Stratix III ALM Logic Structure's 8-Input Fracturable LUT

Stratix^® III FPGAs leverage Altera's highly successful and innovative adaptive logic module (ALM) logic structure (shown in Figure 1) to provide the most efficient logic fabric ever in any FPGA.

Figure 1. Stratix III Device ALM

Each Stratix III device ALM has eight inputs with a fracturable look-up table (LUT), two dedicated embedded adders, two dedicated registers, and additional logic enhancements. These features enable the ALM to implement select 7-input LUT-based functions, all 6-input logic functions, and two independent functions consisting of smaller LUT sizes (such as two independent 4-input LUTs).

Figure 2 shows the different LUT configurations that a single ALM can support, and Table 1 describes each ALM configuration.

Figure 2. Fracturability of a Stratix III Device ALM

Table 1. Stratix III Device ALM Configurations
Configuration	Description
	One Stratix III device ALM can support any 6-input logic function.
	One Stratix III device ALM can be configured to implement two independent 4-input or smaller LUTs.
	One Stratix III device ALM can be configured to implement a 5-input LUT and 3-input LUT. The inputs to the two LUTs are independent of each other. The 3-LUT can be used to implement any logic function that has three or fewer inputs. Therefore, a 5-input LUT and 2-input LUT combination is also available.
	One Stratix III device ALM can be configured to implement a 5-input LUT and a 4-input LUT. One of the inputs is shared between the two LUTs. The 5-input LUT has up to four independent inputs. The 4-input LUT has up to three independent inputs. The sharing of inputs between LUTs is very common in FPGA designs, and the Quartus® II software automatically seeks logic functions that are structured in this manner.
	One Stratix III ALM can be configured to implement two 5-input LUTs. Two of the inputs between the LUTs are common, and up to three independent inputs are allowed for each 5-input LUT.
	If two 6-input functions have the same logic operation and four shared inputs, then the two 6-input functions can be implemented in one Stratix III device ALM.
	One Stratix III device ALM in the extended mode can implement a subset of a 7-variable function. The Quartus II software automatically recognizes the applicable 7-input function and fits it into an ALM.

Table 2 provides links to additional information about Stratix III FPGAs.