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Single Bit Full Adder
The 16-bit ALU uses 16 of these full adder
circuits, in a ripple adder configuration. ALU16 maniplates
the carry bit between its second and third instance of this adder,
when it executes the functions that compute branch targets.
Because these adders are used in a ripple carry in ALU-16,
(as they are also used within ALU8's adder),
the addition is the slowest portion (timing critical path) of
OSU8's circuitry. For Xilinx, this should probably use the
fast-carry logic within the Xilinx CLBs, or custom CMOS this should
probably be a Manchester carry or carry-lookahead type adder.
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Schematic
CPU Programming