units, etc.
+
+The sequencer instructions in turn contain indices into these configuration
+registers. This way, every sequencer instruction can select a configuration for
+the entire Montium for the cycle during which the instruction is executed. This
+also means that the Montium is reconfigured on every cycle, for maximum
+flexibility and performance.
+
+Using a two-level configuration register scheme ensures that when a (part of) a
+particular configuration is reused in more then one sequencer instruction, it
+does not have to be duplicated entirely. Only the index pointing to the right
+configuration register (which is a lot smaller) is duplicated in multiple
+sequencer instructions. This does of course limit the amount of different
+configurations that a single program can use and thus limit the size of a
+Montium program.
+
+\subsubsection{Memories}
+The Montium contains ten memories (two for each ALU). Each of these memories has
+its own Address Generation Unit (AGU), which can generate different memory
+patterns. This means that the instructions or CRs never contain direct memory
+addresses, only modifications to the current address. Each memory simply reads
+from its current address and offers the value read to the interconnect (which
+can then further distribute it to wherever it is needed). Writing works in the
+same way (though a memory can only read or written in the same cycle TODO: Is
+this true?).
+
+\subsubsection{ALU's}
+The main processing elements of the Montium are its 5 ALU's. Each of them has
+four (16 bit) inputs, each with a number of input registers. Each ALU contains a
+number of function units, a multiplier, a few adders and some miscelaneous
+logic. Each of the elements in the ALU can be controlled seperately and data can
+be routed in different ways through configuration of multiplexers inside the
+ALU. The ALU has two output ports, without registers. Additionally, there is a
+connection from each ALU to its neighbour.
+
+The ALU also has no internal registers, so data travels through the entire ALU
+in a single cycle, to arrive at the outputs before the end of the cycle. This
+means that the ALU can perform a lot of computation in a single clock cycle. For
+example, using four of the five ALU's, an FFT butterfly operation (two complex
+multiplications and four complex additions TODO: Right?) can be exected in a
+single clock cycle. The downside of this approach is that the data will have a
+long path to travel over, which limits the clock speed of the design.
+
+\subsubsection{CCU}
+The CCU controls communication with the external world, usually a
+network-on-chip. During normal operations, the CCU can take values from the
+interconnect and stream them out onto the NoC, or vice versa. Additionally, the
+CCU can be used from external to the Montium to start and stop execution and
+move configuration registers, sequencer instructions and memory contents into
+and out of the Montium.
+
+\subsubsection{Interconnect}
+The central part of the Montium is the interconnect, which is a mostly connected
+crossbar of lines. There are a total of 10 global busses in the interconnect, to
+which every input and output port of the various components can be connected.
+This way, every output of the memories, ALU's and CCU can be routed to every
+input (provided that there are enough global busses). Additionally, each pair of
+memories belonging to a specific ALU can be routed directly to the inputs and
+outputs of that ALU, without requiring a global bus.
+
+\subsection{Design changes}
+Currently, the Montium design is experiencing a major overhaul. During work with
+the original design, a number of flaws or suboptimal constructs have been found.
+In particular, the ALUs are capable of performing a large number of operations
+in a single cycle, but since they operate sequentially, this severly limits
+clock speeds. In the new design, the number of ALUs is reduced, but each ALU is
+subdivided in multiple parallel-operating function units.
+
+This approach requires computations to be properly pipelined to be efficiently
+use all those function units in parallel, but since data only travels through
+only a single function unit in each cycle, this allows for much higher clock
+speeds than the old design.
+
+During my internship I have mainly been working with the old Montium design, and
+unless otherwise stated, that is what is meant when referring to the "Montium".
+Some of the work has been done with the new design in mind, but only during the
+final weeks of my internship I have been involved with the new design enough to
+see most of the picture. See section \ref{Pipelining} for more details.
projects / matthijs / projects / internship.git / blobdiff