Instruction Decoders and Combinatorial Circuits

The instruction decoder is arguably the most important part of a computer's
CPU. Although the CPU is often thought of as little more than a glorified
calculator in which arithmetic operations are done, such math work is
actually the job of the Arithmetic Logic Unit (ALU), a particular section of
the CPU. The real job of the CPU is simply to follow instructions. The most
fundamental operation of a computer's CPU is to receive opcodes and to
execute the instructions that those opcodes correspond to. For a CPU, the
first step in carrying out those instructions is to use the instruction
decoder.

The instruction decoder is the circuit that decodes an opcode. As an example,
on the 6502 CPU, the opcode for the CLI (clear interrupt) instruction is
01011000 in binary. The CPU receives this as a set of digital logic signals
on the data bus. How does it go from getting these signals to executing the
procedure for the CLI instruction? Answer: The instruction decoder receives
the opcode and uses it to activate the correct instruction procedure.

An instruction decoder is essentially a form of combinatorial circuit. A
combinatorial circuit is a circuit which has several inputs, and combines the
state of all these inputs to determine what the output will be. Logic gates
are combinatorial circuits, because their output depends on the state of all
the inputs. Memory address decoders are also combinatorial circuits, because
although they have several outputs, only one output is active at a time, and
the active output will be different for every combination of inputs.
(Combinatorial circuits are not to be confused with combination circuits. A
"combination circit" can mean several things, but usually refers to a circuit
which has both series and parallel architecture, also known as a
"series-parallel" circuit.)

The instruction decoder has several inputs. These inputs come directly from
the data bus; Whenever the CPU has to perform an instruction, the opcode gets
sent to the instruction decoder first, and the instruction decoder will
process all the binary digits that make up the opcode to decide what the CPU
will do next. Obviously, since each instruction has a different opcode,
meaning a different combination of 1s and 0s, each possible combination of
input bits for the instruction decoder means a different output from it.

Exactly what format the instruction decoder's output takes may vary. In some
CPUs, it may be simply a one-bit output that gets sent to a separate circuit.
In such a situation, each CPU opcode has a separate physical circuit that is
triggered by the output from the instruction decoder. In other CPUs, the
instruction decoder's output is another set of bits which form a pointer to
the instruction's location in "micro memory".