1.2.3. Arithmetic Mode
The ALM in arithmetic mode uses two sets of two 4-input LUTs along with two dedicated full adders.
The dedicated adders allow the LUTs to perform pre-adder logic; therefore, each adder can add the output of two 4-input functions.
The ALM supports simultaneous use of the adder’s carry output along with combinational logic outputs. The adder output is ignored in this operation.
Using the adder with the combinational logic output provides resource savings of up to 50% for functions that can use this mode.
Arithmetic mode also offers clock enable, counter enable, synchronous up and down control, add and subtract control, synchronous clear, and synchronous load.
The LAB local interconnect data inputs generate the clock enable, counter enable, synchronous up/down, and add/subtract control signals. These control signals are good candidates for the inputs that are shared between the four LUTs in the ALM.
The synchronous clear and synchronous load options are LAB-wide signals that affect all registers in the LAB. You can individually disable or enable these signals for each register. The Intel® Quartus® Prime software automatically places any registers that are not used by the counter into other LABs.
The carry chain provides a fast carry function between the dedicated adders in arithmetic or shared arithmetic mode.
The two-bit carry select feature in Stratix® V devices halves the propagation delay of carry chains within the ALM. Carry chains can begin in either the first ALM or the fifth ALM in a LAB. The final carry-out signal is routed to an ALM, where it is fed to local, row, or column interconnects.
To avoid routing congestion in one small area of the device when a high fan-in arithmetic function is implemented, the LAB can support carry chains that only use either the top half or bottom half of the LAB before connecting to the next LAB. This leaves the other half of the ALMs in the LAB available for implementing narrower fan-in functions in normal mode. Carry chains that use the top five ALMs in the first LAB carry into the top half of the ALMs in the next LAB in the column. Carry chains that use the bottom five ALMs in the first LAB carry into the bottom half of the ALMs in the next LAB within the column. You can bypass the top-half of the LAB columns and bottom-half of the MLAB columns.
The Intel® Quartus® Prime Compiler creates carry chains longer than 20 ALMs (10 ALMs in arithmetic or shared arithmetic mode) by linking LABs together automatically. For enhanced fitting, a long carry chain runs vertically, allowing fast horizontal connections to the TriMatrix memory and DSP blocks. A carry chain can continue as far as a full column.
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