The operation of a processor is characterized by a fetch-decode-execute cycle. In the first phase
of the cycle, the
processor fetches an instruction from memory. The address of the
instruction to fetch is stored in an internal register
named the program counter, or PC. As the processor
is waiting for the memory to respond with the instruction, it
increments the PC. This means the fetch phase of the next cycle
will fetch the instruction in the next sequential location in
memory (unless the PC is modified by a later phase of the cycle).
In the decode phase the processor stores the information returned by the memory in another internal register, known as the instruction register, or IR. The IR now holds a single machine instruction, encoded as a binary number. The processor decodes the value in the IR in order to figure out which operations to perform in the next stage.
In the execution stage the processor actually carries out the instruction. This step often requires further memory operations; for example, the instruction may direct the processor to fetch two operands from memory, add them, and store the result in a third location (the addresses of the operands and the result are also encoded as part of the instruction). At the end of this phase the machine starts the cycle over again by entering the fetch phase for the next instruction.
Instructions can be classified as one of three major types: arithmetic/logic, data transfer, and control. Arithmetic and logic instructions apply primitive functions of one or two arguments, for example addition, multiplication, or logical AND. In some machines the arguments are fetched from main memory and the result is returned to main memory, but more often the operands are all in registers inside the CPU. Most machines have a set of general purpose registers that can be used for holding such operands. For example the HP-PA processor in Hewlett-Packard workstations has 32 such registers, each of which holds a single number.
The data transfer instructions move data from one location to another, for example between registers, or from main memory to a register, or between two different memory locations. Data transfer instructions are also used to initiate I/O operations.
Control instructions modify the order in which instructions are executed. They are used to construct loops, if-then-else constructs, etc. For example, consider the following DO loop in Fortran:
DO 10 I=1,5
...
10 CONTINUE
To implement the bottom of the loop (at the CONTINUE
statement) there might be an arithmetic instruction that adds 1
to I, followed by a control instruction that compares I to
5 and branches to the top of the loop if I is less than or
equal to 5. The branch operation is performed by simply setting
the PC to the address of the instruction at the top of the loop.
The timing of the fetch, decode, and execute phases depends on the internal construction of the processor and the complexity of the instructions it executes. The quantum time unit for measuring operations is known as a clock cycle. The logic that directs operations within a processor is controlled by an external clock, which is simply a circuit that generates a square wave with a fixed period. The number of clock cycles required to carry out an operation determines the amount of time it will take.
One cannot simply assume that if a multiplication can be done in
nanoseconds then it will take
nanoseconds to perform 
multiplications or
that if a branch
instruction takes 
nanoseconds the next instruction will
begin execution nanoseconds following the branch. The
actual timings depend on the organization of the memory system
and the communication channels that connect the processor to the
memory; these are the topics of the next two sections.