1.2.4. Shared Arithmetic Mode
The ALM in shared arithmetic mode can implement a 3-input add in the ALM.
This mode configures the ALM with four 4-input LUTs. Each LUT either computes the sum of three inputs or the carry of three inputs. The output of the carry computation is fed to the next adder using a dedicated connection called the shared arithmetic chain.
Shared Arithmetic Chain
The shared arithmetic chain available in enhanced arithmetic mode allows the ALM to implement a 3-input adder. This significantly reduces the resources necessary to implement large adder trees or correlator functions.
The shared arithmetic chain can begin in either the first or sixth ALM in a LAB.
Similar to carry chains, the top and bottom half of the shared arithmetic chains in alternate LAB columns can be bypassed. This capability allows the shared arithmetic chain to cascade through half of the ALMs in an LAB while leaving the other half available for narrower fan-in functionality. In every LAB, the column is top-half bypassable; while in MLAB, columns are bottom-half bypassable.
The Intel® Quartus® Prime Compiler creates shared arithmetic chains longer than 20 ALMs (10 ALMs in arithmetic or shared arithmetic mode) by linking LABs together automatically. To enhance fitting, a long shared arithmetic chain runs vertically, allowing fast horizontal connections to the TriMatrix memory and DSP blocks. A shared arithmetic chain can continue as far as a full column.
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