54 Computer Architecture and Organization
SUMMARY
Digital circuits are based on Boolean algebra, which allows only two states 0 and 1, which may be
electrically modelled as two speci c voltage ranges. Its basic operators are AND, OR and NOT. Other
operators are NAND, NOR, XOR and XNOR. NAND and NOR gates are designated as universal gates
and all other gates may be constructed by using any one of these two gates.
In sequential logic circuits, outputs are dependent on previous states and behave in a dynamic fashion.
Most, not all, sequential circuits are clock based in one way or other. Combinational logic circuits do not
exhibit such a behaviour and rather static in their function. Decoder, multiplexed, adder and so on are
examples of combinational circuits.
This design is a laboratory tested circuit. If the reader is interested in fabricating it, then it
must be constructed and tested module-wise, using a logic probe. The common practice of
completing all wire connections on the breadboard and then switching the power on, is not
a good practice and generally leads to failures and frustration.
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Figure 3.30 Complete circuit of 8 function 4-bit ALU
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